Whilst this post refers to the 1972 OIF BSA B25SS Gold Star, it also applies to earlier bikes, although the thread on the oil pressure switch may be different. Check yours with a thread gauge and make sure you don’t fit a tapered thread, when your old switch had a parallel one, and vice versa; you could crack your crankcase!
The original OPS
If you are the lucky owner of a BSA B25SS Gold Star or similar, you will know that it has an oil pressure switch (OPS) fitted at the front of the right hand side of the engine. It’s in a somewhat vulnerable spot, so may get damaged…or simply fail…necessitating replacement. The other engines in the BSA single range, such as the B40, B44 and B50, do not have an oil pressure switch fitted. Note that the Triumph TR25W range is essentially the same as the BSA B25 bikes.
The switch does nothing else but illuminate a red light on the headlamp bowl, to let you know the oil pressure is very low. Usually, the light will go out as soon as you kick the bike over.
The B25 engine relies on a good supply of oil to the big end and other critical components, so if your oil light is on when the engine has been started, stop it immediately and trace the problem.
If your switch has failed, the manufacturer’s part number is 60-3719 and the OEM part is PS 5330/1/03. This is a normally closed switch, with a 27 tpi 1/8″ NPT thread, with a switching pressure of 3 psi. There are a number of sellers offering replacement OPS on eBay which are incorrect. One such seller is ‘wayao1’…although I don’t believe he is doing this deliberately.
The incorrect new 45328 OPS
The switch I originally bought from him, didn’t match the numbers on any of the pictures on his listing, but it was marked ‘45328’ and that was all…I couldn’t track it down on-line. The switching pressure is marked on the OPS as 0.4 bar, which is approximately 6 psi; about double the correct item. I also discovered that the threads on the one he sold me were in fact 28 tpi 1/8″ BSP; also incorrect. The switch was returned.
According to my research, the oil pressure on the BSA B25, if the engine is in good order, is around 10-15 psi at idle and about 55 psi at 4,000 rpm. If the engine and oil are really hot, the pressure at idle may well be a little less. The incorrect switch was set to switch at around 6 psi, which may well be reached with a slightly worn engine on a hot day, when idling. The correct switch pressure is 3 psi; if the oil pressure is less than that, you have a problem, a worn big end being the usual culprit.
Feked OPS
I then did what I should have done at the start and checked the usual on-line suppliers. It turned out that Feked had exactly the part I needed. So, if you need one, the link is here. But do make sure you order the premium 27 tpi option.
Just so you know, after all this, my oil pressure switch was actually fine…there was an air lock in the pump.