OPR valve shims and spring specs

justin scroope · Feb 8, 2026

marshg246 said:
Where did you get the OPRV?
What pressure did you see with the shim(s) and uncut spring?

Andover Norton, Invoice date 08/11/2022, Order Number SO230134, Invoice Number 126784. On first start up, a 100 PSI gauge was fitted to the cylinder head oil supply at the timing cover, bike started, and the needle hit the 100 PSI peg. "It was not just a gentle caress." The following is from my service notes. Bike running, high oil pressure, no shims, fitted 2nd copper washer 0.85 mm thick under cap, oil pressure still too high, oil pressure spring free length was measured at 29.7 mm, and shortened to28.5 mm, oil pressure reading 80 PSI. (i considered this still too high, winter at Wollongong, i,m guessing about 15 degrees celsius at the time.) Test rode bike 21 miles and checked oil pressure cold, 100 PSI ? (don,t know what happened there), shortened the spring to 27.1 mm free length, no PSI noted, 10 weeks/524 miles later oil pressure checked OK, no pressure reading noted. 65 PSI would be my target cold.

marshg246 · Feb 8, 2026

justin scroope said:
Andover Norton, Invoice date 08/11/2022, Order Number SO230134, Invoice Number 126784. On first start up, a 100 PSI gauge was fitted to the cylinder head oil supply at the timing cover, bike started, and the needle hit the 100 PSI peg. "It was not just a gentle caress." The following is from my service notes. Bike running, high oil pressure, no shims, fitted 2nd copper washer 0.85 mm thick under cap, oil pressure still too high, oil pressure spring free length was measured at 29.7 mm, and shortened to28.5 mm, oil pressure reading 80 PSI. (i considered this still too high, winter at Wollongong, i,m guessing about 15 degrees celsius at the time.) Test rode bike 21 miles and checked oil pressure cold, 100 PSI ? (don,t know what happened there), shortened the spring to 27.1 mm free length, no PSI noted, 10 weeks/524 miles later oil pressure checked OK, no pressure reading noted. 65 PSI would be my target cold.

I asked because I check each bike I build and I was shocked when I first started doing that because I had two RGM oil pressure test gauges that came from a guy I bought out. One consistently showed under 10psi no matter the conditions and the other was always 90psi or more on the same bike: https://www.rgmnorton.co.uk/buy/oil...th-pipe-take-off-for-rocker-oil-feed_1260.htm

I was sure both were wrong, so I bought another that was meant for permanent installation. I connect it to each bike to test when I'm first starting/tuning them (I have no idea how accurate it is either, but at least it's consistent). Each bike varies a little, but they are all between 65-75 psi when cold with Castrol GTX 20W50 Conventional and they all either have a new AN OPRV or AN components and they all have the standard spring and two shims. If they are outside that range, I fix whatever is wrong, but I have never shortened a spring and I have never had a problem with an AN unit. I have worked on bikes that had stainless steel units that did not work correctly, and I replaced them with AN units. Yes, the oil pumps are fully serviced when building the bike. Once up to temp, the OPRV does nothing in normal riding weather and probably nothing in very cold weather.

I have come across bikes with:
1) OPRV pistons that are stuck - some open, some closed
2) The shims under the cap, not in the piston
3) Wrong sealing washers
4) Weird springs

All that said, anything you buy from anyone can have a problem. None of the pumps or OPRVs (no matter the marketing) are actually tested individually so it sounds like you simply got a bad part, or your gauge has a problem.

BTW, if a standard spring that you shortened and no shims, then your piston can move quite a bit, there is nothing to close it, and your spring is not stable in the unit. I would study that if I were you.

Carl H · Feb 9, 2026

justin scroope said:
I fitted a new OPRV assembly and a new timing cover (mk 3 design), and ended up shortening the free length of the spring by 2.1 mm to lower the cold oil pressure down to 65 PSI (without shims). It is worth the effort of checking the cold oil pressure on the first start up and not taking the risk of blowing the crankshaft oil seal inside out or dislodging it,s circlip.

So when your bike is fully warmed up and after a fast ride , what is your oil pressure at idle and at 3000 RPM. All my bikes would have about 80 PSI cold at about 1500 RPMs even with the OEM settings, and barely make 45 psi hot even at high RPMs.

Cold oil is thick and always shows higher oil pressures and lack of flow even on my 1985 Supra the gauge runs very high at start up and drops way down when warm. If you have never had an oil pressure gauge on the bike, you'd never know what is going on, cold or hot.

The designers had a reason to set the blow off the way they did in the OEM parts they made or used. All I know is Commandos have a great oiling system that does the job and can hold up to extreme racing conditions. I always hook up a gauge on a bike and test hot and cold and warm up the bike at low RPMs when I drive for the first few miles.

justin scroope · Feb 9, 2026

marshg246 said:
I asked because I check each bike I build and I was shocked when I first started doing that because I had two RGM oil pressure test gauges that came from a guy I bought out. One consistently showed under 10psi no matter the conditions and the other was always 90psi or more on the same bike: https://www.rgmnorton.co.uk/buy/oil...th-pipe-take-off-for-rocker-oil-feed_1260.htm

I was sure both were wrong, so I bought another that was meant for permanent installation. I connect it to each bike to test when I'm first starting/tuning them (I have no idea how accurate it is either, but at least it's consistent). Each bike varies a little, but they are all between 65-75 psi when cold with Castrol GTX 20W50 Conventional and they all either have a new AN OPRV or AN components and they all have the standard spring and two shims. If they are outside that range, I fix whatever is wrong, but I have never shortened a spring and I have never had a problem with an AN unit. I have worked on bikes that had stainless steel units that did not work correctly, and I replaced them with AN units. Yes, the oil pumps are fully serviced when building the bike. Once up to temp, the OPRV does nothing in normal riding weather and probably nothing in very cold weather.

I have come across bikes with:
1) OPRV pistons that are stuck - some open, some closed
2) The shims under the cap, not in the piston
3) Wrong sealing washers
4) Weird springs

All that said, anything you buy from anyone can have a problem. None of the pumps or OPRVs (no matter the marketing) are actually tested individually so it sounds like you simply got a bad part, or your gauge has a problem.

BTW, if a standard spring that you shortened and no shims, then your piston can move quite a bit, there is nothing to close it, and your spring is not stable in the unit. I would study that if I were you.

I agree. As i,m only at amateur level, operator error on my part is always a concern. The bike is still being road tested (currently 2800 miles) and i plan to have 3500 miles on it before any long distance touring is done. The OPRV assembly looked good to me, a nice sliding fit of the piston in it,s housing, after shortening the spring there was minimal piston movement when assembled by hand on the bench. I should have documented measurements like piston inner diameter to inner diameter of the cap, and i didn,t find out what the spring free length should have been before i shortened it.

justin scroope · Feb 9, 2026

Carl H said:
So when your bike is fully warmed up and after a fast ride , what is your oil pressure at idle and at 3000 RPM. All my bikes would have about 80 PSI cold at about 1500 RPMs even with the OEM settings, and barely make 45 psi hot even at high RPMs.

Cold oil is thick and always shows higher oil pressures and lack of flow even on my 1985 Supra the gauge runs very high at start up and drops way down when warm. If you have never had an oil pressure gauge on the bike, you'd never know what is going on, cold or hot.

The designers had a reason to set the blow off the way they did in the OEM parts they made or used. All I know is Commandos have a great oiling system that does the job and can hold up to extreme racing conditions. I always hook up a gauge on a bike and test hot and cold and warm up the bike at low RPMs when I drive for the first few miles.

I have only checked cold oil pressure while stationary. I,m not brave enough to permanently fit an oil pressure gauge due to the risk of failure of the assembly and/or accidentally bumping the connection at the head, (by me) and loosening it when working on things like the head steady. I,m sure there are many benefits with fitting an oil pressure gauge in a perfect world, but i.m an imperfect mechanic.

Fast Eddie · Feb 10, 2026

justin scroope said:
I have only checked cold oil pressure while stationary. I,m not brave enough to permanently fit an oil pressure gauge due to the risk of failure of the assembly and/or accidentally bumping the connection at the head, (by me) and loosening it when working on things like the head steady. I,m sure there are many benefits with fitting an oil pressure gauge in a perfect world, but i.m an imperfect mechanic.

I think you’ve got the prioritisation the wrong way around.

An engine only runs cold for a very short while and (hopefully) under minimal stress. Hot oil pressure is therefore far more important than cold.

You are correct to be concerned about oil seals when cold, but provided they are not being damaged (and on the vast majority of standard Nortons they are not) then there is little to worry about.

But without checking, you do not know what the impact of your modification is on hot oil pressure…

IMO you should either do some proper checks or revert to standard.

OPR valve shims and spring specs

justin scroope

marshg246

Carl H

justin scroope

justin scroope

Fast Eddie

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