M20 block

[dwight]

What is the safest bore and largest bore an m20 block can be bored?

Thanks

[Poorboyz]

Are any of the cylinder walls scared? What is the reasoning why you're wanting to bore it?

[dwight]

rEPLY

Are any of the cylinder walls scared? What is the reasoning why you're wanting to bore it?

Looking to put in over sized pistons, no scars engine had 550K miles on it and still ran but compression was dropping. lOOKING TO DO THE FOLLOWING
s-52 CRANK
135MM RODS
86MM PISTONS
284 CAM
USING MY M20 BLOCK AND 885 I HEAD

THANKS FOR YOUR INTEREST

[digger]

86 is pretty safe, abs max i heard was 87mm

[dwight]

Thanks Digger for your input.

[MattAvino]

ooh look, a local

[SkiFree]

86 is pretty safe, abs max i heard was 87mm

I tend to agree here, however the power difference between an 85mm bore and the same pistons in the 86mm is not enough to warrant much stress or thought. 87mm in my opinion is on the "not worth it" side of the "power vs risk" sliding scale. The only way I'd do it would be on a race engine which I wouldn't expect to last.

[dwight]

Thanks for your reply ski free.

[digger]

I tend to agree here, however the power difference between an 85mm bore and the same pistons in the 86mm is not enough to warrant much stress or thought. 87mm in my opinion is on the "not worth it" side of the "power vs risk" sliding scale. The only way I'd do it would be on a race engine which I wouldn't expect to last.

there are some very good reasons for a bigger bore (not just a small cubic inch increase) but you need to consider the subtle details to get it to work otherwise the losses can outweigh the benefits.

when i spoke to MM they said there is plenty of meat to do 87.5mm in the walls but limiting to 87mm allows a margin for rebore. they have done many 87mm bored engines but usually limit things to 86mm to be able to use the stock headgasket and that is the only reason. having seen how close the oil gallery is to the bore i wouldn't be thinking about 87mm at all