Timed breather

One purpose of a breather is to reduce crankcase pressure. Rex Bunn advocates one check valve allowing fresh air into the engine on the upstroke and another allowing the crankcase fumes to exhaust on the downstroke.

There would be a partial vacuum if there were no blow-by past the pistons and rings. Once bores are worn, the timed breather may fail to cope with the blow-by and you get a positive crankcase breather which will naturally tend to force hot, thin oil past the various gaskets and seals.

Even with present technology there is always leakage between rings and bores of 0.5 - 1.0% (blow-by). However, gaskets and seals should be able to cope with typical crankcase pressures, 'thin' oil should not leak out. Head gaskets manage a gas tight joint with combustion pressures of approx. 600 p.s.i.

I think four cylinder engines and other designs do not subject the crankcases to the large pressure swings associated with a single or 360 deg parallel twin. All engines, for a few years now, do have to meet emission laws as you state hence the closed breather system.

The design of the timed breather results in a negative pressure in the crankcases, thus reducing leaks, but also means there is less resistance for the descending pistons. This is cancelled out somewhat though on the upstroke because the pistons then are working against a vacuum.

Disadvantage with the later breather is the crankcase is at positive pressure on the piston downstroke and then at atmospheric pressure on the upstroke. The gases to an extent are travelling back and forth along the breather tube.

I favour a through flow system where piston blow-by gases are flushed out of the crankcases by inlet and exhaust check valves. Google Bunn Breather for more info.

Not essential, merely desirable.A 4 cylinder engine does not have significant crankcase pressure fluctuations and a fair sized vent to atmosphere works fine. Similarly, a big vent to atmosphere from a Commando engine will work OK too, despite the crankcase pressure fluctuations. It's more elegant to have a timed breather (mechanical or reed valve) and if you have one you don't need such a large vent to atmosphere to prevent crakcase pressure build up. Royal EnfieldCrusaders had a nice little one way valve breather, but for the Continental models a large diameter pipe from the crankcase was added as the original small valved breather was found inadequate at prolonged high speed (hot thin oil, lots of blow-by, oil widdling out from the barrel base gasket) - and not to mention the inadequate con rod, but that's another story.

Hi Colin, It is a long time since I pulled a Dommie engine apart but from memory the spring that holds the moving and fixed discs of the breather together to maintain the sealis not particularly strong and if a big vacuum was somehow to be realised in the crankcase, this spring has the potential tocompress enough to break the "seal" and tend to relieve the vacuum.