Assembling Oil Pressure Relief Valve

Stephen Hill · Aug 17, 2025

I am re-assembling the oil pressure relief valve. The valve did not have any shims in it when I disassembled it. I decided it needs one shim to bring the amount of piston freeplay within specs (Hemmings, White, etc). The parts manual shows the shim located at the end of the spring opposite to the piston. In this position the shim would be floating around inside the large domed nut. It makes much more sense to me to put the shim inside the piston, where it fits snugly and is held in place by the spring.
Am I missing something here?

PeterJoe · Aug 18, 2025

Stephen Hill said:
I am re-assembling the oil pressure relief valve. The valve did not have any shims in it when I disassembled it. I decided it needs one shim to bring the amount of piston freeplay within specs (Hemmings, White, etc). The parts manual shows the shim located at the end of the spring opposite to the piston. In this position the shim would be floating around inside the large domed nut. It makes much more sense to me to put the shim inside the piston, where it fits snugly and is held in place by the spring.
Am I missing something here?

Stephen,

On my 1974 Commando the oil pressure relief valve assembly had one shim and it was installed inside the piston. You may have a shim/shims inside the piston as well. Go ahead and clean the piston with solvent really well, then smack the piston on a table surface really hard and the shim should pop out. It's really easy to look inside the piston and think that there is no shim because it is a close fit inside.

Peter Firkins

Stephen Hill · Aug 18, 2025

Thanks Peter. The relief valve in my bike definitely didn't have a shim installed. I looked at the Hemmings engine build video and Mick installed the shim inside the piston. It is the obvious place for it. Despite the parts manual that shows the shim installed at the other end of the spring, somehow floating around in the domed head of the nut. Duh.

velo54 · Aug 19, 2025

Shim at the top. Reduces unsprung weight!

Stephen Hill · Aug 19, 2025

The piston was frozen at the top of the bore of the valve with minor corrosion on the piston wall. This would mean the engine was running on full pressure all the time. Seems this would be irrelevant on an engine with hot oil, more of an issue with a cold engine. What are the symptoms of an engine with too much pressure at start up? Over-oiling at the top end? I am pretty sure I don't have any blown seals.

kommando · Aug 19, 2025

Blown seal where the end of the crank enters the timing cover is the normal consequence , the seal edge inverts so reducing the pressure it will hold and the oil leaks out reducing both the flow and pressure to the big ends. The spinning crank will help what oil does make it so the big ends will take their time before issues appear.

Tornado · Aug 19, 2025

Stephen Hill said:
The piston was frozen at the top of the bore of the valve with minor corrosion on the piston wall. This would mean the engine was running on full pressure all the time. Seems this would be irrelevant on an engine with hot oil, more of an issue with a cold engine. What are the symptoms of an engine with too much pressure at start up? Over-oiling at the top end? I am pretty sure I don't have any blown seals.

Blown off rocker feed lines are a possible consequence of over pressure. Note the OPRV only controls max pressure, not normal operating pressure as some presume.

Stephen Hill · Aug 20, 2025

Got it. This is why I said the valve frozen at the top of the stroke would have no affect on a hot engine, because the oil pressure would be below the operating range of the valve.

Assembling Oil Pressure Relief Valve

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