I have dome pistons that are advertised at +13.8cc. I put them in the bore .2 in the hole and cc them, I only get -6cc. Bore notch may take up a cc or 2.

What's up with that? So I take the piston and wrap it tight with masking tape and fill the relief to meet the dome, takes14cc. .095 dome on a 4.29 piston comes out to 22cc or so for a measured net of +7cc. Take away a cc for the bore notch and I'm back to my measure of 6.

I need to set the compression, advertised -13.8 and measured -6. What am I missing?

Fyi, I measure the untouched combustion chamber and it spot on 121 as advertised.

Domes are + CC. Dish are - cc.

You'd have to make a mold of the top of the piston and then fill( liquid cc measure) that.

or put piston to tdc, fill that,

Take that cc and add to your gasket added cc, and then add comb chamber cc , and that will all equal your total combustion area cc.

Compare that to your single cylinder cid and that will equal the compression.

SB, these are domes and that's what I did.
First I put the piston in the hole .182 with a bit of grease to seal. Took 37cc to fill plexiglass on top. Calculated 43 for a net of +6cc.

Tape around piston, fill with liquid to top of dome I get 7cc. Assume the 1cc difference is the bore notch at top of bore.

Looks like I need to use +6 as measured not +13.8? These are a small .095 dome, no dish here.
Thanks

SB, the dome sits above the deck so I can't use that method.