to overbore, or not to overbore

My machine shop guy says my engine is about .005 too far for my .040 over pistons. The car had 40k miles on it when the oil pump quit and it I tore it down before anything failed other than a bearing.

He said it wouldn't be that big of a deal to use the pistons, but said that it will likely develop piston slap and possibly consume a large amount of oil as it got some miles on it., but that since we plan on spraying the motor that it'd actually be ideal. I only plan on a 100-200 shot.. as they are cheap speed pro forged pistons.. I'm running a scat forged crank, and eagle h beam rods. I might go ahead and get the pistons coated.. just for fun while they are out.

My question is, why is really loose clearancing in the cylinder bore ideal for nitrous.. or is it? I trust this guy... as hes done alot of work for me before, but I'm still unsure exactly why it would be ideal to have a piston looser in the bore for a nitrous application over N/A. Do you think I should go ahead and go .60 over and get some new pistons? I'm on a budget here.. still trying to decide what the best move would be.

Thanks,
Josh

You don't indicate what kind of a motor it is in your post, but if (as your sig suggests) it's an LT1 you definitely should sonic check it if you plan to go more than 0.040" over. As far as piston to wall clearance, the correct clearance will depend on the piston used. But in general, an increase of .001-.002 compared to NA is typical for nitrous or forced induction use.

So, .005 extra is pretty loose. But I think it will work fine and if you are on a tight budget I'd say try it. I bet lot of people have motors in their car this far off and never know it.

Rich Krause

from the signature
1999 Red Appearance Package Z w/
12 bolt, 4.10's, ram 910 clutch, tons of suspension, Hooker Headers, Lanes Duals, Torque Thrust II's, Glasstek 4" Cowl hood, FMS 236/239 .601/.610 111lsa Cam, LS6 Intake, Asp Pulley, Ported T.b.

419rwhp - With Belt
428rwhp - Without Belt

Originally posted by rskrause
You don't indicate what kind of a motor it is in your post, but if (as your sig suggests) it's an LT1...
Um Rich, I'm betting he's got an LS1 unless he has changed his sig VERY recently.

This is the engine in a 1984 camaro z28. It is a 350 bored .40 (71 010 block). The buildup consists of scat forged crank, eagle h beam rods, and speed pro forged pistons. I will be using ported AFR 190 heads, and a 274h cam. Pondering going with a different cam, however. What is the advantage of going .001-.002 over for a nitrous piston? Is it due to the extreme cylinder pressures?

Thanks,
Josh

The idea is that with nitrous or forced induction the piston heats up more and therefore has greater expansion. The idea is to keep the piston from scuffing or even seizing at peak operating temperature, hence the recommendation for greater cold clearance.

Rich

Ahhh.. awesome.. I bet that this is a must considering the typical characteristic of most forged piston to contract/expand with temperature more than hypereu.

Thanks,
Josh