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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

⦁ 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)

⦁ 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.

⦁ 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

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.

Many thanks for posting this as it's something I'm interested in doing to my 72 A65L when I strip the motor out later in the year.

I'm curious as to why this brings about the improvement it does on the OE valve, especially so as the later piston valve was considered an improvement over the earlier type. I read one of Mr Mike's posts about this and I think his explanation was down to the hysteresis of the valve, I had to look that up and from what I read the hysteresis of a hydraulic valve is the difference between the initial pressure needed to open it and the pressure for it to close once already opened. Assuming that my understanding is correct, is it the rerouting of the excess pressure that is the fix or is it changing to a different valve?

John,
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

There are two problems with the stock OPRV, leakage past the piston (or ball) and leakage past the threads. My opinion is the ball was better because it is easier to get a seal against a ball seat than a piston in a bore.
There are only three threads between the end of the OPRV and the cross drilling into the sump (early) or return drilling (late). If they had put a short flat nose on the OPRV and matching step in the bottom of the OPRV cavity, an O-ring could have been used to seal the threads.
Whenever the drain port opens there is a drop in pressure. If the pump is not supplying enough pressure to keep the port open the piston will move back to close the port. This is more of a problem with the Triumph plunger pump where there are large variations in oil pressure with each pump and refill of the chamber.

I've been doing a similar thing ona65's for a long time. (20+ years.)
Blank the return hole as per Mike's explanation..
use an old type ball oprv.
drill and tap the end for a 1/8 bsp fitting.
run the pipe to the rocker feed not back to the tank
the spring can be changed to suit.
open the rocker feed fitting oil hole out a bit and if you want drill a small hole in the exhaust
rocker shaft centre boss. (allows a bit more flow).

Bob's your mothers brother.

That's similar to what Ferrari V12's from the fifties and sixties did. The nose of the OPRV carried an o-ring that sealed in the bottom of its port.

Years ago I developed a stepped washer with a conical back face to seat in the BSA OP relief port. It provides a ledge where a carefully sized o-ring can seat so the end of the OPRV seals against it when fully tightened. Having said that, it is fussy to do because those ports are not all the same depth and the taper seems to vary as well so the seating washer needs to be custom sized for the particular bike. Sealing washer thickness is critical too.

If the engine is in good condition, OP will improve greatly. A '67 Hornet I rebuild some years back could only manage 15 LBS hot OP. After fitting the 0-ring mod, hot OP went up to 50psi.

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

You are doing this to correct a design flaw or because of excessive bearing clearance ?

A bit of a design flaw and a stack up of manufacturing tolerances. In particular, the location of the OPRV drain in relation to the end of the threads on the OPRV. It can vary a fair amount.

BTW - I have seen a set of cases with less than 2 full threads between the drain hole and the end of threads.

I have found eliminating the sealing washer and using a thin section O-ring on the OPRV helps seat the OPRV deeper into the cases. I also seal the threads with Permatex Hy Tack.

HillBilly Bike,
When I rebuilt my A65 I was very meticulous to get the bearing clearances right at the low end of the specified acceptable range. Even with these clearances, the oil pressure was on the low end of what I like to see on a plain bearing motor. That's what prompted me to experiment. Even BSA acknowledged low oil pressure by changing to a piston type OPRV and an improved cast iron pump. The pump produces enough volume however their OPRV design left much to be improved.
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.
Cheers,
Mr Mike

With an end fed motor it's best to also run a small pipe teed back to the return as above about 2000rpm
the valve is dumping huge amounts. (well, if the engine is any good anyway....)
With my standard engines i have a small tee in the return line and a tee in the head feed, that way
the head can be supplied either way depending on the pressure from the relief valve and the relief does
not get backed up by the head restriction.

Started messing about with this mod 20 odd years ago when racing these old crates, it does work very well.

Does anyone know the THREAD FORM for the OPRV cavity. In my sketch above I measured the nominal diameter and the pitch but did not know the form of the threads. I checked BSF, BSW, UNC, UNF and none of them match the nominal diameter and pitch I measured (7/8 x 16). My machinist (brother) used the OPRV I supplied to measure and cut the threads on the plug. They were perfect.

Mr Mike

7/8"-16 60 degree thread form is what I cut mine to. If the case were 55 degree they would be wobbly when started.

David,
Thanks, just what I needed.
Mr Mike

Just use the outer shell of an old valve and tap the inner whatever size you want.
Why mess about cutting funny threads. Beezers used a lot of Strange unified threads i think.

NickL,

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 not...it'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

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.

Nate,
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

Thanks Mike, I’m actually doing this today since I have the rear fender off for a minor repair. The return line is a little swollen and cheese graded a little from the clamps. Thanks for all the help.

Nate.

Nate,
You could remove that hi pressure return line and place the little OPRV in the other end right next to the tee coming out of the OPRV cavity. It would have to be securely clamped in place but that would make the downstream part of the hose low pressure. Just a thought. Just make sure you use hose rated for high pressure. Also check the line going to the gauge. When I was first started working on this change I had the gauge threaded into the tee at the OPRV and had to look down by my foot to see the pressure. I am convinced that this little change will greatly extend the life of these bikes that universally suffer from lower than desirable oil pressure.

Mr Mike

perhaps I missed something completely

but where do I buy one of these small OPRV's

Hey mike,

I think I’m going to keep your oprv in the position it’s in now. After looking in that area around the “T” it’s really tight and with the exhaust being right there I don’t want to chance anything...I like this bike and don’t want to “Jinx” myself as one would say.

I do have High pressure hose as a replacement. Thanks for the idea and if I figure out a way I will post.

Take care.

Nate.

lemans,
The are available from Lee Company in Westbrook, Ct. I will see if I can get you a part number from the shop tomorrow. When I was working on this project a salesmant sent me a couple samples for free, so they are not very expensive.

Mr Mike