Can some one tell me what a stock 1991 305's piston to deck clearance is? I can't find any where how far down in the hole it is. Thanks

 

it usually runs around .020 to .025

 

Thanks. Thats what i thought but i was doing the measurement where you add 1/2 the stroke + rod length + comp. height of piston and then subtract that from the deck height which i am assuming is 9.025 and i was getting like .040 to .045 down in the hole and i did not think that sounded right. Can you also tell me what the piston volume would be on the stock pistons? They are the small dish with 4 valve rekiefs.

 

Your math is absolutely correct. That's how it should be. In the real world..... you need to check. Wouldn't be the first time a spec was "fudged"
by someone, somewhere along the way.

And there have been plenty of GM motors that came with pistons sitting a lot further than the typical .025" in the hole. A true 1.56 piston compression height will get you exactly .025" in the hole with stock crank, rods and an unmilled block.

 

Thanks again Damon. Well thats what i thought and when i was figuring it up with the comp height of a speed pro flat top it was coming out loke .040 down and i thought that was a crap load!!! I was also wondering, I am going to use the stock small dish 4 valve relief pistons that came stock in the 1991 block. It origanally had 187 casting heads on it which i know are 58cc. With this combo the comp ratio was supposed to be 9.3:1 or so. So if i use the 601 castings milled 0.10 which should put me at about 51 to 52 cc. what is this going to put me at comp. ratio now? Thanks in advance to any replies.

 

Head gasket cc?

When you're down to brass tacks like this the difference between a 3cc steel shim and an 9cc thick composite head gasket makes the difference between 10:1 and 9.3:1. I calculated those two compression numbers with standard bore, 52cc heads, pistons sitting .025 in the hole and 10cc worth of piston dish.

FYI- Keith Black has reasonably priced hyper pistons with a -12cc d-dish that have a true .025" deck height (1.56" deck clearance). With a 6cc (GMPP .028") head gasket, 52cc chambers and these pistons you would be at 9.4:1 with a standard bore. If you used a set of KB flattops and unmilled 58cc heads, eveything else the same, you would hit exactly the same number. With a 30-overbore on the motor all these compression numbers go up by .1