OPRV expiry date

[DAVE BRADY]

Hi,

When did you last check the condition of you OPRV?  Have you ever replaced it or even just the spring?
Over time, the spring must loose some of its 'springiness' so that the pressure at which the valve opens becomes less and less.
So unless you are going to measure the resistance of the spring what is the rule of thumb for checking it or what would be good mileage to renew as a matter of course.
I took one out of a high mileage A10 and found that I could squash the spring in quite easily with finger pressure.  Too easy I think so new A65 type is sitting on the bench waiting to be fitted.

Dave.

[Dean Southall]

I guess the OPRV spring has an easier life than valve springs so as a rule I'd suggest changing it when you change the valve springs ( twice a century?)

[LightningAndStarfire]

Best way IMHO is to have an oil pressure gauge. Unfortunately only easily accomplished on the late models from 1969 onwards. However, there is a service sheet from Hap Alzina describing how to modify an OPRV enabling to study the function during startup.

[Allan G]

Knowing what the oil pressure is only tells one side of half a story,

Your TS bush could be nipping up and the bike would show fantastic oil pressure.

Until the engine is a full working temperature, the oil pressure gauge cannot be trusted, ESPECIALLY if your using a SAE 40 or 50 in the motor. The gauge would show fantastic pressure but thats resistance against the pump and the thickness of the oil struggling to pass through the OPRV cavity.

What it does tell you is that the pump is working and it has the ability to hold pressure.


Dave, I think a good rule of thumb would be 10000 miles, a new OPRV isn't expensive.
In 1970, the factory were supposed to fit OPRV units with a 75PSI blow off, however they didn't and fitted them with 50PSI units instead, saving the 75 PSI units for A70's and Rocket 3's

There is an article in the Library called Detail Improvements 1970 Season, showing this

[DAVE BRADY]

Hi,

I would have thought a bit more than 10,000 miles but definitely something to put on the service sheet even if only the spring is replaced - this is the cheapest way of restoring it.  If a well used one opens at, say, 30 psi what could the result be?  Once the engine was warm it will hardly be achieving this pressure anyway so this would probably be fine unless being ridden faster.
Using the theory of 10psi per 1000rpm, working the engine hard and over 3000 rpm could result in not enough oil pressure from the pump but at these revs the affect of the centrifugal force of the rotating crank should ensure that there is sufficient flow though the crank and the oil wedge between the bearing surfaces of the big ends would be maintained.

I agree re the oil pressure gauge.  To some degree it is the same as the oil light but showing max. starting pressure rather than min. running pressure.  Also, I suppose that it could show something catastrophic has just happened if the pressure suddenly goes up - rotated TS bush or blocked/seized big ends or an oil pipe has failed.
I assume that the reason to have a pressure release valve in the first place is to prevent too much pressure in the engine.  It is, I suppose, possible for too much pressure to distort the soft bearing surface of the big end shells so shortening their life and leading to premature failure.
With this idea in mind why would A65s, A70s and R3s need more pressure in 1970 than 1969?  Possibly higher compression but again once warm the pressure would drop anyway.

Dave.

[Allan G]

I think if the pressure wasn't able to blow off then it could also apply back pressure to the pump and cause it damage??? Hydraulic lock??

10,000 isn't a high mileage, however if you say that the expected life of an engine with a bush is 20,000 then servicing or replacing the oprv at half that interval might help increase service life of the engine. Least that's where I was going with it.

If its the piston type it would certainly be worth while inspecting its condition in that time.... IF its a stainless one I would inspect it each year, I've seen some bad wear on those which would stop me ever using one again.

"why would A65s, A70s and R3s need more pressure in 1970 than 1969?", good question. Its what was in the article, It is library watermarked which is why I cannot share it. However as we know the factory fitted a piston valve but still at 50PSI, the A70 and A75 got 75 PSI. The way the A75 feeds its oil is totally different to the A65, this might have a bearing on why it needs more, with the A70 it was likely to try and improve oil pressure and engine reliability during racing - as you say pressure will drop as oil gets hot.. it would have been really "thin" at the thrashing the flat track racers would have been giving it.

That Said I fitted the 75PSI unit to my 823 as I wanted to guarantee as much as possible that the bottom end on a bored and stroked motor would last whilst still using the timing side bush.

[Mike Farmer]

 

:)Honestly, never even thought about it

Mike

[DAVE BRADY]

Hi,

The 1970 manual mentions the tendency for the spring to weaken after prolonged use, what ever that is, and replacement of the spring.  So a serviceable item it would seem.

The 1971 manual mentions a 50 psi blow off but refers to the oprv as "a self contained unit and cannot be dismantled".  So no servicing possible but obviously it could still be replaced but no indication as to mileage.

Dave. 

[Roy]

I have a test rig for pumps and p r v ,I have tested newly made stainless unit and found it leaking at 20 lbs ,
After investigating it turned out the threads were under size oil was paint by them before valve open ,
Tested a 60 odd year old original and that was 50 lbs , the pumps on rig are visible and when pressure build up I can see oil bleeding from the joints , where as pressure is good we also need flow ,and some journal when ground loos the little flats where oil comes out and reduces flow

[DAVE BRADY]

Hi,

Interesting re the flats on the journals.  I remember reading that it a slight countersink is beneficial for the same reason.
SRM test their oprvs and they tend to be a bit tight to compensate for worn or slightly over size threads in the cases.  I had one case that was tight and SRM lent me the correct tap to sort it out whereas another case it was just fine.

Dave.

[Roy]

I have a test rig for pumps and p r v ,I have tested newly made stainless unit and found it leaking at 20 lbs ,
After investigating it turned out the threads were under size oil was passing by them before valve opens,
Tested a 60 odd year old original and that was 50 lbs , the pumps on rig are visible and when pressure build up I can see oil bleeding from the joints of pump , where as pressure is good we also need flow ,and some journal when ground loos the little flats where oil comes out and reduces flow

[DAVE BRADY]

Hi,

Thanks for the clarification.  Can I ask how recently you test a stainless one and found it lacking?

Dave.

[Roy]

Last stainless valve tested ,was approx 3 years ago .