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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
[Linked Image]

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]

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]


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]
(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.

Attached PDF document
A65oilpressuresystem.pdf (1.39 MB, 23 downloads)
Last edited by Morgan aka admin; 04/20/22 4:47 pm.

Morgan Johansson
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A more expensive and long winded way of doing the 30 minute job i described in the same thread.
With the downside of having high pressure lines outside the case.
Great eh.

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My 'O'ring on the original valve body is still working except the ball cannot blow off enough oil when cold and 110psi is a bit high till it gets hot. I'll do a piston one and see what that does.


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End fed motor Mark, you need to pipe the exhaust away.
That tiny relief valve would be the same on an end fed engine,
just not big enough.
You would need to open up the relief gallery with a standard valve.
Or use lighter oil.
Mind you don't turn the seal inside out, a fairly common problem.

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Originally Posted by NickL
A more expensive and long winded way of doing the 30 minute job i described in the same thread.
With the downside of having high pressure lines outside the case.
Great eh.

Nick, It is really harder to describe the change than to do it. I agree that it puts a high pressure line outside the cases, however, an alternative to this is to install the little OPRV in the tee fitting threaded into the case. It is designed for press-in applications.

Just talking out loud.

Mr Mike

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The little oprv is just too small for both cold oil or an end fed engine.
You could put a couple in a block in parallel i suppose but it would be
marginal even then.
It's much easier to just modify an old type relief valve, they work a treat.
I fail to see why you find it so abhorrent, the job is so simple and neat,
a 90 deg 1/8bsp fitting and a drill and tap. You can set/change the spring
easily if you want more or less pressure for some reason, the valve is
internal, the exhaust can be routed wherever you wish etc.
Why make it complicated by having to turn a funny thread? and adding
the problems already noted.

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Originally Posted by NickL
......That tiny relief valve would be the same on an end fed engine,
just not big enough........
Thinking along those lines, it's worth remembering that an end fed A65 engine has one less bearing to feed so the net effect is to increase oil delivery by approximately 50% to the rod bearings. This means the thread leak around the stock OPRV is less of an issue but the relief capacity of an OPRV modification is more of an issue. It would be interesting to get some OP info on end feed modified A65's.

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When i raced one, we ran R30 oil, it would dump a huge quantity from
the oprv. I piped it to the head and had a small hole in the exhaust rocker central
support to allow a little more flow. That way the pressure would only get to around
100psi when cold and run at 70 or so during use.
For the road bikes i've done, the exhaust is piped to the head and teed into the return
as most blokes run 20-50 oil and when cold pressure can be excessive. Even my own
road bike , which has a t/s bush will still get to 100+ when cold without the tee. The a65
pump is better than most people think if it's built properly.

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We all agree that the design system by BSA left something to be desired. We now have three systems that get oil pressure where it ought to be. Nick, with regard to that threaded bushing, one can use the earlier plug and just tap it for 1/8th NPT.

Mr Mike

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BSP is just so much more civilised.


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