By-pass valve

[Phil C]

Just browsing through the instruction manual for my 1965 B40F. In the section headed "Lubrication system" it says:  " A by-pass valve (D) ensures a constant pressure in the system. Surplus quantities of oil are discharged into the crankcase. In no circumstances should the cap if this valve be removed by the private owner." Crikey. What's that about, I wonder. The manual appears to trust the owner to have a go at other tasks such as decarbonising,  tappet adjustment, etc, so I wonder why the owner is advised not to touch this by-pass valve. Any thoughts?    Phil

[Phil C]

Btw, I made a typo: "if" should of course be "of". Just in case it didn't make sense!   Phil

[LightningAndStarfire]

It doesnt make sense. Perhaps in BSA factory experience many owners had been tempted to tamper with the spring in an attempt to raise system pressure.

[Pete C]

They probably just didn't want owners messing about with a critical component. Get the tappets wrong and you burn out a valve or go deaf from the clatter. Mess up the oil flow and you could sieze the engine.

[Calum]

I wonder if it is adjustable. My guess would be them being worried that someone messing with it might accidentally lower the relief pressure and potentially starve portions of the engine of oil. Pressure relief valves on any car engine I have worked on are usually non adjustable and rely on the set length and rate of a new spring. Obviously not great as springs tire over time but pretty fool proof.

[JulianS]

I suspect BSA were concerned that owners would assemble the spring the wrong way around. See first service bulletin.

Then they changed the spring so it would fit either way around. See second service bulletin.

[Phil C]

As ever, Julian, many thanks. And of course to others who responded with their thoughts.   Phil.

[JulianS]

Looks like some  though it was the gearbox level plug................see below from BSA.

[Phil C]

Interesting article. Thanks again.  Phil