Marine 273 build
Tr Benj
That Cylinder Head Chart is correct.
But those CC Volume Numbers are not what they left the factory at.
The #920 Castings {1967 Closed-Chamber} left the factory around 63 CC's.
The #675 Castings {1968 and later Open-Chamber} left the factory around 68 CC's.
Many guys will get the #302 Heads to run, because they have a tight 'Heart-Shaped'
Combustion Chamber much like the #920 Cylinder Head, but with a 'Swirl Port Design'
and a {62 CC Combustion Chamber}.
And the #302 Heads have induction 'hardened' exhaust seats.
I imagine that you're running a {340 Grind} Hydraulic Camshaft.
Unfortunately, the 273 loves a 'Solid Camshaft'.
More to come,,,,,,,,,,,