Improper assembly is a possibility and it should be checked against the exploded diagram above. If the piston does not uncover the relief port there will be no pressure limiting. There is a mistaken belief that hot oil pressure can be raised by using a stiff spring or by blocking the piston and this is a mistake. Typically the pressure relief does not have any effect on hot oil pressure which is usually a function of oil supplied vs oil leaked out past the bearings and can only be increased by using thicker oil or tighter clearances. The primary function of the pressure relief is to reduce cold oil pressure at startup. To do this it has to be functioning properly. The piston has to be free to move and the spring has to be the correct one. Stock oil pressure usually runs around 45-50lbs and the stock relief may open at that point or it may not. Going a little stiffer on the spring is acceptable, up to around 70-80psi when cold. Jim
Remember the part where the OP tells us that someone drilled and tapped the pressure relief or filter bypass valve cap for a threaded line? That has to be corrected before anyone can speculate on what's causing pressure or filter blow off or whatever. If either of those circuits arent working properly the oiling system cant self regulate.
