Anyone know what kind of piston's these are? It's the original motor in a stock L82 that was rebuilt a few years ago. Trying to get an idea of what kind of compression I'll have with my 64cc heads.

 

I have never seen pistins with tops like that but I haven't seen everything yet. I would guess that if they don't come up past the deck at TDC and a stock stroke you would be aqbout 9.5 -1 compression with your 64cc heads.
Just a guess !

Bill

 

I think you got a "what piston is this" answer on your other thread...not sure if you were able to find the details on the volume, etc.

With the engine in the chassis, there's really only one way to estimate CR. It's typically "close enough" - or at least, as close as we can reasonably get.

For a typical flat-top piston, it's pretty easy:

1. Using a dial indicator, find TDC.

2. Using a depth micrometer, determine how far down in the bore the piston is. If you don't have one (a cheap one is better than nothing) then you can "guesstimate" with a precision straightedge and a feeler gauge...at the cost of accuracy. Still better than a complete guess

3. Carefully fill any valve reliefs with clay and carefully shave them level with the piston. Remove them, and using a graduated cylinder determine their volume. This is done by filling the cylinder to a known level with water, dropping in your clay molds, and measuring the difference.

4. At this point you have the deck height, bore size and piston volume...close enough

You have an interesting challenge in both the "turbulence" dimples and the small dome. The dimples I think you'll need to measure one and "do the math" to come up with total cc. The dome you can take a mold of with clay, level it out as best you can and use an eye dropper to fill just level with water, counting the drops. 1 drop = .05 cc.

It's a fair bit of work, but IMHO worth the effort. Choosing the right head gasket is key to achieving the right quench and a street-friendly CR...and the right quench is worth real power.