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MG Midget and Sprite Technical - Head gasket again!

I begin to wonder if it's wise to uprate a small bore engine for road use. OK I did mine 14 years ago, but it's generated a catalogue of problems. Tuftrided crank bent after 5k miles, brand new Metro cylinder head cracked, valves seized and bent, cam and followers wore out and pushrod bent. Another new Metro head was purchased at considerable cost, and the 4th gasket has just failed. After the last one I had the head skimmed - it was 4 thou out. That was only 2k miles ago and it has gone again. They always fail between cylinders 3 and 4. I had the block decked when I built the engine, and the pistons machined flush.

Last weekend on an 80 mile run up the M3 it overheated and blew off half its coolant. No idea why. I set the mixture I think spot on according to plug colour. Problem was that the temp gauge sensor is in the head and when the coolant level drops below it the reading is misleading. Today however the coolant level was fine just after the gasket failure.

I don't think I have done anything outrageous by way of mods. I closely followed Vizard's recipe for a road 998 and the only departure is that I'm using a 276 cam not a 270. CR is 10.5. Performance is excellent when it's running properly - good torque for a 998 (helped by the long ram pipe I made). Carb is single SU HIF44. I wonder about mixture control, as I am plagued with unstable idling again. Does it lean out too much at top end and run hot as a result? Maybe another rolling road session will have some answers.

Before anyone asks, I am using good quality copper gaskets.

L B Rose

who have you used for rolling road setup?
David Smith

Brian Slark in Amesbury, but that was 14 years ago after the engine was first built. Didn't go well. I have been reading around the forums and there are lots of problems with copper gaskets on small bore engines. Composite or monotorque ones seem to be better, but are very hard to find.

L B Rose

Im no small bore expert but I steer clear of copper gaskets. They always used to be the performance gasket to go for but for the 1275 at least the standard black ones are now preferred. Not sure what your choices are but I always use Payen gaskets. Id maybe try the Payen gasket from the later 998 mini or metro.

John Payne

I had a number of HG failures after my engine was tuned some years ago. Copper gasket was a complete disaster but after fitting a Payen BK450 (mine's a 1275) gasket in 2010, I've had no further problems - so far!
Like John, I definitely recommend Payen. Mine came from MRA Minis.
Peter Blockley

Are you running too hot? Smaller engines are particularly susceptible to overheating. Coolant problems or running a weak mix perhaps. Richen it out on the HIF and take it from there...I too am a fan of Payen..

Bad luck firstly, but i wonder if i am the only person that would choose to uprate from 998 to 1275 as an alternative to the money and aggro of tuning the smaller engine. I get the originality thing but your engine is far from that now, so why not go for displacement ?
It sounds like you have done all the right things, but maybe check that the calculations are right for the CR again once the head is off, i wonder if its higher than you think and that could be an issue with your symptoms.
just a guess.
P Bentley

Les- sounds like you're having a bit of a bad run there
Seeing as you've had it on the rollers we will assume that mixture and ignition timing are ,or should be ok
When it got hot I reckon that was the trigger for the blown gasket, so you need to find the cause of that---blocked radiator, stuck or sticking thermostat etc
When you get the head off, as well as checking the head for straightness, because you have had the block decked it will pay to have a good measure up with a straightedge, in particular between 3-4 where that gasket blew just in case the block has been machined with a trough- As well as running the straightedge along the block go 45' and at 90 accross the block just to make sure it hasn't been machined with a concave in it--same with the head
With the copper gasket, I chased a problem with a mate's B for quite a while, we were running a competition copper gasket
The problem wasn't so much because it was copper, but finally when it was compared to an ordinary over the shelf gasket most of the coolant holes were missing, specially between 1/2 and 3/4 and that's where it was blowing all the time, between 3/4 just like yours
I'm a converted Cometic gasket fan now but if you go that way on an A series I think you have to be carefull on some combinations of head and block and check that one of the rivets holding the gasket together can get sandwiched between the head and block and has to be removed before fitting
William Revit

As it's 8 years since your original rolling road set up, suggest you contact Peter Burgess to arrange a session on his.
He may also have an opinion on head gaskets.
Jeremy MkIII

Unless I've read it wrong it was on the rollers years ago and it didn't go well then (car or session?), and a lot can happen and change in the years.

In my experience it would be a good value to take the car to Peter Burgess for advice, rolling road assessment and setting up/tuning - even if traveling from Wiltshire.

ETA: I was typing whilst Jeremy was posting but I'd also ask Peter for gasket advice (and I read the number of years wrong).
Nigel Atkins

If the head has been heavily skimmed you can run out of thread for the nuts when torquing the head up. So that the clamping force is far less than expected. Otherwise decking the pistons can be an issue if you have crank whip + any wear on crank bearings + little ends and might explain how the crank got bent. Did the valves seize with cast iron guides? Runnning a remote header tank may help if it's pushing water out you need to find the cause.
f pollock

Maybe have a look at the Mini Spares composite one?

If you do end up using a copper one make sure you retorque or the gasket will eventually blow.

Weak part throttle/moderate throttle is an excellent cause of hgf on A series. Weak wot usually kills the pistons.

Peter Burgess Tuning

Carrying on from P Bentley's post my Mk1 came with a very tired 950 engine in boxes that had obviously had all forms of mods over the years and had reached the end of its life. The PO had then gone with a 1275 and that's what I'll be fitting.
If it's being kept standard for originality then fair enough but if you've got a different head, manifolds, carbs, filters etc then maybe a bigger engine is an option. The HIF will be perfect for it as well.
John Payne

Thanks for all these ideas guys. The reason for using the small bore was that it was a spare engine I had lying around, rather than for originality. At the time I was still on drum brakes and thought the insurers would bleat about a bigger engine on the same brakes.

The crank bent not long after the original build. It was straight when assembled but a few 1000 miles later there was 4 thou of runout on the centre main. I was told this was memory effect. After another regrind it has been fine and oil pressure is always nearly 60 psi hot. I had the bearings out recently for another reason and they were fine.

The C-AEA647 gasket is out of stock at Minispares and everywhere else AFAIK.

Good idea re stud thread length. I'll check.

Yes I did say it's running hot. The rad has been recored twice in the last 10 years, but maybe there is gunge in the block. The coolant always looks crystal clear though. I'll check how straight the block is.

The thermostat is new.

I won't go into detail publicly about the rolling road session but if anyone wants to email me (lesrose at ntlworld dot com) I'll be happy to spill a few beans.

As I still have the original engine, I'm wondering whether I should rebuild that and get the car back to matching numbers. I wouldn't go back to original tune - the H1s are a pain and I have sold them anyway. Right now the performance is just enough to be great fun when it's running properly.

L B Rose

Whatever engine set up you decide on Les, suggest you have it set up by Peter - he will restore your faith in rolling roads.
I have no connection with Peter other than as a satisfied customer - twice.
Jeremy MkIII

have you done a thorough cleaning and (multi-)flushing (and multi-back-flushing) of the engine block with the block tap (or plug) removed - and clean and flushing of the heater matrix.

If you've not seen my simple but thorough system(s) I can email details to you.

My engine was reconditioned but I still got lots of muck and crud out of it first time I done a thorough clean and I was surprised to find on a subsequent clean there was still some grit coming out of the block drain.

As you know the good book will show you the location of the block drain. :)
Nigel Atkins

The Payen website lists 2 head gaskets for 1275s. AF 470 and BK 450. According to them, "The difference between AF470 and BK450 is that the AF470 has the oil feed grommet and the BK450 does not. Other than this the gaskets are the same." My question to you all is what is the "oil feed hole"?
J Bubela

The oil feed hole is the oil supply at the front of the block which supplies oil to the rocker assembly and consequently the valves and cam followers. It is located in the front left corner of the block IIRC when looking towards the front of the car and oil comes from the front cam bearing. Some info here
David Billington

Remember not to overtighten the cylinder head bolts. Overturning crushes the fire rings and spreads the pressure over the whole of the gasket surface instead of concentrating it around the parts around each cylinder which take the greatest stresses. It's easy to think that tightening up a bit more than specified will improve the clamping effect on the gasket when it in fact has the opposite effect.

Here in Kent Masc we have adopted a different approach to tuning by adopting A.C.Dodd.whom like Keith Calver is better known in mini circles.
After arranging an 'audience' several members engines have been improved by de-tuning.
Burn Vizard as it is about 20 years old likewise the workshop manual and drivers handbook.
Rebuild your 948 under the tutoraledge of A.C. At his base in Dorset on one of his 'Saturday' Clubs. He can fit new cam bearings or show you how. Rebore your block and more. His input in our group has creared many a smiling face.
I could turn his ideology and input into War & Peace but I will keep it brief.
He has his own camshaft range although his 'Road Torque' has proven the most popular.with CR reduced to around 9.5 : 1.
Look for him on Facebook where his daily posts are really interesting.
We have had several tuning days with him also.
Alan Anstead

Sorry chaps I have been busy - back now and thanks for the comments. Had to wait 5 days for new gasket to arrive - an ordinary composite one this time. I am always very careful and torque the nuts exactly to 40.

I tried the time-honoured Radflush and nothing significant came out of the system. Haven't done a long enough run since, but on short runs it still goes over 190F on the calibrated gauge. I'm off to the Lymington show on Sunday so that should be a hot day and long enough for a test. I'll travel with spare water! If still overheating I'll backflush, but frankly that was done only a few years ago and the coolant is always crystal clear. When I built the engine (over 10 years ago now) I had the block boiled out but when first run a lot of orange gunge still came out of it. None of that for many years now after multiple flushings.

Thanks Alan for advice re AC Dodd. Don't know him, and will follow up. He doesn't seem to have a website, and his FB page doesn't have contact details. I'm not on FB and don't want to be! Dorset would be fine for me here in Salisbury.

Any thoughts on ally rads? Are the Chinese ones any good? But over some 45 years with these cars overheating has hardly ever been a problem with the standard one.

Yes Nigel I would like to see your flushing system. My email is <lesrose at ntlworld dot com>.

L B Rose

its simple but thorough, nothing clever or sophisticated, I suggest you use the 'more thorough' and 'flushing and reverse flushing' sections of the notes.

This is exactly the weather to be doing the job, messing about with cold water.

Notes in the (electronic post) now.

Nigel Atkins

Just thought, apart from the petrol type you use, what exactly is your cooling/heating system made up with. Sorry you've probably put it before but I can't remember. What pressure is your cap, temp of water stat, type of rad, expansion tank, heater matrix and tap fitted, type of water pump, type of coolant, etc.

Oil cooler (and stat) fitted.
Nigel Atkins

If you contact me at alan dot anstead at btopenworld dot come I will pass you Adrian (A.C.) Dodd's contact details.
If you can access his Facebook through someone else it is quite an educational journey. Here in Kent we have certainly learned a lot and reaped the rewards.
Alan Anstead

Nigel - rad cap is 7psi, water stat is 82C, cooling fan stat is 85C, rad is bog standard Frogeye (recored twice, last time 7 years ago), heater matrix cleaned maybe 15 years ago, usual alloy water pump, blue silicate anti-freeze at about 35%. Oil cooler but no stat.

I just checked the timing with the strobo and it's spot on as per distributor spec. Mixture is if anything very slightly rich according to plug test - which also shows timing marks exactly where they should be. I usually switch off with open throttle to avoid a misleading reading.

L B Rose

quick thoughts only -
. have you checked the rad cap,
. has it been replaced, perhaps just before current issue (or during, after, not for many years)

Sorry I can't remember exact details, but it seems IIRC you have a 950 water system plus electric cooling fan on a souped up 950 engine (with long standing misfire(?)) so perhaps the margins have always been narrow but adequate until now.

Does your rad have the extra shroud piece of later ones, presumably not if original.

Is the electric cooling fan in front or behind the rad, is the engine fan still fitted.

I think Alan, as usual, has a good point, about the souping up of the engine (despite the slur on the good book!) but I'm also thinking of Tim's blocked engine cooling drain point which (may, but not heard back yet) have been the cause of his 1500 overheating not that I'm saying your cooling system is blocked just about restoring the margins. Engine performing well and cooling system(s) performing well, no overheating.
Nigel Atkins


You can always read my vehicle profile :). But seriously, it's a 998, the misfire was very brief, and it went away with a new distributor cap and Powerspark module.

The rad cap was new maybe 5 years ago, and doesn't blow off when the fan kicks in at idle.

What's the extra shroud piece? I have the curved piece at the top, and flat sides.

The electric fan is at the front, and there is no engine fan (main reason for the electric one). The overheating was at 70mph on motorway, not at town speeds. At speed the engine fan would not add anything to ram effect (possibly the opposite). As I have said, this is quite a new problem as the electric fan almost never came on before.

L B Rose

Why not try removing the stat and see what happens. I don't use one in mine but it does have the sleeve to ensure the water flows correctly. Mine is a 1275 in a frog but the same cooling set up as you except i don't have an electric fan. Even touring in switzerland and france this year it rarely got over 170 degrees F in ambient temps of 80 F plus.
Bob Beaumont

I do read the Vehicle profiles and find them useful, doesn't list cooling system details.

Sorry I forget details like your misfire being solved.

Extra shroud is metal pieces at sides IIRC.

I'm with you on the fans, (same as mine) just checking.

TBH I forgot what the problem was at 70mph (like Tim's).

Apart from a moveable blockage what about water pump, is it shifting sufficient volume, if you're emptying the system out you could perhaps turn the engine over to see what pumps up and out below open pipe/hose after gravity.
Nigel Atkins


Thanks, it may be worth checking the water pump. All this will have to wait until after Sunday's show. Today has been spent polishing.

L B Rose

Good idea Les, the coolant will flow easier with less resistance so less heat build up.

But don't leave any rags or polishing materials inside the cooling/heating system to cause any blockage. :D
Nigel Atkins

The Lymington show was a very hot day again. No overheating and no water loss, but not a fast run over the New Forest. The run up to the Basingstoke show last Saturday was faster and I filled up on the way with super unleaded. Temp still steady but the air was about 15C cooler. I'd say that while it doesn't overheat, it's still running a bit hotter than I remember. It was always just over 190F on the gauge, which I have calibrated several times after refilling the sensor. Now it's nearer 200. No time to flush system right now as I'm rebuilding rear springs!

L B Rose

well done on reporting back on both threads.

Try a couple of tankfuls of the super unleadeds for their cleaning packages to take effect too. Pity it wasn't a faster run to see if the additives and higher octane made any difference to the feel of the engine's performance, I always feel that there is a very slightly different sound and very slightly more torque but I could well be kidding myself perhaps it's just me happier using the stuff rather than the car.

I tried to ween myself off the harder stuff and just use the standard fuel with a couple tank fulls of fuel system cleaner added once a year instead but I'm not at ease doing it this way.

I think the air being so hot before didn't help, other than to highlight the possible issue.

Keep going, get the rods out if need be, show it who's boss!

Nigel Atkins

What temp thermostat have yo got in there
I'd be poking a 70-72c (160-165f)in there for the hot weather
Any hotter than that and the whole system gets too hot and it just keeps building itself up and up
William Revit

Is your temp gauge electric? I only ask as for a long time I was using an electric gauge and was plagued with what seemed like chronic overheating. Recored the rad and did everything known to man in an effort to drop the temperatures. In the end I realised the gauge required a voltage regulator to read correctly. That and proper burping of the system were the answer.
f pollock

The gauge is the old bourdon tube type. I refilled it with ether and calibrated it versus the cooking thermometer (and yes I got caught by 'er indoors!).

The thermostat is 82C. Can't see how a lower value would help, as once it's open it stays open. A lower one would just open earlier.

L B Rose

82 is when it starts to open, operating temp is usually a few deg above that
I think you really should try a lower 70-72 and it will run about 170 on your gauge
Because it starts to open earlier the radiator gets more chance/time to work it's tricks and keep the whole system cooler
If your cooling system (radiator)is up to scratch the thermostat doesn't get to be fully open it floats around controlling the temp'
William Revit

Daja vu, I'm sure I've agreed with Willy before about the control of the water stat over engine temperature - with Willy explaining it better.

IIRC Les's car is a souped up 948 with 948 rad and electric fan.

By the book IIRC the standard 948 had a lower stat rate and lower running temperature, of course Les's car isn't standard.

IIRC correctly there's a 2 degree variance in water stats within spec, an 82 (180f) starts to open as Willy's put at 82 and fully open at (IIRC) 10 above so 92 (198f). Allowing for the 2 degrees that would mean the opening range of an 82 could be between 80 (176f) and 94 (201f).

In my non-technical thinking, if the two water stats were equal it should mean that a lower rate water stat would have the control range at a lower temperature coupled with the electric fan control being set lower it would mean control at lower temperature.

Keeping to the lower temp range may make it easier on the cooling system to control.

This of course assumes the rest of the cooling/heating system can keep the cooling within these ranges.

I've never understood where you'd be best taking the engine running temp from but wouldn't 190f (88c), let alone 200f (93c) be too hot to get the most engine power from what I've read about Minis, IIRC(?) 70c for power 90c for mpg.

Nigel Atkins

Willam's comment makes sense. As mentioned I don't use a stat at all. it rarely gets above 165F running around but because I live in London with traffic it can creep up to 190F or higher sometimes. I did have a 82C stat and it regularly went over 190F hence why I removed it. Never tried a 72C stat. The cooling system is in excellent condition (new rad and hi capacity pump) and regularly flushed and refilled with bluecol every 2 years. This year I also had the exhaust manifold ceramic coated and it seems to help underbonnet temp. Certainly when in France this year the engine temp did not go over 180F even with ambient temps at 35C.
Bob Beaumont

Thanks chaps, interesting comments. I'll try a 72 stat and see what happens.

L B Rose

you might even be able to go lower, if the stats are available, see -

The Driver's Handbook for the (standard) Frogeye has -
"Normal operating temperature - 164F. (73C.)"
Coolant "Capacity - 10 Imp. pints (5.68 litres)"

My 174 Haynes has -
"Thermostat setting 948cc models - 65-70c (149-158F)".

The rad pressure caps at 7lbs.

Perhaps running at a lower temperature might help with Les's misfire (IIRC).

Nigel Atkins

72c stat, using same opening range as before -

72c (162f) to 82c (180f)

and allowing for +/- 2c
70c (158f) to 84c (183f)
Nigel Atkins

I had persistent overheating issues on my 1275 Mk4 which the so called 'specialist' failed to cure even after pulling the engine apart and replacing the HG and various other components that in hindsight I should never have agreed to have replaced... Once bitten twice shy.
After help from many on here, and following Nigel's coolant flushing/rev' flushing procedure, together with multiple oxalic / sulphamic acid treatments, the engine temps are much better, usually around 80-85c, whereas before it was regularly over 95...
I also replaced the exp' bottle press' cap, thermostat (82), all gaskets, and finally refilled with a 50/50 mix of std MEG coolant.
Over the winter I'll prob' replace the rad or at least have it re-cored, not really sure what other stones would remain unturned after doing all of that!
Good luck with yours, hope you finally get it sorted.
M Weller

Nothing wrong with 80/85.Even 95 isn't really overheating on a pressurised system.

Last year I fitted a new radiator and I didn't notice much running difference with my eyes on my CNH gauge but I did notice the new rad seemed to help cooler quicker from town driving to open road driving. I hardly drove the car last winter so can't say how much cooler it was than with the old rad over winter which would sometimes have the gauge showing nearer the C.

I updated my cooling cleaning notes to make them slightly more readable (well relatively) but also to include more info on draining and getting the residue liquid out on each drain and flush as so much is still left in the system especially with the 1275 with bottom cross pipe but also the rad and heater matrix.
Nigel Atkins

I'm on my second tank of 97 octane and no overheating, but weather much cooler of course. Gauge steady at about 195F. The el cheapo gasket still holding up! Will probably have to wait for next summer to know if this is solved, but what's the betting it rains for months?

L B Rose

I'm on my second tank of 97 octane and no overheating, but weather much cooler of course. Gauge steady at about 195F. The el cheapo gasket still holding up! Will probably have to wait for next summer to know if this is solved, but what's the betting it rains for months?

L B Rose

You didn't fancy 99 octane and the cleaning packages of Tesco Momentum or Shell V-Power then Les.

I know there are other 97s but just mentioning BP Ultimate, when I put it in my midget it made the exhaust sound a bit louder and rougher, unless the fuel has changed or it was just my imagination.
Nigel Atkins

Well Nigel I could try the really expensive stuff but the cooling seems OK at present. Do you think these fuels clean better than a separate additive?

L B Rose

Very simple, totally unscientific, no proof, answer - yes I do.

As it'd be cheaper and easier to fill with 95 and add a half bottle of cleaner every year or six months I thought I'd give that a try and did for a while.

Out of no more than habit, IIRC for decades, I normally use STP Complete Fuel System in my cars. It used to say use every 6k-miles now it has every 4k-miles. A 400ml bottle treats 50-60 litres of fuel so with a nominal 25-27 litre A-series tank you could use one bottle over 8k-miles or put in a double dose in one go to empty the bottle and forget about it.

When I went back to using Tesco Momentum 99 and occasionally Shell V-power instead I thought there seemed just a little more torque lower in the rev range but I could well be kidding myself and it's a placebo effect as I've never tested it.

When I take my car to Peter, Keith and the rollers I make sure I go there with a tank full of cheap 95 and the previous tank full of the same as I want my car set for when we're away in the mountains of Wales or the Cumbria sort of area when the engine will be working hard on whatever petrol we can get which never seems to be Shell V-Power and rarely Tesco Momentum 99.

I don't use either of those two to keep the car cool but I'm sure it would help a very little.

Now I normally use Tesco 99 but will still occasionally use the STP anyway as a (lazy) part of servicing and also because I do still have to sometime use 95 and of various brand names.

When I had my MGBs I used to like Elf petrol too but the 1800 BGT preferred Shell Optimax as it was then (98 octane).

All subjective of course.
Nigel Atkins

On re-reading the post - I mean I think I get a little more low end torque with both Tesco 99 and Shell V-Power than when using a 95 petrol.

Nigel Atkins

This thread was discussed between 22/07/2018 and 06/09/2018

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