Although I did not use the trial head gasket in the stock motor. I went and made another one for my 3.5hp workhorse motor. This motor is my every day use mower that has seen many hours of abuse. Already heavily modified this motor outperforms the 4hp. Upgrades include; 4300 RPM governor spring, increased valve lifter clearances (for more compression), 0.020" ignition advance keyway, roto-cap exhaust valve, racing oil, pure gasoline, thin copper head gasket. You'll notice a bump in compression but not as drastic as the stock motor due to it being already heavily modified.
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