I'm sure much has been written about this topic down the years - but I'd like to revive it after a recent comment on a Commando thread.
A certain Forum Contributor suggested all this talk of Taps in the oil Line to the engine, with microswitches to ensure the engine could not run until the tap was opened was old Hat. He suggested Automatic units were now available......so I posted the following note in reply to him:
Mark Hawkins .....you poked a stick in a termite mound, by suggesting an automatic anti-drain valve to cure wet sumping.
Although the Velo one was adapted by Mick Hemmings for use on Norton's, you can't get away from the fact that it relies on atmospheric pressure (14psi) to lift the ball off its seat, as it is located BEFORE the oil pump.
The MkIII item is located AFTER the oil pump, where it has full pump pressure (100psi?) acting on it.
Another thing to consider is that a tap, when open, has no restriction to the flow of oil.
A non-return valve (like the Velo item) will always have a ball & spring in line with the oil flow.
BUT....fitting a tap, although superior, has a downside.
As stated above, it must be fail-safe, with a microswitch cutting off your ignition until the tap is opened. A reminder like the 'Remove before Flight" tab airplane pilots have over their Pitot Tube covers....its a pretty critical item!!
