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MG Midget and Sprite Technical - piston bore damage

I've just remove the cylinder head on my 1275 midget. I have a new metro (non mg) head which I have been tidying up for the last couple of weekends. Its all nice and clean and I've had it skimmed to up the CR.

On first inspection the cylinder bores looked in good condition cross hatching pattern still there and no lip and the top edge. However in one locate in both number 2 and 3 cylinder there are two vertical scores, approx 5mm wide and 5mm apart over virtually the full piston stroke. From running my finger across them I'd say they are very deep.

What has caused this, miss fitting rings?

The piston heads are marked AE21251 +060, which I assume mean +60 thou over sized pistons.

Can I have the block rebored or have i reached the limit at +60, also if the score marks are so deep will they exceed the depth of rebore.

Can i get the bores relined, this is all starting to sound expensive.

I guess I need to start looking for a new block.

I am experiencing one of those Midget ownership low points that seem to come along ever now and again, so your advice will be much appreciated.
tm wainwright

+60 = 1330 cc and there is an overbore which gives 1380 but IIRC the bores have to be offset for the latter and I'm not sure if you can do it on a +60 block. Somebody should be along soon with a definitive answer!
David Smith

its usual to have a discoloured suface here as its part of the ribbed casting underneath. but it shouldn't be rough or uneven inside the cylinders if thats what you're saying.
Nick

I think you can still go to the 73.5mm bore without offsetting. Swiftune will supply Omega pistons at a good price. Tell Nick I told you... lol

Mark.
Mark T. Boldry

BTW, just FYI... the 21251 code is the standard compression piston of 8.8:1 CR if you go the 21253 code that will upgrade your compression ratio to 9.75-10:1 CR


prop
Prop

Thanks for the responses

Nick, This is more than roughness. Its like some thing has worn a gouge in the steel 0.5 to 1mm deep. It looks a feels horrific

How can I confirm if a 73.5 piston will be large enough to account for the grand canyon running down my cylinder.
tm wainwright

Scoring in the cylinder bore is a sign of what we call picking up. In others words slight siezing and IMO has caputed the block. A competent A Series machinist would likely tell you what your options would be after inspecting the block. If you offset bore then cylinder 2 and 3 would be honed away from the centre, although I suspect your trouble would be that the score lines are towards the centre?
Bob (robert) Midget Turbo

On the basis that reboring any more than +60 is a problem, what about cylinder liners. Is this a realistic option.
tm wainwright

if your at +060 and have a 0.5-1mm score then i'd say your block is kaput. You can have liners fitted (some original engines have them anyway) but for the cost you might as well search for another block. when going to 73.5mm then it is usually preferable to offset bore, which can't be done from anything more than a +020 block, so not an option for you. As Mark points out (and he's an A-series guru) you can have 73.5mm on the original centres but take his advice and find someone experienced to look at it for you. Nick Swift is the closest to you, builds some of the best a-series engines in the country but chrages accordingly. but i certainly wouldn't trust any old machine shop to do a proper job in this instance as the block must be perfectly flat if you run 73.5 on normal centers as otherwise you will run into head gasket trouble between 2 & 3. good luck.
Nick


Have you considard just getting another block in good shape and having it re-bored to +.60, line bored, new rings ect. and having everything transfered over.

After all, it appears your just looking to freshen up your engine and ran across a bigger problem, And your intention was never to go for a new high performance rebuild.


prop
Prop

In NZ, we have gone away from offset boring, with no apparent problems.

Grooves often feel deeper than they really are, so don't jump to conclusions as to whether to scrap the block just yet. Get a machine shop to check it out first. Also if there are serious scuff marks on the pistons (which would indicate significant oveheating) get them to crack test the block.

A further thought is to suck it & see - It is surprising just how well an engine will perform with nasty looking grooves like this if the rest of the bore is still parallel. Found this out years ago when 1000+ miles from home we had a circlip drop out of a 1098 engine. Major grooves down the bore courtesy the floating gudgeon pin, and a huge dent in the bore where it came to rest at the top of the stroke, rattled around a bit and hamered at the bore.

We managed to track down a new piston, but being broke students we didn't have the money to get someone to do the whole job for us and having only a tent to work in we opted for a get-you-home bodge. Honed the bore, fitted the new piston ... and to our youthful glee we found that the engine didn't consume a drop of oil all the way home, and still went like the clappers.

Hence I'm thinking suck it & see: If the engine were out then go ahead and fix it properly. But since you only have the head off it may be worth simply refitting it, checking the compressions and (if OK) waiting till it starts using oil, oses compression, or the engine needs to come out for something else. The only difference in cost is a few gaskets and an hours's work.

Paul Walbran

Thanks again for the advice

The original reason for changing the head was low compression and oil consumption/smoking. I thought foolishly that it may have been a valve issue.

Where would I find a spare block? There's nothing on ebay.
tm wainwright

I've added a picture of the damage to the cylinder.

What do you think has caused this?

cheers

Tim


tm wainwright

Tim,
My guess is you have cracked pistons which have lead to the gudgeon pin travelling across the bore. Skirt failure of this sort is not unknown with Hepolite pistons. The engine will need a full teardown and inspection and there may well be enough associated damage to warrant a new short engine - not just a replacement block.
f pollock

Would you still bet on a cracked piston if the same damage was present in both number 1 and 3, and to a must lesser extent in number 3.

I have resigned my self to engine out a hopefully a rebore. Why would I need a new short engine and not just a rebore / replacement block.
tm wainwright

Tim where in Essex are you?

If you are my neck of the woods I would be happy to come and give you a hand...
Toby Anscombe

Toby,

Thanks for your kind offer, your assitiance would be very much appreciated.

I'm in Barking

my email is franandtim-AT-tiscali-DOT-co-DOT-uk

cheers

Tim
tm wainwright

Tim,
Apart from cracked pistons,the only other possibility is the pistons were installed into worn little end bushings. Unlikely but possible. But whether you'll need a short engine or block depends on all the associated damage. With long term gouging like this the rods will certainly need bushing. In addition you can reasonably expect the crank and cam journals to have been fed some of the detritus from the cylinder walls. So once you cost in crank regrinding,bushing,pistons,bearings,oil pump,timing gears and gaskets a short engine is often the cheaper option.
f pollock

That definitely looks like gudgeon pin damage, I had virtually identical damage many years ago on a 948 sprite which I didn't assemble correctly. I know the 1275 has a different type of gudgeon pin assembley but if it comes loose, that is what happens!

I'd be happy to help as well if you're not too far away, Graham.

Graham P

That could happen if the gudgeon pins are not central in the little ends. The 1275 has pressed in pins and they are fairly hard to do without the proper tool. If not put in centrally they could stick out slightly from the piston and cause the wear pictured. Might be that.
John Payne

Cetainly looks like gudgeon pin damage. As well as not being in the right position, another fault (not common) is for the inteferance fit in the rod to be lacking.

1275 Midget blocks are not common, so don't automatically give up on your block - once stripped get it checked to see if it is still sound and if it will clean up at 1380cc (73.65 bore, which is a further 1.5mm from .060)

if all else fails, there's always a Marina block or you could convert an east-west engine which would look like the original Midget but is a more expensive change)
Paul Walbran

Tim.

Most certainly a gudgeon pin moved in the rod. You will need not just a rebore or worse,a block, but also a set of rods. You could get the new pistons machined for circlips and the rods honed to fully floating pins but that is yet a further expense.

As mentioned above, Midget blocks are getting scarce now and Marina ones are getting difficult to find too. I have a spare Ital engine (24000Mls) but it is still in the car and is for sale as a complete car... (still a cheap option though).

I'm sure there will be someone on hear a little closer to you who can help out.

Mark.
Mark T. Boldry

There are a couple of A+ short engines on EBay at the moment, one in Linconshire and the other in Warwickshire. Search using "Marina Engine".

Could be an easy way of getting a good bottom end.
Graham P

The Ital was A+ but the Marina wasn't....
David Smith

Not that it is neccesarily relevant, lightening the small end of the con rod too much has the same effect, allowing the gudgeon pin to move, we had to put PTFE buttons in the ends until we could get new rods done when we ran a race engine
Jus a warning
Cheers
Neil
Neil Williams

Tim,

I suffered exactly the same damage recently, and also am already at +60. Having done much research into cost effective solutions, IMO these point are worth considering;
* 73.5mm piston are expensive, i.e. ~£250
* Standard piston up to +60 are cheap i.e. £75
* 2nd hand Midget engines on eBay are available, expect to pay £150 to £300.
* Marina engines are much less expenisve typically £50-£100.
* A Marina engine will 'look' slighly unoriginal in your Midget engine bay, due to the filter position. But then you save ££.

Rgds James
James Eastwood

I'm currently looking at purchasing an exchange short engine with the machining done and the pistons fitted. It initially seems an expensive way to go but by the time I have purchased my replacement engine from ebay and had all the machining done, plus the purchase of new pistons it may not be that much more.

cheers

Tim
tm wainwright

I might have a solution for you! Drop me an email at n DOT j DOT veuger AT ncl DOT ac DOT uk. replace the usual anti spam gubbins of course!
Nick

Tim,
I think you need to bear in mind the engine block you have may not be acceptable as an exchange item. Consequently there's likely to be a surcharge if it can't be overbored or linered economically.
f pollock

This thread was discussed between 17/08/2008 and 27/08/2008

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