Oil pressure release/relief valve

[Oggers]

I am uncertain of just how and when my Pressure release/relief (PRV) valve should operate and seek enlightenment!

The PRV on my old nail of a 55 G9 Matchless twin is located within the timing case and is essentially a plunger with a captive spring ostensibly pressing against the timing case cover and pushing the plunger mating face against the seat. This seat is at one end of a filter housing - the other end of which is a ball valve with a weak sping, which ostensibly allows pressure to build, releasing said ball valve and allowing oil to flow through the engine at pressure.  This ball valve seems to be in order, and I also cleaned out the PRV tunnel when the timing case was apart last.

However, you may recall I am having problems with the dynamo seal - very close by the PRV. Essentially I am pretty sure it is leaking oil despite a new cork seal and plenty of silicone. With the dynamo removed and on starting the bike, it is very clear the PRV is relieving and spraying masses of oil around the vicinity of the dymano seal, probably overloading it and causing the leak seen. I am not certain of how the PRV behaves when up to running temperatire

Oil is Castrol straight 50

Questions

Is the PRV supposed to relieve at start like this? I appreciate the oil is thick, but surely the weaker ball valve spring would be first to go to allow oil to circulate?

 Is the oil truly circulating through the engine at start, the PRV just relieving excess oil? Would there be any merit in extending the PRV spring to possibly increase the relief pressure/the point at which it relieves- thereby reducing the amounts of oil splurging into the timing case? 

Given the above observations, can I expect the PRV to close when up to running temperature. I guess I am in trouble if it does not! No obvious sign that it doesn't when out on the bike!


 

[iansoady]

This seat is at one end of a filter housing - the other end of which is a ball valve with a weak sping, which ostensibly allows pressure to build, releasing said ball valve and allowing oil to flow through the engine at pressure. 
 

Not quite - the PRV, as its name implies, releases excess pressure. Under "normal" operation it's closed, making the oil flow through the main and big end bearings. It opens if the pressure seen at these bearings exceeds the design value and in fact that oil is generally used to lubricate timing gear etc.

It's fairly normal for the valve to open with cold (especially straight 50) oil. I would actually consider that to be too heavy for most bike engines - my ES2 and other bikes I've had all seemed perfectly happy on 20W/50 which of course is much thinner at start up. Although some will disagree.

I'm slightly alarmed by the "plenty of silicone" remark - this stuff has a horrible habit of going where you don't want it to.

It wouldn't be a good idea to just increase the spring value. Are new ones available from the Owners Club / Steve Surbey?

[Oggers]

Ian

This seat is at one end of a filter housing - the other end of which is a ball valve with a weak sping, which ostensibly allows pressure to build, releasing said ball valve and allowing oil to flow through the engine at pressure

This is my set up though - and how the manual says it works. I understand that a PRV relieves on excess pressure - that is what it is there for - and at start up, I would expect oil to flow through the path of least resistance - which is surely at the other end of the the filter housing where the weaker ball valve resides.

It may be prudent to change out the PRV as you suggest though.....

[john.k]

Even AMC themselves admitted the early twins had excessive oil pressure when cold......... F.W.Neill says it can force the crankcase halves apart at the filter............So I would say if you are happy the let the oil warm up ,a considerable reduction could be effected,say a relief set at 50 lbs ,or even 45.................Its funny to think that the two AMC bikes ,were near opposites ..........Norton twins had zero filters  ,or even gauzes,whereas Matchless/AJS twins had filters everywhere......that need to be serviced regularly.

[Oggers]

Even AMC themselves admitted the early twins had excessive oil pressure when cold......... F.W.Neill says it can force the crankcase halves apart at the filter............So I would say if you are happy the let the oil warm up ,a considerable reduction could be effected,say a relief set at 50 lbs ,or even 45.................Its funny to think that the two AMC bikes ,were near opposites ..........Norton twins had zero filters  ,or even gauzes,whereas Matchless/AJS twins had filters everywhere......that need to be serviced regularly

.

Well. I think it that depends where you think the leak is coming from. I still believe it is from the dynamo interrface rather than the crankcase or filter joint. Looking at the PRV piston, I am further inclined towards this view. There are two small holes which would force the oil out in 2 jets of oil when it relieves. If one of those jets is directed to the dynamo joint I can easily see how it would leak at that area. So - my solution, block off one hole, and re-insert the valve with the one free hole pointing downwards, away from the interface.

On relief, PRV is pushed outwards, oil forced through PRV piston bore/holes into timing case as before. Only difference is pressure will not drop as quickly as previously - but then so what?

[john.k]

The other side of the AMC twin story.........Norton twin cranks are often standard ,and with little wear.....Matchless/AJS twins its rare to find a reusable crank............but I believe the reason was blocked filters neglected by careless owners............some blame the cast nodular iron cranks ,but these cast cranks work well in cars..