I measured 3 different styles of 3.0 pistons.

1997 3.0 (supposed 9.3:1)
1994 3.0 (supposed 8.9:1)
198x 3.0 (supposed to be 8.85, its a full skirt piston with thicker old fashion 1.5, 1.5, 4mm ring pack)

Every single pistons measured 12.5cc dish, even though they are completely different castings.

I have measured a stock head in 2006 and I believe it was 47cc. Heads I ported for myself back then were around 51+cc (BIG...) Ed Kelly measured and found 46-47cc for all chambers, and I am assuming the motor he measured was around a 1990 model.

Measuring OEM used headgasket, this leaves 3.0L SOHC 12 valve compression ratio

8.19:1 for 46cc head
8.3:1 for 47cc head.

Super low! Its too bad the top ring land and intake valve relief are so close together or we would be able to make a lot more stock piston power safely.

If you look at JE brand shelf pistons sold for vR4, which has a 43cc chamber (much smaller),
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Part # Bore Oversize Stroke (mm) Rod (mm) C/D (mm) Head CC's C/R With .040 (squish) Dome CC's

321330 91.1 STD 76 141 31.5 43 8.0:1 -15.5
321333 91.1 STD 76 141 31.5 43 9.0:1 -6.5
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Shows how deep our 12.5cc dish is for a much larger combustion chamber!!!!!