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Original Post (Thread Starter)
#861708 10/26/2021 3:50 PM
by DMadigan
This alteration to the oil pressure control system on A65/50 motors will greatly improve your oil pressure both at idle speed and normal riding. The change gave me 18-20 psi at hot idle and 45-60 psi at cruising speeds. Here are the modifications:
⦁ Remove the existing OPRV from the cavity.
⦁ Using an 8-32 tap, tap threads in the return port. This port is simply a drilling at an angle on the left side of the cavity. With the sump cover removed flush out the aluminum chips thoroughly with fluid and compressed air. Once it is clean and threads are dry, install a 8-32 allen set screw into the threaded hole. Use medium (blue) Loctite to secure and seal threads. (See pic below). This blocks the pressure side from the return side
[Linked Image from]
⦁ Fabricate cavity plug per sketch. A plumbing supply store will have a brass plug that is large enough to cut the new threads and tap the 1/8’s pipe thread. (See sketch)
[Linked Image from]
⦁ Using a suitable gasket, thread the plug into the cavity. Install a Tee into the 1/8ths tapped hole in the plug. The forward end of the tee connects to your gauge, the rear side of the tee will be your new relief pressure return line.
[Linked Image from]
⦁ Cut thru the return line from your sump to the tank. Install another tee in this line. This will allow the overpressure oil to return to the tank. Now, take the new miniature 60 psi OPRV and install it in the end of the tubing coming off the rear of the OPRV cavity. Push the new OPRV into the line far enough so that there is enough tubing (about a half inch) to connect the end of the hose to the tee in the return line. Make sure you put in in the correct way round so it relieves pressure. Install a small hose clamp on the tubing around the OPRV to prevent oil from passing around the OPRV. Clamp it tight because it will see high pressure oil and you do not want it to move inside the hose (See pic of new OPRV. It is the little OPRV on the left in the pic). I used double hose clamps on all of the tee’s. Use tubing capable of high pressure on the high pressure by bass line to the low pressure return line. The OPRV’s are made by Lee Company and are used for bleeding off tank and vessel pressure but work perfectly on an oil system. This miniature OIPRV is nothing more than a small ball and spring type SS relief valve
[Linked Image from]
Figure 1 (minature OPRV on left)

⦁ Double check all fittings are tight and hose clamps secure. The small orifice in the new OPRV will not pass enough oil when it is cold so you have to run the motor at idle until it warms. If you rev up the motor cold you will see pressure on you gauge between 80 and 100 psi. Once the oil warms it will maintain pressure between 18psi at idle and 50-60 psi at high speed. Because of its miniature size, the new in-line OPRV restricts flow even when the relief ball is lifted off its seat but once the viscosity of the oil drops as temperature rises it works perfectly.
Liked Replies
#862203 Nov 2nd a 11:35 AM
by Mr Mike
Mr Mike
Just a point on rerouting the by-bass oil to the top end. Make sure that this is in addition to the existing top end oiling. Remember the OPRV does close off to barely a trickle and in my test closed off completely at around 10 psi. You wouldn't want an intermittent supply of oil to be the only oil going to the top end.
Mr Mike
2 members like this
#861745 Oct 26th a 07:31 PM
by Mr Mike
Mr Mike
Hysteresis is just part of the problem. This system fixes leakage by the piston (or ball) and also through those three threads that separate the pressure side from the return. The new valve is so small that it restricts flow even when the ball is off the seat. Loss of pressure due to hysteresis is quite small. This new valve opens at 50 psi and closes at about 10 psi do to the hysteresis effect.. I observed the hysteresis effect by running the motor and I saw when it finally closed off at about 10 psi. My idle pressure went from about 7-8 psi to about 20 psi with the change. When I closed off the bypass hole with the set screw I had any bypass oil going thru my return line and I could see it drizzling into the tank. I actually clipped my return line from the pressure side to the tank so I could see the volume of return oil.. It returned a small amount compared to either the ball or piston which meant all the oil was going through the main bush and rods...where it should be going. I think the piston system is an improvement from the ball but it just leaks down the pressure too much. When I first tested my idea I installed a manually adjustable needle valve. I could close off the bypass completely and have 20 psi at idle but at speed it made 80-100 psi which is too high, so I knew I needed relief and the pump was adequate. The only down side which I mentioned in the writeup is on cold starts my relief is too small and I have too much pressure except at idle. So when I rode the bike I warmed it up for a few minutes before taking off. Today I only have one BSA left in my shed, my B50 so no problems with oil pressure.

How this helps anyone with a low pressure A50/65.

Mr Mike
1 member likes this
#861801 Oct 27th a 01:15 PM
by Mr Mike
Mr Mike
It's good to see that others were unhappy with the low oil pressure in the A50/65. I think if I still had an A65 I would reroute the bypass oil to the top end. That's a better way to do it. I just didn't think of it at the time. I was so happy to get good oil pressure on my A65. Now the bike is on its second owner after me and still has great oil pressure.

Mr Mike
1 member likes this
#862342 Nov 4th a 12:09 AM
by Mr Mike
Mr Mike

I did just what you described with the early OPRV and it worked fine, however, I have brother who is a retired tool maker with a couple of lathes in his shop. Why's cleaner and I did it right. The later model domed OPRV was not so good to work with as the earlier ones.

Mr Mike
1 member likes this
#862700 Nov 8th a 08:44 PM
by Nate stuch
Nate stuch
I currently own Mikes old 66 and it still runs great with great oil pressure and a solid stream of oil return back to the tank. And yes when it’s cold I do let it idle for a few minutes in the driveway and I take it easy for the first few miles. Great bike.
1 member likes this
#862906 Nov 11th a 03:44 PM
by Mr Mike
Mr Mike
Just a word of advice not only to you but to all bikers with pressure gauges. I never liked the idea of a hi pressure oil lines outside of the engine case. My conversion not only has the line to the gauge but also the line that routs pressurized oil to the return line. Check your hose condition regularly for bulging, poor connections etc. If one of those pressure lines pops off the pressure on the bearings goes essentially to zero and the oil is pumped to the street.

OTOH, good oil pressure is key on any plain bearing motor.

Mr Mike
1 member likes this
#864282 Nov 26th a 03:16 PM
by Adam M.
Adam M.
What my friend in Poland does with those valves

[Linked Image from]

But it's quite involved mechanically with making a base for a fiber gasket, I simply locktited the threads down from the groove in the valve.
1 member likes this
#864318 Nov 26th a 11:26 PM
by NickL
Dave Madigan built a really good oil pump for these using gerotors.
No-one was interested, maybe it's because he didn't anodise it blue
like the srm ones? As i said, dumb owners are the problem.

If you want an a65 to work well with the plain T/S bearing you will definitely
need to spend some hours rebuilding the oil pump or replace it with either
a properly rebuilt one or a new one. Then fit an oil filter and sort out the
relief valve, some are worse than others.
By the time you've done the bush properly you may as well have done an
end feed needle roller conversion anyway so that relieves 30%+ of the demand
on the oil pump.
1 member likes this
#864313 Nov 26th a 10:43 PM
by NickL
The piston type is more likely to stick or seize with debris than the ball valve.
The advantage of the piston one is that it does vent more.
They probably changed over as they used the same valve on triumphs.

With cold oil either valve does not vent enough pressure but it's no big problem,
as for gauges i use one when i build the engine to check then get rid of it, another
thing to cause leaks.
Oil pressure can be something people obsess about, certainly at idle, loads of old
cars and bikes would ride quite happily on 20-30psi and tick over at 2 or 3. At idle
there is virtually no load on the bearings anyway.
Failures on these motors were normally caused by dumb owners not oil pressure.
1 member likes this
#864705 Nov 30th a 12:56 AM
by Mr Mike
Mr Mike
Thanks Bill, I haven't seen or heard from you for a long time. Hope you are doing well and still doing those hi mileage rides on these old bikes. I am just a weekend short hopper when the temp is above 65. Got the sailing bug and now have more sailboats than bikes. I am gonna make a ride somewhere this spring. Good hearing from you again.

Mr Mike
1 member likes this
#863723 Nov 21st a 02:00 PM
by Mr Mike
Mr Mike
The company is Lee Company, 82 Pequot Park Rd, Westbrook CT 06498 (800 399-6281). The part number for a 50 psi relief valve is CCPI8010350S. Remember the key to this project is the elimination of the BSA OPRV and the leakage which causes the loss in pressure. The pumps, later ones or the early alloy pumps make enough volume and hence pressure but the pressure is lost to leakage leaving the bearings short changed for oil.

BTW this company uses BARs as a unit measure of pressure so you will have to have a conversion factor or table available to convert to PSI.

Mr Mike
1 member likes this
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