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MG Midget and Sprite Technical - Decisions, decisions... advice please

Morning fellow spridgeters

Well, after my recent coolant issues, and suspected HGF (big bubbles in exp' bottle, very high HC level in coolant), I left the car with our local A-Series specialist.

Long story short, the HG itself was OK, no apparent breaches. However, they did say some of the bolts/studs were looser than they should have been, and there was some obvious staining on the top of the block.

Anyway, I was told that it may well have the wrong pistons in it. Without removing a piston, he did various measurements / calcs, and advised CR was 8.5:1. Now I'm aware the std CR in a UK car of the time should have been 8.8:1, with an optional 8.0:1. He described them as heavily dished, almost like van pistons! They weren't original, obviously, so perhaps in the not too distant past, someone had a set of pistons on the shelf that they were only too pleased to offload onto somebody...

So, option 1 is just to reassemble with fresh gaskets and later / flanged? bolts, clean up the intake etc and set up correctly on the RR. With my existing spec; 1309cc, 266 cam with 1.5 rockers, bigger valves, and the existing twin 1 1/4's, he reckoned he should see around 75hp, perhaps more.

Option 2 was take the opportunity while the head is off to drop the sump and fit higher compression pistons (around 10 or 10.5:1), and reassemble as above, perhaps fitting an HIF44 and RC40 at the same time. With this set up, I was told I could expect 85hp, perhaps more.

So, my question is, under the circumstances, would it be a good idea to swallow the extra £5-600 and get the pistons done now, or leave as is with low compression spec? Do A-Series engines perform noticeably better with higher compressions?

I do fairly spirited road driving, occasional track days, and plan on doing the odd autosolo with my local motor club. Car has Ford 5spd, FL suspension, Luminition, bigger front discs and LCB manifold.

Although I bought the car 'well', I hadn't planned on something like this so soon into ownership, but at the same time, I want the car to be right. So, sort now, or pay more later on?
M Weller

No brainer, they love compression. Do it now, will never cost you less and you won't regret it.
Just make sure you get the right pistons!

10.5:1 A+ pistons on the market now have a similar looking top to 9:1 pistons. The dish is about the same volume. Where the CR is made up is that the compression height is greater. This is a better arrangement that the old flat top HC pistons as it provides more squish around the edge due to the piston being closer to the head at TDC. More squish gives better combustion characteristics and so handles the higher CR better.
Paul Walbran

option 2 I would suggest.
P Bentley

Yep, a set of 6cc dish pistons from say minispares is the way to go. Assuming the head has not been skimmed will give you around 9.75CR
Bob Beaumont

Turbo it!
Karl Bielby

I did think of turbo, but not sure if there's anybody in the south who offers a drive in drive out solution. And I know there was an mg metro turbo but at the back of mind is a nagging doubt that a midget should always be NA...

So seems like it would make sense to just bite the bullet and be done with it, and revel in my enhanced performance!
M Weller

Supercharger. Moss or VmaxScart?

M Crossley

I find that the incorrect pistons would be installed... thats a huge stoke of luck

the most common replacemt piston is AE 21253 HIGH compression pistons they have a deep dish ...probably what you have currently

AE 21251 is the same piston as the 21253 but a lower compression by way of paul described

Im at .60 over and the equals 1330 cc

so if your at 1310 cc thats about .40 over

Prop and the Blackhole Midget

yes get the engine work done as you'll start with a fresher sheet and other stuff might be discovered, which may be better to know about sooner than later.

Might sound daft but is the head already lead free, if not that'd be a must for what you want.

Personally I wouldn't rush for the single carb or very high compression - but I'm not interested in sports use, or could afford it.

Never get too carried away by figures but I'd have thought you'd get more than 85hp without the single carb. Not that it matters because it's how the car goes that matters and if you want to go full sports route then you'd be stripping weight too and possibly making the car less fun and practical for road use.

Before anyone else mentions it (like Daniel :) ) you can get books about this sort of thing -
Nigel Atkins

You also need to check out the bores. No point chucking in expensive new pistons into worn bores, at best it will smoke or worse you'll break a top ring on the step.

It will also need a good hone if fitting new pistons, with the crank fitted I'd be wanting to know that none of the honing residue had gone into the oil ways.

If it were me I would save any major work for a full rebuild when you can start off with a new bottom end and nice new bores.
john payne

Rushing I missed the cost so didn't realise you'd be restricting the work on the engine, I'm not sure what you can do in stages but you'd be better with a full rebuild if you intend to trash it, which will go way outside your present budget.

So you could consider option 1 and stick to road use for a good while to see if anything else needs doing on the car, to sort other niggles and get used to the car and the car used.

Until the bubbles, to some extent at least, you seemed to be enjoying the car as it is, you can improve the whole car (and not just the engine) with use, servicing, maintenance and repair making any future engine improvement even more more balanced to the rest of the car.

When you get the engine fully rebuilt you'll also no doubt at the same time want (need) to get other work done/checked on other components and possible replacements/upgrades, the expense can turn into a long piece of string - better to have planned stages (allowing for not nothing the exact extent of any stage).
Nigel Atkins

The engine was fully rebuilt, to the tune of 3.5k only 3 yrs ago by a well known mg specialist whilst with the PO, which is partly what attracted me to the car in the first place, as I (incorrectly) assumed that would be at least one area where I wouldn't need to throw money out in the short to medium term...

It has unleaded inserts already, though I do tend to run it on 97 Ron.
M Weller

Unless you already have more detail I would want to know more about the present specification and condition before making any decisions.

Firstly, they say the pistons are wrong, giving a low compression. But what is the measured dish capacity of the installed pistons? Low compression could be that the head has been worked on and opened up the chambers, and the fault doesn't actually lie with the pistons.

The HGF could be from a warped head and this needs to be checked accurately. If the chambers have been worked on then skimming the head could solve both problems at pretty minimal cost.

Unless I have missed something in the discussion, I think you definitely need to get more checking, explanation and calculation before making a decision.

Did you think it lacked power?
Do you have the funds to spare?

If both of the above are answered with a yes then fit the pistons, carb and exhaust.

If you where happy with the performance just fix it and drive it.

In both circumstances I would advise to use the minispares competition head stud/nut set.
About 50 quid and with them never had a HG fail

at that amount of money you'd expect the engine to be very good, sounds a lot of money even if you say it quick.

Things that spring to my mind in no particular order -
. all the receipts do relate to that engine
. could be minor faults
. could also be other minor faults as yet undetected
. some fault from running-in (if required)
. I've now gone blank (as usual).

As usual Guy puts up a good point about more info. You don't want to find out how long a very long piece of financial string is.

Not that I know (who) the engine builder is or who told you what they think is wrong now, they may well both be very good experts and do excellent work. I do know that some experts don't always do a good job no matter how much they do or charge.

If you've recently driven other Midgets in good condition of use then you'll know what sort of power to expect from yours subject to number of miles since rebuild.

All being well it'll be a bolt down and use - all the very best with it.

Nigel Atkins

Another thought

K series... it probably wont cost that much more...that will punch out some power and considering what a decent rebuildable A series goes for on evil bay it make a good the budget

Prop and the Blackhole Midget

Maybe the first thing to do is to contact the 'well known MG specialist' (with the engine number and any receipts you have to hand) and just talk to them about it.

I would imagine they would have some records about the spec they built it to...
Andy Phillips (frankenfrog)

Morning all thanks for your comments. It did have head work - gas flow / porting, at the time of the rebuild, including the 266 cam and 1.5 rockers. It also has circa 35.8? / 29.5? valves but the head must have had these already as there's no mention of these on the invoice.
When I get home later today, I'll pull out the invoice and see if these is anything else of relevance on there.
I've already spoken to the engine builder concerned (before I bought the car) and he just confirmed mostly what was on the invoice. He did say he didn't much like midgets and a-series - preferred b's and c's!!!
M Weller

Really important to measure not just the dish but bore volume (via the compression height, or if assembled the distance down the bore at TDC) as well as chamber volume.
There are a number of versions of 1275 pistons, don't jump to conclusions without measuring CR properly.

But do aim for 10-10.5, they do love it.
Paul Walbran

I really don't understand this. It doesn't stack up. A £3500 engine rebuild is extremely expensive and suggests that the work was very extensive at the time. It seems unlikely to me that the engine builder would have used "the wrong pistons".

Your present "specialist" says it has a low (calculated) CR but you don't appear to have the actual measurements that he has calculated that from. I would ask for those first: specifically what is the combustion chamber volume and what is the piston dish area. The other figures can be reasonably accurately estimated. If the CR is low it is most likely due to either one of these volumes being too great.

One question though - was this engine ever fitted with a supercharger? That might explain the specification with a low CR. It could also explain the high rebuild cost at £3500 if it included fitting a supercharger! Maybe it was built for that and has subsequently been changed back to carbs.

Incidentally, my car has dished pistons (6cc) and yet runs at just under 10.5:1 CR, so don't assume that dished pistons will produce a low CR. It all depends on how the head has been worked.

I'll check invoice later and get the measurements taken on Tuesday , he did tell me but with all the other info / stats I've forgotten what he said... 16cc springs to mind hence why he thought they were van pistons. But rather than speculate any further I'll get all the data together and report back later.
M Weller

Joining the undertone of questioning the information coming from 'the specialist', how does one measure the compression ratio of an assembled engine?

I can imagine ascertaining the cylinder's swept volume might be fairly straightforward (volume of ejected fluid from the cylinder during BDC-TDC piston displacement) but how does one ascertain the combined volumes of combustion chamber and concave dish (if any) in the piston head ?

Just curious . . . .
Andy Hock

I used this:
Andy Phillips (frankenfrog)

just regained consciousness after reading £3.5k on the engine rebuild.
P Bentley

Andy H - they took the head off - see post 1.
David Smith

I think Andy H means without taking the head off.

How about a strong but flexible balloon fed through the spark plug hole and filled with fluid?

Something extremely strong but thin.....

Who said condom!!!!! come on, own up.

Rob aka MG Moneypit

Afternoon all, didn't get to speak to engine man today but he's back in tomorrow so will talk to him then, and confirm dish capacity, bore & chamber volumes etc.

I've dug the invoice out for the engine rebuild from 3 years ago (previous owner). It's not the most descriptive, and some other bits have been 'bundled'in with the engine work, so the £3546 is not all the engine rebuild, but most of it is... No mention of fitting new pistons etc...

Also in there is labour for gearbox rebuild (T9) at £375+VAT, and fitting hi-torque starter motor, all new hoses & a vernier. Labour for removing engine & box, cylinder head gas flow and lead free, fitting cam & rockers, refitting everything and running in, was £1815 + VAT. Camshaft was £205, Vernier £147, Rocker Assembly £150 (all inc VAT).

So, not sure if this is going rate or not, but bear in mind they are South East, so probably a bit dearer than all the guys up north...

I'll post the other info tomorrow.
M Weller

OK, from tat it would seem it hasn't been intentionally designed as a low compression engine to take a supercharger as I had speculated.

From the information as it is known I would be surprised that someone would have built it as a relatively enhanced performance engine and intentionally planned for a CR of only 8.5:1 Not impossible, but I think most engine builders familiar with an A series would probably be aiming for something rather higher than this. So either the original engine builder made a miscalculation, or the current "specialist" has got his sums wrong!

Are there no numbers on the piston crowns?
Dave O'Neill 2

I'll get him to check the crowns as well
M Weller

Morning all, spoke to Richard this morning, and now have a clearer picture of the situation.

Now, I understood most, but not all of what he was saying, so forgive me if not all of this is in the most logical order...

Head / Chamber Volume (valves sealed off and filled with liquid) - 19cc

Bore? Volume - 21cc (piston dish approx approx 11-12cc, balance area above piston @ TDC to top of block)

Two above = 40cc, head gasket = 3.8cc, total = 43.8cc clearance volume, giving 327.39cc per cylinder, and an approx CR of 8.5:1.

He said standard bore should be 70.6mm, but with approx .40 oversize mine is 71.6mm. So my engine is 1309cc.

He said ideally he would like a clearance volume of 32.739cc to give an approx CR of 10:1.

So, it seems the problem with my engine as is (excluding the issue with the loose studs), is that the pistons don't come higher enough up the bore at TDC - no idea why, we didn't discuss that.

He said ideally, if he was rebuilding the engine from scratch, he would take up this excess in clearance volume by fitting 6cc pistons and taking a small amount off the block.

As it is, with block still in situ, he said he could fit some 6cc pistons, and take a small amount off the head, though he doesn't normally like taking anything off an otherwise good head, just in case of the need to skim in future / HGF etc.

So, there it is, hope this gives the experts on here a little more to work with, and perhaps gives you a better idea of which of my two options would be the best way to go.
M Weller

With Metro-spec pistons (6cc and are 1mm higher from pin centre to top so come higher up the bore) you wouldn't need to take anything off either block or head.
You'd need to confirm dims with the current pistons out, if you can measure that I have both Midget and Metro ones in stock I can compare with for you.
Paul Walbran

Hi Paul, thanks for that. What specific dimensions should I ask for?
M Weller

You have me at a disadvantage right at the moment, I am out of town for a few days so I can't re-measure to confirm, but my notes say the compression height (from the centre of the pin to the top of the piston at its outer edge) is 38.6mm for the Metro spec pistons, and 37.1mm for the std Midget pistons. If you get stuck let me know, my supplier (in UK) has stock and I can probably get sent direct.
Paul Walbran

Interesting detail.

There is a chart in Vizard's book which lists volumes between the piston crown and the block surface for various bores. So for a +0.040" bore a piston to block clearance of 10 thou gives a volume of 1.02cc From this your piston / block deck volume of about 10cc (21cc total - 11cc piston dish) would mean the piston crown at tdc is around 0.10" below the block deck. (maybe 0.09" if you also allowed for the ring land) Either way that is quite a lot!

The actual volume of the chamber at 19cc is quite low for a 1275 head and suggests the head has been skimmed - especially if any combustion chamber reshaping has been done and is probably about as low as one could reasonably go.

The choice of a deeply dished (11cc) piston and possibly a short crown could possibly be intentional in order to provide adequate clearance at tdc for the valves if it has a high lift cam combined with the 1.5 ratio rockers and the heavily skimmed head.

Your options may be limited. If you can identify the original engine builder they may have records. A discussion with them about the design logic could be worthwhile before you start out on changing to different pistons which might just clash with the valves!

Im backing guy 100%...

The head is way beyound salvage at this 19 cc thats alot of shaving and resurfacing. 22 to 24 cc is the norm head combustion chamber volume

if you want to keep this head... id be looking at cometic head gasket ... they can dial in the CR because they come in all differant sizes

11cc piston dish.... thats extreme imho

im wondering if this started out as a super charged engine then along the way the direction took a sudden turn for an asperiated engine

any chance the mechanic is just doing an eyeball guesstamation

Considering you will need another new head built just like you have at around $1000 .... plus the lower end ...maybe revisit the k series idea agian. Alot more horse power plus 5 main bearings instead of 3 means alot more reliablity..

if i did a k series id dump the efi and ignition system and stick 4 SU hs1 carbs with long induction pipes on it and a crank trigger ignition system ... may loose a few horses but it would still be cool as the north pole...and save a wad of cash in the mean time

Prop and the Blackhole Midget

No, I wouldn't say there is anything wrong with that head, and certainly wasn't suggesting it should be scrapped. But at that capacity it does suggest to me it has been skimmed as much as it can be. There wouldn't be enough metal to contemplate any further skimming.
What I had in mind here was that the reason for the HGF hasn't yet been explained, but if it is down to a poor surface, warping or some other fault with the head, then there isn't any scope for resurfacing it.

The other effect of heavily skimming a head is that it brings the open valve heads that much closer to the piston crowns at tdc. Combined with high lift cam lobes and 1.5 roller rockers that could be why it was necessary to use a deeply dished piston with lots of clearance below the block deck.

Hmmm, interesting but rather disheartening at the same time, will get Richard to clarify the points made above thanks for the input. Starting to think the old girl may not have been such a good buy as I initially thought...
M Weller

Original volume was 21.4cc, yes it does creep out a little if opened out around the valves. 19cc indicates 30-40 though taken off from memory so there should be scope to clean up the face. Check by measuring the overall head thickness.

Guy, the piston/valve interference is worth considering. Being away from the office at present I don't have the figures with me, but I have plotted piston position v valve head position on the same graph for 20 deg or so either side of TDC (an interesting exercise, try it some time) but from memory there's plenty of clearance for all but the wildest cams. (At 20 deg, the piston has moved 2.5mm down the bore from its highest point, by 30 deg it is 5.5mm down the bore and well out of the way)

A typical fast road cam will have around 150 thou valve lift at TDC on 1.5 rockers, 190 thou max while the piston is in the -20 deg -> +20 deg danger zone. That won't even bring it to the head face so it would be well clear of the piston.
Paul Walbran

I may be envisaging problems when there aren't any then! I was looking for some sort of explanation why, assuming it was done by a competent specialist, it might have been built that way. Just trying to interpret those figures and I know that valve clearance can be an issue on some performance engines. But it could easily be checked with a bit of plasticine on the piston crown.

is it possible to find out from the previous owner, or perhaps engine builder, what the idea was with the engine in relation to the car.

Did, perhaps, the direction of the car as a project change or was it always to be a Q/sleeper style car that is standard looking but fast underneath. Or perhaps the engine need unexpected work and things just spiraled on.
Nigel Atkins

Paul... you know i worship the ground you walk...


im not sure about your 150 thousand spec ... i fear its a little short

on mine... ive got 1.5 rockers with the SW 05 swift tune cam ... and i cant remember exactly but i want to say im at around .350 lift .... in fact i used short brass bullet valve guilds just because i feared it was to close

i remember vizard in his bible saying anything beyound 1/2 lift inch is waste so i think they get fairly close

ill look up my valve lift spec tomarrow

Prop and the Blackhole Midget

Hmmm... i maybe wrong on mine as well accordinv to this site the stock valve lift is .318

which seems a bit much to me considering stock sheet metal rockers are around 1.25 to 1 and 1.3 to 1

i dont know... im slaying my own dragon inspired windmills at the moment so i may be off base in my thoughts at the moment.

Prop and the Blackhole Midget

Sorry paul....

my bad... i reread your post several times and now i get what your saying

Btw... i did plot my cam when i built my engine with with a dti and graph paper at 5į intervals .. and it was a fun exercise... sadly paper is not time frindly and desolved not long after

Prop and the Blackhole Midget

Thanks everybody for further comments / insight. I'll try to get hold of Richard again this morning and put some of this to him.
M Weller

Reading between the lines( you can always email me but discretion will obtain for all), I maybe wrong but this could be a head we have supplied to an engine builder. 19cc is spot on for our road spec with std 8.8 spec pistons being used. On a +40 all other things remaining the same the CR will be approx 9.6:1 spot on for the cam and high lift rocker choice (sort of spec we would recommend). I think head gaskets are usually 2.8 cc compressed even ones allegedly 3.8. Even if it isn't one of our heads the ccs are on the nail!
I cannot see, readily, big dish 1275 pistons available so if that is the problem it seems odd. We have seen one or two rogue BLMC blocks where the bearing saddle to block face is greater than usual, must be a Friday afternoon block! We have seen this on A and B series blocks. Always good to measure TDC to top of block before engine stripped then piston deck heights old to new can be measured and one will know what to skim off the block if necessary.

Of interest were compression readings taken? I would expect around 170-180 for the spec as it should be.

The valves will not hit OE height pistons with this spec. Valve faces will be approx 5 mm from head face plus 1 mm gasket and usually around 2 mm from TDC to top of block.

Prop, we can and have in the past run down to 13cc on full race heads, this can only be done on good pre A+ heads. 16 cc is fairly common still on Full Race heads. On the 13cc one has to use non projected plugs or the side electrode can strike the piston at TDC! People still fit non projected plugs in A series thinking they must be better but I feel it is a throwback to when one had no choice. The projected ones have always shown more bhp on the rollers.


Peter Burgess Tuning

Hello Peter, thanks that's very interesting. Tell me, what dish would you expect on a STD 8.8 piston that you refer to? On a +40 you say approx 9.6:1, so where do you think the problem lies with regard to my 8.5:1? Is it simply the bore volume that's the problem? Too short compression height pistons? Too dished? Too great deck clearance caused by compression height or dodgy block as you allude to above?

FYI the compressions were 175/180/165/180.

Do you think it's unlikely then that the pistons were specified to get round potential valve strikes due to the heavily skimmed head?
M Weller

The head is not heavily skimmed! It is usual workshop practice for a mild road A series 1275 engine. Try not to go down the route of emotive words, they can send you down blind logic alleys of worry.

Your CR readings seem about right for 9.5ish:1. I would have though around the 145 mark for 8.8 and even less for 8.5! We go over the 200 psi with 10.5:1

Piston dish usually 8 cc I think.

There is no way the pistons would have been lowered to prevent valve to piston contact.

I do not know why your engine has a problem, if it was in front of me I would measure and compare with other engines but I have to rely on what is written.

Peter Burgess Tuning

There speaks the expert! I certainly wouldn't argue with Peter!
As I said, the head should be fine not "way beyond salvage" as Prop described it! In saying I thought it was fairly heavily skimmed I did say this was assuming the chambers had been worked on, which would increase their volume and skimming would be needed to get it back down to 19cc which is about 2.5cc less than a standard unmodified 1275 head.

From the measured figures quoted by the specialist, a 71.6 bore gives a 1312cc engine, so 328cc swept volume per cylinder, and with a total chamber of 43.2cc that calculates out at a CR of 8.5:1 (8.49:1)

Standard 1275 pistons have a 6cc dish. Hepolite made a 1275 piston with 11cc dish, though I have no idea how or if the gudgeon pin to crown varied from standard. If the 11cc dish pistons were changed for standard 6cc ones and everything else remained the same, then the calculated CR would go up to 9.68:1 I think that is what I would do, and leave everything else the same

All this is just maths, but does assume that the measurements your engine specialist has given are correct and accurate.

My apologies if my speculation about valve clash being a possible reason for the deep dish pistons. The experts, Peter and Paul, clearly both think this I am talking nonsense. Anyway, with large valves simply using deep dished pistons wouldn't really help! I was basing this on my experience of rallying minis nearly half a century ago when we used an engine with pockets machined into the piston tops to give valve clearance. But thinking about it, that was a 997 engine so clearance issues may have been different.

Did you get a clear opinion on why the head gasket failed in the first place? Does that need some remedial action or is it just a matter of replacement and reassembly as far as that is concerned?

Guy, nothing wrong in going outside the square, there lies the answer on a number of occasions! And it was an interesting excursion.

One of our own Midget engines has some deep dish (~11ish cc IIRC) pistons which sit higher in the bore (compression height about the same as the Metro) so despite the extra dish the CR is standard (with std head volume of course). I understand from PO that they are Aussie made ACL pistons. As the car had been laid up for a decade+ before we got it, they would have been produced quite some time ago.

Paul Walbran

Usually 11 cc dish pistons are for the 1380 conversions. Interesting about the deep dish Australian pistons.
I wasn't criticising your maths Guy, all spot on.
I am not sure what was measured to give you the figures and what wasn't.
Not my engine build, may be one of our heads depending who built it but for Uk specs the Cr should have been ok. The compression readings hint at correct CR.
Peter Burgess Tuning

The apparent looseness you mentioned right at the beginning can be a result of the head nuts running out of thread. I ran a 16cc head at one point and the standard head nuts could be tightened up beyond the threads and torqued onto the studs as it were. So the effective clamping was less than spec.


Are you sure you were machining pockets into the pistons and not the top of the block?

It is common on small bore engines - 850, 948, 997, 998 & 1098 - to machine pockets into the block when using big valves and high lift cam.
Dave O'Neill 2

No, no pockets in the block. But a wedge shaped channel across the top of each piston.

120 car night stage plot and bash rallies in Wales. Good fun and at that time affordable!

Did the 997 use a raised-crown piston like the 998 Cooper?
Dave O'Neill 2

I suppose, Guy, without flat tops you would need to get the pistons near the top of the block then slot to save valve to piston contact. lift around TDC can be lots on race type cams.
Years ago I worked on a drag race Harley, we had massive by huge valves (2.25" inlet) and were a little worried about valves hitting each other around TDC, I said all ok as it takes 6 mm lift for thenm to collide at TDC, my friend says, Bum! lift is nearly 7 at TDC! We reduced the OD of the ex valve head and put some large chamfers on the firefaces. The bike went well as it snapped the drive belt as soon as the throttle was snapped open!

Peter Burgess Tuning

Guy - your calculated CR ( based on 71.6 bore) tallies with what I've been told. So, and I aim this at Peter as well, say I left everything else as is, but changed the pistons, to say 6cc from the current 11 ish, what would be the (very) approx increase in power? And would I need to get / be advised to get a taller compression height piston than is currently fitted to take up some of the slack above TDC? And if I do this is there any risk of valve strike?
M Weller

Reducing the total chamber volume by replacing 11cc dished pistons with ones with a 6cc dish would in theory increase the CR to 9.68:1. But there are other unknown variables that could alter that theory.

A replacement piston with 6mm dish might also have a different height from gudgeon pin to the top of the piston. Without removing the present piston you just don't know. Unless there are markings on the piston which enable you to identify the manufacturer and get hold of the specification.

The other thing that could alter, though maybe by not a great deal, is the gasket thickness when re-assembled with a new gasket, possibly of a different make and thickness.

I really don't know about likely increase in power. Peter is most definitely the one to answer that one!

No risk of valve strike. If your measurements are correct you need to raise the CR with pistons.I have said several times the compression readings do seem about right for 9.5:1 but it seems to illicit no response? If the pistons are 11 cc you would be better off with the smaller CC ones dont worry about changing compression height. Beware of taking 6cc as gospel,all my literature says the AE 21253 type piston dish is 8cc.
I do not know what bhp increase you will get, have you had it on a dyno when it didnt have a gasket fault? More than anything you need to have it running and not blowing gaskets!
Peter Burgess Tuning

My engine, with good rings produces around 205 psi(wet) and has a theoretical CR of just under 10.5:1 so I would agree with Peter that you readings of up to 180 would suggest an engine with mid 9.5 CR. Which given the other engine mods is what one might well expect an engine builder to aim for rather than the 8.5.

So, mystery! How confident are you in the figures the specialist has provided? Are they actual measurements or has he "guestimated" them?

The compression estimates quoted are well out from what I've been told by my engine guy, hence no response from me. I'm not technically proficient enough to counter one way or the other and until I speak to Richard again on Monday, I won't be able to put it to him either. Ultimately if you Peter say it should be 9.5:1, then who am I to disagree? Perhaps there's another variable that I haven't been made aware of, or perhaps one of his calcs was out?

Either way, it seems to me now that unless I'm happy to stick to 8.5:1 (or 9.5:1) then a piston has to come out to ascertain exactly what is in there / compression height etc, will check crowns first for any reference.

Thanks again for the input guys much appreciated.
M Weller

This has been such an absorbing read, and I've enjoyed nearly every line of it. But I found I had to go back to the top, to find out what the original problem was.

And then, reading down your posts only, I was not able to get a sense of whether you want to do this. I mean, do you itch to get your engine as good and powerful as it can be? I suspect lots of us here would have the engine out before tea time, while others would buy a new gasket, bolt it up, and get out there on the foggy road for a lengthy test drive before Evensong.
Nick and Cherry Scoop

Hello Nick, I want to get the car back and enjoy what little decent weather we have left this year!

Problem is I'm an engine luddite compared to most of the guys on here!

Yes, I could instruct Richard to just throw it back together with new gaskets, studs etc and I'd be back out enjoying my 65hp (what it made on the rollers when it arrived with my 'coolant issue') midget within a week or so.

But I'm well aware that 8.5:1 is not really where it should be, at least with the other mods to engine and chassis etc. So, the point of this thread is to better understand, via the knowledgeable bunch on here, what the options are with regard to getting it closer to the 'recommended' CR of 10:1, and potential benefits.

I know enough to realise that it is sometimes prudent to do certain jobs while other work is in progress, spend more now to save in future. Equally the car has already cost me more in the first three months of ownership than I had anticipated, so any extra expenditure, particularly when well into the hundreds, has to be really necessary. Ie if going for the 6cc pistons will only produce another 5hp, then 5-600 quid isn't VFM in my mind. However, if it's going to give me another 12-15 with better response etc then financially it makes more sense, and could make the car simply more fun to drive.

Then on top of all of this there is the possibility that Richards measurements are off, and this setup, backed up by my compression readings, is actually already at 9.5:1, and that there is another problem altogether that might explain my rather poor showing on the rollers...!!!

So, a lot of factors for a luddite like me to get my head around.
M Weller

depending on where you read it the original car when new was claimed to be about 64 and I'd guess many standard Midgets owned by the average classic car owner now would have a true figure of nearer 50(?) than 64. Power figures are useful marks but don't be too absolute about them. I realise that you'd expect more from an engine of the spec you'd thought it was and as I put I thought the tuning guy saying 75 was being on the cautious side. I think what you have now is nearer a bitter disappointment than a complete disaster.

You also have to balance purchase costs with and/or against repairs, maintenance, servicing and improvement costs.

A good value car can be one that costs seemingly a lot to purchase or one with a lower purchase cost but higher subsequent costs (of course there are many exceptions to both as I well know). Rarely does an ordinary purchaser get a low priced car that doesn't need more spent on it to make it a good value car. Usually a high priced car is based mainly on its cosmetics so regular servicing, maintenance and use are even more important.

Having the engine right is of course the main base but for the power it's also needs the ancillaries being balanced and in good order as well as regular servicing, maintenance and reasonable distance use.

It's not that often that a new owner has the car running the way they want soon after purchase.

I've just been told i've Got to go now, cheers
Nigel Atkins

Hi Nigel, hear what you're saying, all very true! I'm happy the rest of the car is in good shape, brakes are very good, gearbox all fine. Tyres will be replaced end of this month, just need to figure something out re the rear FL telescopic setup, as even on softest setting it's still too firm. So, really the only reason it went it in was to sort the coolant issue, so I didn't have to manually transfer coolant between expansion bottle and rad before every journey with a syringe! All the other engine/piston related uncertainty has arisen as a bi-product of my desire to have a sound, reliable cooling system...

Prior to last week I had no real complaints about the performance, yes the idle was erratic, but I'd been told that the jets were slightly bent, and the car felt a bit 'flat' compared to what I expected based on engine spec, but I wasn't unduly concerned at this stage, and had hoped to use it a fair bit more before getting someone to check out those issues. There we are, that's classic cars for you, all part of the challenge...!!!
M Weller

Hi Mike,
with the FL rear are you sure theyíre fitted correctly, otherwise they might 'bottom out' see photo for bottom fixing (fitting instructions might be available on t'web).

As regards tyres, Iím the first to say get good tyres but unless your present tyres are bad why not stick with them until other things are sorted.

I think you probably mean the needles might be bent so yes theyíd need replacing.

As for classic cars, no disrespect because Iím a major culprit of buying dear and selling cheap, but much of the problems with classics cars are previous (and present, not meaning you) owners and the expectations of novice owners or those like myself who donít do enough research or search far and wide enough and test drive enough cars before buying. In my case despite experience and (should) knowing better.

At the moment itíd be best to investigate sticking with it but if you donít/canít spend too much then experience has taught me, and I have done it despite spending far too much, to cut the losses and to sell despite the loss but I think youíre probably nowhere near there.


Nigel Atkins

Just wondering - - I think you said that Richard was more used to, or happier working on MGB engines? I understand that the standard pistons for an MGB have either an 8cc dish, or a 13cc one for low compression spec engines. I wonder if Richard put in 11cc as an estimate for your pistons as a mid-point compromise between these two.

Hi Mike,

Might need a degree of tact in unravelling this! You want to keep Richard on your side! I guess the anomaly worth asking his opinion of, is that the dimensions given calculate out correctly at 8.5:1, yet the compression test at 180psi would be typical for something nearer 9.5:1

MGB is 6.25cc and 11-ish
Paul Walbran

OK Paul. I looked them up for a 3 bearing MGB engine, and 8 and 13 is what I found. Different for different markets perhaps?

Evening gents. Guy, no the specialist who preferred b's and c 's to midgets was the original engine builder from three years back. Richard is a die hard a-series man, and has been for many decades. I've had no reason to doubt anything he's told me so far, the only thing nagging away at the back of my mind is the correlation between cylinder compression readings and CR. I'll ask him about this tomorrow.
M Weller

OK. But as you say, that apparent discrepancy is worth investigation.

One thing - there is no direct relationship, at least in as much that you cannot use a compression test to calculate the actual compression ratio. The reason is that the compression test depends on other factors such as the quality of the ring and valve seal, the volumetric efficiency, efficiency of the intake and so on. The CR as you appreciate depends on actual, precise measurement and maths.

However it is hard to think what might cause the compression test psi to be higher than normal for any particular compression ratio. Lower, yes, but not higher unless it was done with a really excessive amount of oil added through the spark plug hole.

You also have to bear in mind the compression pressures for the head mods/266 and hi lift rockers will be lower than the std cam/rockers and head set up due to the point of inlet valve closure and the flow capability of the head especially the valve seat/valve shape.
As Guy says, if it is genuinely lower than std CR it has to show lower compression readings than the standard setup unless it is tested with loads of oil or tappets are way to tight which would also cause poorer running.
Peter Burgess Tuning

I meant way too loose for tappets, trapping more air in!
My books give me 120 psi cranking for a 1275 Midget engine which seems low to me, we can't win can we?
I suppose the compression test gauge could be high but you would have thought the operator would be suspicious of so many high CR engines once testing the 200ish psi genuine MG Metro and other A+ high CR engines maybe showing 240!
Peter Burgess Tuning

If all the tappets were loose it would reduce the amount of incoming charge as well, so less to compress and probably not then give the abnormal high psi reading either. Maybe if the inlet settings were correct and the exhaust REALLY slack so the exhaust valves didn't open at all then the engine would work like a pump with the psi just going up and up and up and up .....and bang! Another HGF! ;-)

Don't worry Mike! Just mind games! Best just to talk to Richard and maybe the original engine builder as well if you can track him down. And anyway, Nick makes a good point. If you were happy before with the engine performance the new "knowledge" of the low CR figures shouldn't really change that.

You could probably get as much or more improvement in performance and driveability by getting a it on a good rolling road session for 1/4 the price and a lot less trouble then getting the pistons replaced. And after replacing pistons you would probably start having doubts about whether you were getting the benefit of all that cost and so need to pay out for a RR session on top!

The engine wasn't built by the person I thought may have done it being a C specialist so maybe not one of my heads either.
Peter Burgess Tuning

Seeing as I have no beef with them, an am not one to point fingers anyway, I see no problem in saying that the original engine build was done by MG Motorsport.

Based on everybody's views here and Richards too, I'm going to leave the pistons alone and just reassemble with new gaskets etc and better bolts/studs with flanged heads giving greater surface area. Richard also said he would clean up the inlet manifold and fit some full radius stub stacks inside the standard air filters. Yet to decide if I should fit an RC40 before it goes on the RR for final setup.

May see 70-75hp with all this, which based on the state of much of the road network down here and the overly firm rear susp', is probably quite sufficient.

Anyway, thanks once again to everybody here for all the help, the forum really is a very useful resource for a mechanical numpty like me.
M Weller

if the valve clearance, plugs, timing and carbs are sorted and set up and the other things you mention done then you might find the 70-75 is a slightly conservative figure.

Same as having a standard twin box exhaust fitted if your car has the original saucepan air filters it's an example of things being a bit mixed between performance and appearance/originality. As with clean paper elements fitted the original saucepan air filters are fine for a standard or original engine set up but performance engines usually have different filters.

I don't know how much the stub stacks are going to cost you (you can get very low and very high priced ones) but if you want to ask Richard if the K&N plate filter (the one with 'Midget' on) with rear plate with stacks built in (wrong word but I can't think of correct description) would be better I have one for sale if you wanted it, just email me for more details (and honest assessment of it).
Nigel Atkins

Now you have said who built it it may be one of ours!!!! If the head is marked MGM near the thermostat housing.....

I feel you are making the correct decision to be honest. If you have a copper head gasket you must retorque, if you are using a Payen black gasket then we do not retorque, we use Wellseal on the metal parts of the gasket.
Not wanting to teach Richard to suck eggs but don't polish the manifold or it will make the engine run poorly.

Peter Burgess Tuning

Its been an interesting discussion. I know I hold forth as if I know what I am talking about, but my experience is limited and I have learnt a good bit more about these things here too!

I would be interested in a confirmation of both the piston dish capacity (is it really 11cc?) and also the piston top to block surface height at tdc as either of these could be significant in the CR calculation. But it's only to satisfy curiosity.

It does occur to me, that even at the lower 8.5 calculated CR its only a little less than "stock". But you have high lift rockers, 266 cam, big valves? worked (gas flowed?) head and other modifications that even with a near standard compression engine, have the capability to produce well over the standard engine output anyway. Just right for those Devon lanes and a blast over Withypool Common. ;-)

<< don't polish the manifold or it will make the engine run poorly >>

What is the cause of that? I've never heard of it.
Nick and Cherry Scoop

Flow is nil at the manifold wall. If the manifold is rough cast or fairly rough 'flapped' with say 60 grit emery cloth you are ok as there is quite violent eddying at the boundary layer twixt wall and air/fuel flow. If it is polished the fuel tends to drop out of the air going into the engine giving very poor combustion, especially at light throttle, once into the inlet ports a smoother finish has less effect, the same obtains in the combustion chambers, smooth is not bad. On a few occasions we have found it impossible to sort the fuelling only to discover polished manifold. Rough it up and away we go!
The same thing happens if the bridge to the main jet is smoothed in an SU, very poor pressure drop on the jet and subsequent poor fuelling.
Peter Burgess Tuning

Thank you, Peter. Perfectly satisfying explanation for a simpleton, and perhaps even a spur to further study.
Nick and Cherry Scoop

Morning all, thought I had top and tailed this last week, when I decided to leave pistons as they are, as to get 10.5:1 CR the top of the block would have needed to be skimmed, as well as fitment of 6cc pistons, so engine out etc etc all rather expensive.

Anyway, after a very quick flying visit to the garage yesterday, it was suggested that another way to achieve a higher CR without removing the block or taking anymore from the head, was to fit flat top pistons. Richard wasn't sure he could still get them in the required size, but the thought had just recently crossed his mind, and whilst I'd already decided which route I was taking, he felt I ought to be aware of another (possible) option.

He did say that technically you can lose some torque compared to a dished piston, but in reality it wouldn't be that noticeable with my setup, on the road. Based on a back of fag pkt calc, he reckoned it would raise CR to between 10.5-11:1. Now, I reckon I would actually 'feel' that!

So... thoughts on these? Any other +/-'s that I should be aware of? Would they need to be pocketed? Thanks.
M Weller

This thread was discussed between 19/10/2016 and 30/10/2016

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