Hi all,

Currently planning a motor buildup for my STi (using the shortblock already in it). I've got a local machine shop looking into what it's going to take. The plan was to use 100mm forged pistons (.020" over) and then bore out the cylinders as necessary (keep stock rods and crank, new bearings). The builder (one man show basically) said he called both Ross and CP about their pistons and said both manufacturers said he would have to sleeve the motor if using oversized pistons on the ej257 :huh: . This is the first I've heard of this, and I know of at least one company marketing a 100mm piston motor w/o sleeves (Gruppe-S GS600).

Just looking for some insight on this subject from anyone who would care to share it :cool:

Thanks!

We make 100mm pistons and you do not have to sleeve the block for 100mm bore. If you are going to run some serious boost I would recommend it, however it is not needed. We have seen people put some good power down with and without sleeves.

We make 100mm pistons and you do not have to sleeve the block for 100mm bore. If you are going to run some serious boost I would recommend it, however it is not needed. We have seen people put some good power down with and without sleeves.

Thanks for the reply Nick! What would your definition of serious boost be? I'm looking at keeping it under....but possibly approaching 30psi.

close to 30psi.... I would sleeve it. Just think, taking .20" out of the cylinder walls makes it that much weaker.

Iam not saying it wont work but the main question is how long do you want it to work.

If you are really going to do 30 PSI I would sleeve it. I have seen people doing it without it, but Its better to be safe then sorry.

close to 30psi.... I would sleeve it. Just think, taking .20" out of the cylinder walls makes it that much weaker.

Iam not saying it wont work but the main question is how long do you want it to work.

Well, it's a .020" increase in bore diameter, which would be a .010" reduction in cylinder liner thickness. According to a quick search, that means a 10% reduction in cylinder liner thickness (and a much much less percentage reduction in total cylinder wall thickness). Obviously I would want the motor to last as long as possible, but realistically I don't expect it to last forever. If I got 50k miles out of it I'd be pretty happy :confused: .

The 30psi would only be on race fuel obviously. Normal pump gas driving would be quite a bit more tame (20psi +/-).

Bump for more info, I am going with the 100mm's as well..

I will be running .020" over pistons in my motor. I will not be sleeving my motor. I will be running 30+ psi with a 35R.

my junk is being put together right now with 100mm pistons and it will not be sleeved.

Well, it's a .020" increase in bore diameter, which would be a .010" reduction in cylinder liner thickness. According to a quick search, that means a 10% reduction in cylinder liner thickness (and a much much less percentage reduction in total cylinder wall thickness). Obviously I would want the motor to last as long as possible, but realistically I don't expect it to last forever. If I got 50k miles out of it I'd be pretty happy :confused: .

The 30psi would only be on race fuel obviously. Normal pump gas driving would be quite a bit more tame (20psi +/-).

sorry I ment to say .020" ;)

I didn't hear anyone here say you "need" to sleeve. Frankly, I think it does much the same thing as using forged pistons: it helps to minimize potential failure, in this case caused by the cylinder tapering and ovaling which, over time, increases with high boost. How much longer a sleeved motor will last with good compression and leakdown is the $64K question. Unfortunately, the cost differential between simply replacing the slugs vs. sleeving the block makes it a prohibitively expensive option without knowing the answer.

Looking for any long term feedback on this.

I have a 0.020 over sb assembled - new everything w/ balanced stock crank - waiting for heads etc to finish it up.

1. I've been reading/hearing more about what happens w/ bored sb's. My app is pump gas at a conservative 375whp or so for road racing. Any feedback on shortened life or problems?

2. We have another build and a machine shop may have honed the block out of spec. for the pistons. We'd hate to go right to a 20 over motor just for that. I searched for something i read about slightly oversized stock bore pistons for just this.

-Ken

As long as you keep it under 30psi and the tune is good the block should live a long life.I have built and tuned a few subys on the stock cylinders that have seen 30psi plus and +600whp with no problems, as long as the timing is in line and the afr's are just right to keep in cylinder temps down while making power you should be ok.