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MG Midget and Sprite Technical - Engine rebuild advice

Trying to use this time constructively so took apart my spare engine to have a look after 18 years under the bench but thought I would show you the head and ask for any thoughts please . I presume the PO had it eat a piston or something. There is a receipt for a piston and rod in the paperwork . Is it worth trying to get this ground / skimmed out or leave it be ? It’s a nicely modified head so would like to keep it





Mike Fairclough

First impression is that it's had something flying round in there, but then the valves look like it's been pinging bad on those two cylinders but then it's into the ports as well so maybe a combo
Pinged till it broke a piston and then bits went through it-
My guess is that it leaned out under load on that front carburettor and melted the piston or something
It'll need a skim minimum, to get rid of that mark on the face between 1-2 otherwise it's going to blow a headgasket, the rest of the marks shouldn't worry it but the gasket sealing area has to be good If you
re taking it in to get machined they can do a vac. test to make sure the valves are sealing ok, or, if you're looking for something to do you could pop them out for a look

willy
William Revit

I’d definitely get that skimmed, even if you don’t get rid of all the damage you need to make sure it’s perfectly flat and there is no damage where the gasket fire rings sit. Just ask the machine shop to take the minimum off until it’s good. You should be able to find some figures somewhere for minimum overall thickness of the head. Hopefully it won’t already be skimmed to the max.
John Payne

Thanks for the advice I’m pretty certain it’s been skimmed after this has happened, I certainly can’t detect any raised edges to the marks I suppose your right it needs a professional to have a look at it . I know the PO used it on the road for a while before I bought it and I did 3 races on it before I saw this but my crank cracked last time it ran ( yes really !) This head damage couldn’t cause a crank failure could it ?


Mike Fairclough

Mike, AS you probably know,the standard head capacity is 21.4cc. with the standard dish midget pistons it gives a CR of 8.8:1. You probably need to establish what the capacity it is now as those marks look pretty deep. You mentioned you raced with it, did you have any gasket issues? I can't imagine the head damage could crack a crank.



Bob Beaumont

Thanks Bob no I didn't have any gasket issues , unfortunately I can’t get hold of a Burette from work at the moment to get the chamber volume. I believe the engine has 1275gt pistons in that according to Vizard have a 10cc dish . I have just measured the head from face to face and it’s 2.630 but it looks like the top of the head has been skimmed too
Mike Fairclough

The midget and 1275GT both had 11cc dish pistons (part number 8G 2546) as standard. Skimming the standard head to make it 19cc gives a CR of 9.5:1 which was a popular mod. You will need to measure the capacity to be certain what it is.
Bob Beaumont

Mike, a burette may be the proper item, but if you have a syringe that works pretty well too. Or plasticine, or glazing putty?
GuyW

Well thanks for all the great advice on here, using what I had to hand , a syringe a CD case and some grease I got about 16cc into the combustion chamber I presume that’s going to give me a pretty high compression ratio ?


Mike Fairclough

With the standard pistons (11cc dish) it will give a ratio of about 10:1 which is still ok for road use (I run a 10.5:1) However you may still need to skim it to get it flat so not a lot to play with.
Bob Beaumont

It won't need heaps off it, forget about the marks but that little blemish between 1-2 where the gasket sits is the bit you 'have' to have right
Just a light skim to clean that up where the gasket seals and forget the rest of the dents
Were you running air cleaners on it, it almost looks like the front carburettor has swallowed some visitors
I'm thinking you need to get the valves out and check them and the seats for dents as well

willy
William Revit

Thanks Willy here is a close up of between 1&2 there is just a feint mark maybe they had skimmed it as you said , it hopefully looking like it might be saveable ?


Mike Fairclough

Mike
That area looks better in this pic than it did in the other
It sure has had some visitors flying round in there though, but looks as though the surface has been machined since then
Hopefully whoever did it checked the valve seats and valves, I reckon it would pay to pull them out for a look though, if the sealing areas look like the rest of it it won't be much chop, you need to know, and a new set of stem seals is probably the go anyway so no harm to get them out for a peek
It's possibly not the prettiest head in the condition it is in but the damage to it isn't going to stop it working, the seats will be the decider

Something has come in through/from that front carby at sometime, usually that's from running no airfilters

If the valves check out ok I'd be tempted to give it a very light/minimal surface grind anyway, just 2-3 thou to get a nice surface, Whoever did it before gave it a fairly quick/course cut by the look of it---You'd never get all them marks out but just a quick light skim to get the gasket surface nice
William Revit

This thread was discussed between 13/04/2020 and 15/04/2020

MG Midget and Sprite Technical index

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