[Tomchev]

Some 305s had 58 cc heads with 1.84 intake and 1.5 exhaust that would give you around 9.1 compression ratio. the 76 cc heads would give 7.6 compression ratio which would make it a slug. Some 305s heads had 60cc but had 1.72 intake valves. They changed the center intake bolt angle in the later 305s but do not know the year they made the change.