Davey,

The O.S. Max FS-91 Surpass engine has a crankcase vent on the bottom. This is so that air can move in and out as the piston moves up and down. This venting is essential.

The vent also serves the purpose of allowing the excess oil to be dumped overboard.

The vent is located on the bottom of the engine, forward of the rear bearing. This allows the oil to pass through the rear bearing before it's blown overboard.

An additional vent should not be needed on this engine.