ok, so i ditched my last engine after some problems. now i have a 468 short block with a forged steel callies crank (stock stroke) forged gm "dimple" rods with trw/ls6 (7305p) forged pistons hung on them. im getting ready for assembly, an have a question. the pistons are stamped for .060 over bore. i slipped the piston in the bore, and it seems like it has too much clearance to the cylinder wall. the bore for the block is at the proper 4.310, i.e. .060 over. the pistons however only measure in at 4.270, i.e. a .020 over bore. i know that they wont be the exact same size as the block bore, and i also know forged pistons require more clearance than cast or hypers, but i expected closer than .040. i mean, the pistons fit in my old block that is bored .030 over to 4.28.

what im gettin at, is are these pistons too small for the .060 over bore they claim to have?

im going to do some combustion chamber/ port work to my 781 oval port heads to help with compression and flow, but was wondering if it would be wise to mill the pistons some as well. im shooting for 9.5:1-10:1 cr, and will plan the cam accordingly to help.

other parts include an rpm air gap manifold, 850 speed demon carb, roller tip rockers, 3/8" pushrods, msd ignition, t-56 and either 3.90 or 4.11 gears.

wasnt sure if this is where this could go, but any info/opinions would be appreciated.

 

You never bore a block without first having the pistons. The machine shop that bores the block needs the pistons to make sure the wall clearance is within specs. Without the pistons, an overbore is just a guess.

Forged pistons usually have a clearance around .006" and it's measured at the skirt below the wrist pin hole using a very long feeler gauge.

It sounds like you have the wrong pistons or you're measuring it wrong.

 

yea, i feel a bit retarded now...let just say, i measured wrong and leave it at that, thanks stephen! after seeing your message i did some more searching and checking and everything is a-ok. i have .0055 clearance on it, sounds good.