[Henry Schmidt]

Something is not right.

Doing the calculations:
66 mm stroke
89 mm bore
1.0 mm deck
68 cc head chamber ( 2.2 to 2.7 heads are usually close)
19.38 cc dome
Compression ratio = 8.49 ?

Perhaps you meant dome height = 19.38 mm ?

To make 10.5 to 1 compression in a 2.5 engine you would need 31 cc piston dome.