Here is Mr Mike's writeup on the changes to
the BSA A65 & A50 oil pressure control system.
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
(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.