The oil pressure relase valve, the most basic question !

[RichardL]

Hmmm, are those crickets I hear? In that our discussion of heat transfer in alloy heads seems to have induced a general state of somnolence, I thought I would try another topic.

As may be recalled by some of the group at large, my engine is at the machine shop for crank grinding, cylinder boring, skimming, etc. All this started because of oil loss to the left rod journal. The machinist advised me that the spring in the stock pressure relief valve is not strong enough to ensure the 40 PSI of oil pressure that is needed. Knowing that my engine failure was the result of oil starvation, I decided to take his advice and go with the SRM pressure release valve that uses a plunger and is tested at 40 PSI. I don't know that my pressure release valve was the problem, in fact, I somewhat doubt it. However, if I was wrong, I figured that the cost of the valve is very cheap insurance against another failure. It wasn't an easy choice, being that it meant forgoing an original part and putting out more cash. Also, I am sure most of you use the original just fine. I just had to rule it out as problem.

As to what I think the oil problem actually was, well, that's embarrasing. Y' see, I was definitely learning as I went and the sludge trap was a mystery to me. When I did the first rebuild, I did not clean out the sludge trap (not knowing what or where it was).  I don't know what others might think, but I think that there is a remote possibility that after sitting for 25 years, anything living in the sludge trap may have gotten a bit crusty and, thus, I think it was a crusty bit that fouled my oilways. 

My parts will return from the machinist in a about week and I think I might share the unveiling.

Richard

[bsa-bill]

Thanks to those who confirmed the plunger type PR valve works in an A7/A10.
Yep the sludge trap cleaning is a must, remember also that modern oils ( and I use them ) do contain some detergent, detergent is there to hold carbon from your engine ( a natural result of combustion ) suspended in the oil, it may also start to clean your sludge trap for you by slackening the crud collected in there - not good - best to make sure the trap is clean then use slightly detergent oil and change it often, doesn't have to be expensive because cheaper oils will work just fine in our engines.

All the best - Bill

[a10 gf]

Richard wrote

anything living in the sludge trap may have gotten a bit crusty

I second that, early in my ownership, by chance I read something about the mysterious sludge trap (a sludge... some strange furred animal that lives inside BSA engines? And needing a special trap to catch ?). Had a look in there, it was 50% blocked, and some of the stuff along the wall of the trap was hard as cement, no doubt pieces of it could break loose and obstruct everything. Cleaned it out together with all internals of the engine, and fitted the plunger valve. With no sludge or other remnants left in the engine it's safe to use normal detergent oils.

------
btw, here is what I found in the trap 
http://www.cdhm.org/members/pixiwillow/cheshire.jpg

[RichardL]

Yes, sludge is, indeed, evil looking.

I think I mentioned once before that adequate description of the sludge trap, and how to clean it, is a big shortcoming of the Haynes book, ipso facto, early engine rebuild.

Richard

[Brian]

Nasty looking little bugger !!!! Just a bit of probably useless info but C @ D Autos have the plugs that use a Allen key to install, a lot better than the slotted standard type.               Brian.

[fido]

All this talk of sludge traps is scary stuff  I've never taken my engine apart on the rigid so I don't know what horrors lurk inside the crank 

[a10 gf]

It's very good the trap and pressure valve gets all this attention.

Agree with Richard, the Haynes book has a serious shortcoming in this field.

Fido, I did lots of miles with the petrified remnants and 1\2 blocked trap, when I finally opened up the engine, there was no sign of seizing, and the engine was running decently, and any problem I found seemed like normal and expected wear. So it ran fine with what must have been less than optimal oil pressure, but sooner or later, the possible combination of worn timing side bearing, partially blocked trap, oil pressure valve malfunctionning, some stuff getting loose in the trap, worn oilpump. All things to check when one have the engine apart, an interesting job and lots of peace of mind in the process.

The sludgetrap seemed to have never been opened for 40 years, am glad I went in there, I remember having to carefully chop stuff loose with a small hammer and screwdriver and other tools to get it all out. The plugs where next to impossible to get out without destroying them. Getting the hex plugs mentionned by Brian must be a good investment.

btw the sludge trap is refered to here http://www.a7a10.net/BSA/crankcase.htm

With an engine with cleaned internals then then using modern oils, I'd believe the period of use before having to check the trap is increased dramatically.

[LJ.]

I have been thinking/worrying about my pressure relief valve... and the most 'basic question' really has to be... Why is it needed? Surely a good high pressure is of no harm! better than no pressure at all... So why is it there?

 

[a10 gf]

This is the way I figured it out: One could see it as an oil "router" valve as well as a pressure controller. It's job is to give oilpressure priority to the timingside plain bearing which then feeds the crankshaft, and ensure they get whatever oil they can take. The leftover oil is sent trough the valve to less oil-critical parts of of the engine.

As the timingside or bigend bearings wears or any other cause for oilpressure loss at the crank occurs, the valve (due to the spring) will ensure that the oil pressure (=oil flow) to the crankshaft keeps up and compensate for the loss.

So, a blocked valve would result in oil at max oilpump pressure beeing fed to the crankshaft, at the cost of oilflow to the camshaft etc. A full open valve (stuck, or parts missing, or too weak spring) would diminish oilflow to the crankshaft and channel more to the other engine parts.

I'd believe a better and more complete denomination for it would be "crankshaft oil pressure control valve". Or look at it this way, after the crankshaft\timingside bearing has been fed whatever oil it may want, the remnant oilpressure is 'released' and routed to other engine parts.

Hope this helps, corrections and improvements always welcome.