So I bought a brand new block from the dealership and its time to order my pistons. Ive talked to two shops so far, both respected on these forums but i'll leave their names out of it in case people are bias towards one. They have given me conflicting information. Since it's a new block I was under the impression I could order the standard 86mm bore which is what the shop that will be installing them told me. However the place im ordering the parts from recommends ordering 86.5 and boring it out. I thought I would reach out to the community and get everyones thoughts on the matter. To recap, BRAND NEW BLOCK, 86mm or 86.5mm pistons? Thanks everyone.

 

If you buy 86mm pistons and the clearances are too large for some reason, you'll have to go with overbore. You'll need some machining for proper fit either way, most likely. Hence why it makes more sense to go with overbore from the get go. Granted, experience with these blocks is key. If they are generally undersized with respect to aftermarket piston clearance requirements, then it's a non-issue.

 

I'll take this as in general you would recommend overbore but you have no experence with these block specifically so you cant say for sure that the standard bore wouldnt work? To be honest the whole reason I bought a new block was the hope of avoiding alot of machining. But its sounds like its not the best idea to do 86mm. Anyone with experience know with a resonable amount of certainty?

 

You won't find a better bore and cylinder finish than what you receive with a brand new block from Mitsu (or any good OEM), and that's one reason why a new stock motor will on average break in better and burn less oil or zero oil, compared to a built motor bored and honed at a shop. The cross-hatching produced by a the Mitsu factory machines is probably close to perfect 45 degree and uniform.

If you combine this with high quality new forged pistons and rings, I recommend leaving the stock bore. I just built my motor with Darton sleeves (sleeved at Darton), and these come slightly smaller than 86.0 mm so they need to be bored, therefore I went with 86.5 Cosworth pistons.

Use 86 mm and don't bore it, but if you do make sure they have a torque plate and know how to use it right with your head-studs and a head gasket. Also make sure they know how to hone properly.

 

Thanks lbcevo, this is what I always thought. I figured oem's must have much more precise equipment than 'x' auto shop, but the parts supplier gave me doubt. Just ordered my 86mm cossies.