3.0L, 84mm bore, 89.6mm crank?

[nando]

yeah, there's no way the M20 piston is going to work.

see this thread:

http://www.e30tech.com/forum/showthread.php?t=75938

[Raxe]

Took a comparison pic of the M20 and M54 pistons for reference.

[nando]

I think it would be better to machine the M54 pistons regardless. also, I'm certain the M20 pistons won't work, the pin height will be way off with an 89.6mm crank. the rod length is the same, so the pistons would end up sticking way out of the block.

[supernaughtIX]

Chris,
could the m20b25 piston skirts be machined to suit easier than machining the crowns of the m54 pistons?
Or do the m20b25 pistons still sit too proud of the block?
I haven't had any of these combinations together in a block. I can guess that the stock m20b25 pistons might require a lot of removal to clear the weights...but would they work at all?

Maybe I'll just get the m54 pistons and put them on the mill.
Port matching and >272 cam help with gains?
RD headers?

[nando]

you can machine the M54 pistons to work - the crowns are really thick. you'd obviously cut M20 valve reliefs in it.

[digger]

you need the piston dome to suit the 885 chamber. IMO get some pistons from JE they will replicate the correct dome

[acolella76]

You could have the pistons machined down probably ($$$$) but the biggest concern IMO is valve/piston clearance, no? Those pistons are made for 4 valves per cylinder, and doesn't have the valve reliefs like M20 pistons

[Luis325]

subscribed! looking for info on this as well...

[Raxe]

For what it's worth, I tried it out yesterday... B25 pistons obviously didn't work at all as the skirts are too large. M54 pistons on the other hand turned fine with the M54 rods and M54 crank, however at tdc they're about 1mm too high.



Oh well, pretty interesting that it's so close anyways.

[acolella76]

You will need custom pistons of some sort I'm pretty sure.

[Raxe]

I'm rapidly gathering parts together for my 3.0L M20 build, first step was obtaining a complete M54B30. I'm saving the crankshaft and rods, hoping to use the stock M20B25 pistons as I'd rather not change the bore. I recall reading about this setup quite a while ago, but haven't seen any real details in searching for the last few weeks.

I know MM uses the B30 crank and rods along with their 86mm pistons for their 3.2L builds... would the same crank/rod setup with 84mm pistons give me working 3.0L? Or are the stock B25 pistons too tall with those rods and that crank?

Thanks for any knowledge, I'm planning on testing and confirming anyways for future builders but any tips going in are greatly appreciated.