well i just got my block back from the shop, (.03 bore) and i need to get pistons. im using 64 cc heads, and .039 gasket, 3.48 stroke, 5.7 rods. i have been looking at speed pro and silvolite pistons and they list piston head volume at +5,+6. by there cr calculator it puts me at 10.2-1 to 10.3-1 cr. im looking for 9.5-1 to 9.7-1. my problem i think is i dont know my deck clearence so i left it at 0. is there a # to use or do you need to buy a piston to measure and see what pistons to buy.

please tell me im reading in to this to deep and its not this difficult.

thanks mike.

 

Deck clearance will depend on the actual deck height of your block (which will vary from block to block), and on the compression height of the piston, rod length and stroke.

 

To find the REAL compression ratio, you need to know:

- Chamber CC's
- Piston Dome/Releif CC's
- Gasket Thickness
- Deck Clearance

Most "rebuilder" pistons have the pin around .020" higher in the piston (lowering the piston in the bore) because they assume you will deck the block, and that maintains the .025"-ish deck clearance GM builds their engines with. Purpose-built high perf/racing pistons usually don't have that.

The problem with that, is if you don't deck the block, you will end up with .045"-ish deck clearance, PLUS the thickness of the gasket... that will drop the real CR pretty quick.

 

I'll be damned. You're right. If you check Sumit's site you can shop and see the compression height change.
That explains why so many people deck their blocks nowadays.
I always thought that if you bought stock bore replacement pistons that the rings were moved down .020" to keep from hitting the ridge at the top of the cylinder bore. I see now that isn't always the case.
Fortunately my favorite piston (TRW-L2304F) is still available in 4" bore with the correct compression height.

 

flat top pistons dont usually have a positive "dome" cc.. they usually have valve reliefs that are cut into the piston.. some may say a positive number like what my piston manufacturer said: Valve Relief Volume 4.0cc.. that means i have 2 things cut in the piston that equal 4.0cc.. the word relief is a negative volume to an actual flat top piston..


here is another example.. its one of speed pro's dome pistons.. the valve relief volume is a negative number.. which means it bulges out since its a dome piston..
http://store.summitracing.com/partde...0&autoview=sku

 

wow, thanks for all the info but i think im more confused then ever! i understand what you guys are saying but im not sure how to get there. someone rebuilt this motor before i got it because there were male pistons in it. im not sure if it was decked or not. standard deck height is 9.025 ? is there a way to measure ? i have and can use machinist tools. i thought i could put one of the male pistons back in and measure the deck clearence and go from there but i cant find jack on male piston specs.
the pistons im looking at are speedpro 8kh631cp30. being a noobie to rebuilding the lower end of a motor im having a hard time believing i need to make an educated guess at picking pistons, buying them then measuring to see if they do what i need. i understand i may need to deck the block but i would like to get as close as possible.

am i making any sense here?

thanks,
mike

 

What i would do is...

Assume a zero deck and then buy a set of pistons that suits your compression and valve relief needs. Then take your crank, rods, and pistons to your machine shop and have the block cut the block for zero deck.

BTW most shops WON'T bore a block without the pistons. I would have the machine shop make sure that the pistons have adequate and proper clearance in their holes.

 

Yes, standard deck on ANY factory small block is 9.025" and most performance-oriented pistons (not cheap-o rebuilders) will sit about .025" in-the-hole at TDC if the block has not been decked. The compression height for such a piston using a 3.48" stroke and 5.7" rods is 1.56".

"Flattop" pistons will always have some "dish" to them in a small block becuase of the valve reliefs. Most 4-relief flattop pistons will be about 6cc worth of valve reliefs. 2 valve relief pistons are usually around 4cc worth of volume. If it's a single cut all the way across the piston for both valves you're back around 6cc typically.

If you go to a domed piston you will typically need a dome about .100" tall just to offset the valve reliefs and get back to a NET volume of 0cc (what a true flattop would be if there were no valve reliefs).

 

damon, thanks, somehow in reading your post it all makes sense now. think im good for now. or at least have a better understanding.

thanks,
mike

 

Probably better to say that the nominal deck height is 9.025". The exact deck height on a particular block may vary somewhat.