Piston size.

[SLOW-33]

Sorry if there is already a thread for this, I've tried a search but I couldn't find anything. 

I'm going to start rebuilding my RB25DET in the next few months and will be using forged pistons and I beams. This is my first ever engine teardown and rebuild and I'm a bit confused about which pistons I should be aiming for. 

A mate of mine runs an engine shop so I'll have plenty of help in terms of hands on but there seems to be different piston sizes. The head will be serviced with nearly all components replaced, head and block decked, the usual head gasket/arp studs. 

I believe there is a correlation between the head gasket thickness and compression ratio. As such I'm after some ideas on which pistons to go for and what kind of compression ratio I should aim for. End goal for the build is somewhere between 600-700hp atw.

Sorry for the long post, thanks in advance. 

[Murray_Calavera]

When you say piston size, are you talking 86mm, 86.5mm, 87mm? that sizing? 

If that's what your talking about, use the size to match the size of your bores. The factory size is 86mm. If you are rebuilding an old used block, it is likely that it will be slightly out of round and will need to be bored out. The removal of that material will mean you now need to use a piston that is one size up, so 86.5mm. When that same motor lets go and needs another rebuild some time down the track, when you bore it out again you'll be up to a size 87mm piston.

Probably worth mentioning, you don't take your block to the machinist and ask for 86.5mm bores, then go to your parts store and order 86.5mm pistons. Take your pistons and block to the machinist, they will measure the pistons and bore out the block so that the pistons are happy with the piston to cylinder wall clearance. 

As for what compression ratio to shoot for, what fuel do you plan to run the car on? 

Aaaand probably wrong place to post this thread. It'll probably get moved. 

[SLOW-33]

Thanks for the info Murray, being a first timer I wasn't sure how that whole process goes but that definitely removed my confusion. It'll be on E85.

I wasn't exactly sure where to post in, can I move the post myself and of so how do I do it?

Edited March 8, 2024 by SLOW-33
Add info

[TurboTapin]

6 hours ago, SLOW-33 said:

Thanks for the info Murray, being a first timer I wasn't sure how that whole process goes but that definitely removed my confusion. It'll be on E85.

I wasn't exactly sure where to post in, can I move the post myself and of so how do I do it?

Just to add to what Murrey said, you still need to measure your cylinder bores before purchasing pistons. Unless it was rebuilt recently, if you have 86mm bores, purchase 86.5mm pistons. If you have 86.5mm bores, purchase 87mm pistons. If you have 87mm bores, have your machinist inspect your block first before purchasing pistons. It's not advisable to bore past 87mm. 

 

Cheers, 

[SLOW-33]

Legend, thanks. 

[Murray_Calavera]

10 hours ago, SLOW-33 said:

It'll be on E85

I'm guessing you'll be buying pistons off the shelf (not having something custom made)?

Whatever brand you end up going with, I'd pick up the highest comp piston they offer. As a guess you'll end up with something around 9.0:1, but in a perfect world you'd still want to run more on e85.