86.5MM Cylinder Bore?

[EyExR]

Has anyone done this with success?

[Northern]

I remember a post by one of r3v's favorite shops about a true 3.2L M20, so I assume 87mm+ was done for that. A good number of S52 guys to 87mm for NA stuff, so I don't see why the M20 would be much different, other than deck height

[digger]

MM claim they use 87 mm safely but you cant overbore it if it needs rebuild so the block is junk hence staying at 86 for their NA engines.
86.5 mm is probably ok as the wall thickness is comparable to s52 etc at same bore but its not ideal. The thing I don't like about 87mm is the proximity of the main oil gallery which will be about 1mm away from the bore in certain parts, on the m50/52 and S52/4 the oil gallery is moved laterally so doesnt have same risks. It's not in an area that sees rings but the skirts are in that area so maybe it's fine.... You can get the block sonic checked if you're worried, I'd do it as a matter of course if needing to do 86.5. obviously on FI youd never do it as the peak pressures are a bit higher plus the pressure occurs later in the cycle aswell so more area of the bore is exposed to higher pressures

[Northern]

MM claim they use 87 mm safely but you cant overbore it if it needs rebuild so the block is junk hence staying at 86 for their NA engines.
86.5 mm is probably ok as the wall thickness is comparable to s52 etc at same bore but its not ideal. The thing I don't like about 87mm is the proximity of the main oil gallery which will be about 1mm away from the bore in certain parts, on the m50/52 and S52/4 the oil gallery is moved laterally so doesnt have same risks. It's not in an area that sees rings but the skirts are in that area so maybe it's fine.... You can get the block sonic checked if you're worried, I'd do it as a matter of course if needing to do 86.5. obviously on FI youd never do it as the peak pressures are a bit higher plus the pressure occurs later in the cycle aswell so more area of the bore is exposed to higher pressures

I used to agree with the first part, but if you need to overbore to the point you need new pistons, you may as well just spend the piston money on boring a new block. Maybe in the future M20 blocks will be worth more than they are now, but it feels like false logic right now.

Oil gallery issue seems more like a real concern.

+1 to stock bore for FI

[digger]

Smaller bores seal better so many people prefer leaving as much meat as possible