piston deck height variance question for engine builders

[Clanceman427]

Hello, had a quick question for any experienced engine builders out there. I was wondering how much variance you typically see in assembled piston deck clearance. The reason I ask is that this seems to heavily influence the real compression ratio of any combination. I know I could calculate the tolerance stack of (most) of the components that play into this. For example, I use a Merlin 3 iron block that was race-prepped, with a deck height of 9.805 ±.005. The other components influencing the stack would be the crank stroke tolerance, rod bearing thickness (Clevite 77), connecting rod center-to-center distance (Howards), piston pin-to-top distance (TRW l2239NF). It has a GM forged 396 crank (don't know the stroke tolerance on this). I believe I have the tolerance specs on all the other components with the paperwork that came with them, I'd have to look...

When I assembled this engine (7 some odd years ago) I think I quickly measured it with a metal straight edge and feeler gauges, and didn't jot down the measurement. I remember there was some clearance (still some small volume between the block deck and the flat portion of the piston at TDC (these pistons are domed). Just curious if anyone knows a working tolerance on piston deck height, since I can't measure it now without tearing it apart (not an option!). Thanks for any insight. trying to determine how much more power potential i have....

Kevin

 

[Jim F]

If there is significant deck clearance, you can gain some compression by decking the block. Whether or not to do it, is another discussion. Variation from hole to hole should be kept to a minimum for optimal performance, but it in no way has to be perfect to operate well. You can swap rods, pistons etc around and get it as close as possible, but it is nearly impossible to get it perfect without a huge stack of pieces to interchange. Bearing swaps are a moot point, the tolerance there is so small its not worth messing with (oil clearance is another thing!) Stock deck clearances on SBC run anywhere between as deep as .020" down, to around .008' down. Most are pretty deep unless the block has been decked at some point. Dome pistons change the "rules" a little. With flat top, you want the quench height correct which will minimize detonation. The quench height is the distance between the flat head of the piston and the surface of the cylinder head.The variable is the piston to deck clearance and the head gasket thickness. Many times a simple head gasket switch will bring the quench right in (.045" is about ideal in a SB)
Dome pistons, may require different quench heights, as the dome has an effect (detrimental) on the combustion swirl.
The best dome is a short, wide, low ,and flat one, not a high one. High domes block the flame propagation across the cylinder and results in detonation much quicker than a short flat dome.

 

[Clanceman427]

Great info, thanks for the reply! I'll leave it as-is unless I have to open up the motor for some other reason.

Kevin