This tillotson casting has been milled .060 to give it right around a 17.5cc chamber, ported, 25/32 mm valves, 36 lb dual springs installed at 42 pounds and lots of love.

It's perfect for your 308, 356, etc cam and ready to bolt on to make power.

I do not recommend a 32mm for gasoline back yard riding but that's up to you.