The Stovebolt Site Forums The Shop Area Engine Shop 270 GMC piston wall clearance

I have a set of .030 over pistons for a 270 I'm building. What should the piston to cylinder wall clearance be? This engine looks like a recent rebuild, still visible hone marks in the cylinder and the pistons look nearly new and fit snug in the bores. Can I get a reasonable check the old school way with a feeler gauge? This is not a race engine, just a '31 Chevy pickup street cruiser. BUT it will have a turbo on it set to about 12 psi to start with. Also are the old GMC pistons forged or cast. These are he slightly dished ones? THANKS!!
Jay D.

They're cast, and 12 PSI is a lot of boost for anything this old of a stock bottom end. Be sure to give yourself lots of extra ring gap or else the rings will come too close together and break the pistons when the ends make contact while still expanding. A rev limiter wouldn't go amiss here.. are you computer controlling the air/fuel/timing situation?

I haven't figured the compression yet. Its got the D shape combustion chamber so hoping for a low static compression,6-1 OR 6.5-1. 6.5-1 with 12# boost converts to about 11.8-1, so yeah might be to much?? My blower motor has 12# of boost at wide open throttle. It builds as soon as the throttle plates start to open. It has forged pistons in it, blower cam and a 302 head. I did blow a head gasket one time but other than that its a screamer. My boy has a late model Chevy pickup 2wd with some special order 5.3 in it. From about 30 mph to 100 I can dust him by about 3 car lengths. He starts pulling on me about 100 mph. the GMC is gasping for air about then!
Jay D.

It's really cool to see one of these old stovebolts be made to get up and boogie! Refreshing in a sea of LS in everything. (Not bashing the LS scene here, just saying I like to see the 'other engines' side of hot rodding).

jay d - To answer your question on piston wall clearance specs, the 1955-59 GMC Model 100-500 Shop Manual calls for 0.00325" measured at right angle to piston pin (see screenshot below). I can't recall what year your 270 is??? For further info and detail on the 270 maintenance/overhaul and specs, read through Section 8 of the Shop manual for that year group. If you need info from earlier Shop Manuals, let use know what year engine you have so we can see about finding more out.

Please note this is stock cylinder wall clearance and your needs with a blower, and probably more modern piston design/materials, may differ.

I didn't really find much more on your engine question in Stovebolt, but did come across this related conversation in the old "Inliners International" Forums that might be of interest to you.

The lifespan of stock pistons with a buildup like you're contemplating will be measured in minutes at best. Bite the bullet and commission the best set of custom made forged racing pistons available, and follow the piston manufacturer's recommendation for clearance. They're going to rattle like there's a bunch of little guys with big sledgehammers inside the engine, trying to escape!
Jerry