The Oil Pressure Release Valve


Old & New Design

This valve has an important mission, if it gets stuck in the open position, the mains & big end will loose most of the oil pressure. So it's wise to clean it & check it's function regularly. The two different design differs, one is ball-based, the newer is plunger-based. They both open fully at approx the same pressure, but the plunger is less progressive than the ball design. The larger pressure surface & better spring looks like an improvement over the ball system, so I opted to fit the "modern" (1970) plunger type.

Test Setup

A piece of tube + a car tire air valve and a trip to the nearest petrol station will let you test the function. I got a fully open reading at approx 10 psi for both the plunger & the ball valve, but the ball valve started to let pressure trough at around 4 psi, while the plunger, due to it's design, waited for the pressure to rise near to 10 psi before starting to open. These measurements are done with air pressure. I don't think they relate directly to oil pressure. The BSA Service Sheet nr. 202 states the valve should open when pressure "in the supply system" reaches 50\60 psi, I suppose this is the internal pressure, out of the oil pump, not the pressure at the release valve which is placed after the main bearing & big end feeds. Any further comments are welcome.