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I may have confused the issue. No spring referred to the "check valve" in the crank boss of a G80 Matchless engine. No rotary motion at all. It appears that the disc, sucked (no spring) fitting tightly against the base of the fitting on the upstroke, is intended to prevent air from returning into the crankcase (which in this case includes the primary chaincase as there is no crank seal) and the disc is blown open on the down stroke.In all these situations ( timed Atlas, Dommie etc. or late commando with reed valve added) once the engine crankcase is evacuated during the first few crank rotations, there is very little air movement through the breather system as there is a vacuum in the crank case. Only blow-by will need to be evacuated on a continuing basis. Measurements posted showing ~18 inches of vacuum bear this out.FWIW, GM had rear main bearing seal leaking problems for years with the rope-style seal, particularly on truck sixes in heavy service. In the late fifties they added a PCV system that, while not creating a vacuum, reduced the crankcase pressure considerably compared to the previous road-draft tube. Those engines leak a lot less oil. I have two of them one with road draft and one with PCV.
I may have confused the issue. No spring referred to the "check valve" in the crank boss of a G80 Matchless engine. No rotary motion at all. It appears that the disc, sucked (no spring) fitting tightly against the base of the fitting on the upstroke, is intended to prevent air from returning into the crankcase (which in this case includes the primary chaincase as there is no crank seal) and the disc is blown open on the down stroke.
In all these situations ( timed Atlas, Dommie etc. or late commando with reed valve added) once the engine crankcase is evacuated during the first few crank rotations, there is very little air movement through the breather system as there is a vacuum in the crank case. Only blow-by will need to be evacuated on a continuing basis. Measurements posted showing ~18 inches of vacuum bear this out.
FWIW, GM had rear main bearing seal leaking problems for years with the rope-style seal, particularly on truck sixes in heavy service. In the late fifties they added a PCV system that, while not creating a vacuum, reduced the crankcase pressure considerably compared to the previous road-draft tube. Those engines leak a lot less oil. I have two of them one with road draft and one with PCV.
