Morris Minor Owners Club

Dear all, but in particular folk with the technical know-how,

I am in need of a set of oversize pistons for my 10MA engine. ESM has +020", +030", +040" and +060" so that should not be a problem.

However, before buying the right set a set, I need to know first what oversize I can bore to as oversize bore size I only have the following listed as per workshop manual:

64.576-64.635mm bore size, with +0.10” (0.254mm), and 0.20” (0.508mm) taking it to 64.830-64.889mm and 65.084-65.143mm.

No other oversizes are listed in the manual, but I note they are selling larger pistons nevertheless.

Can I assume that the correct oversizes for reboring +0.30”, 0.40”, and 0.60” are therefore 65.338-65.397, 65.592-65.651, and 66.100-66.159?

At the moment I don’t know what oversize pistons I need, at least until the rebore has been done (maybe end of June). There isn't much wear on the cylinders at all, it's just that from piston no.3 a piston ring broke and made quite a score down the barrel, and just honing it would not do it. One engineer friend suggested I might have to go around +040" oversize to get rid of it, but won't know until the rebore is actually done.

Grateful if anyone knows the correct tolerances for the bore.

Thanks in advance,

Joost

Your sizes are correct - but for just one score - don't bother. Just lightly hone the bores and fit new rings !

Thanks for the reply, but it is quite a deep score, and there is also a small ~2mm chip out of the bore. I'd rather get the engine in to top condition again while its out...

Remember a 20 thou rebore only removes 10 thou each side! But don't imagine that a larger rebore will unleash any significant extra power from your engine. It just limits the future life of the block. And check to see if the block has been fitted with liners..... I believe that + 120 thou pistons are available (at a cost!) for the 1098 - but if wanting more capacity - a 1275 is a better choice.

well, absolutely, that's exactly why the mechanic suggested +40" might be needed to remove all damage. Don't worry, I'm not after power, but rather a good repair, while I can. For power I have other cars