How to seal loose pressure release valve threads?

[mikeb]

The threads on my A10’s pressure release valve are very loose – both the upper and lower PRV threads and the female case threads. I’ve measured the existing PRV lower thread OD and its about 18 thou smaller than the new SRM PRV, and the SRM PRV is even a little smaller than an A65 PRV that was nearby. The lower female thread in the cases looks a bit dull and worn. When slightly loosened, the SRM and A65 PRVs both ‘move’ a little in the case thread. The existing A10 PRV rattles around.

I know this can be a source of oil pressure leakage and understand the different expansion rates to the ally cases and steel (or SS) PRV can open this up further, hence dropping pressure when hot. So, when I assembled the engine, I put a silicone sealer around the lower female thread in an attempt to seal it. But I’ve ended up thinking its why my engine appears to wrecked itself from low oil pressure after just 6000 miles. It’s the only thing I can find that likely caused oiling problems.

So how best to remedy this? I’m eyeing up some loctite hydraulic pipe sealers (569 or 542?) but I’d like to hear how others have solved this. All suggestions welcome.

Thanks

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Background note - the engine was fully rebuilt 6000 miles ago by me (low skills) and with someone with high skills looking over my shoulder and doing the clever bits. Sludge trap and everything was cleaned. It got an SRM oil pump, a careful run-in and had oil changes at 50, 500 and each 1000 miles with an oil filter installed on the return after 500 miles, and changed each oil change). I pulled down what I thought was a working engine (for unrelated reasons) to find a partial seize, scoured timing side bush and badly gouged shells. 

[KiwiGF]

I think it’s generally accepted the PRV only operates on most engines to stop over pressure when the oil is thick and cold?

[Swarfcut]

 PRV is in two parts, lower part fits the crankcase, upper part carries the ball and spring. There is a sealing washer between the two components. A sealing washes also goes between the PRV and the crankcase, and in each case the threads do not act as a seal.

 PRV's are usually a tight fit in the crankcase. Always use a well fitting socket and also  a bit of heat for stubborn ones. Hammer and chisel hooligan's way will result in damage, guaranteed.

  Yes, PRV only opens when engine oil is thick and cold. In normal running I doubt hot oil pressure moves it open at all.
 
 Bearing wear and damage to the timing bush more likely from low oil pressure consequential to a PRV stuck open, weak/broken spring, ball and seat probs., so check the inner workings for a start.

 Swarfy.

[chaterlea25]

Hi All,
I have to disagree with swarfy's analysis!!
There are only two or three threads between the high pressure side of the prv and the leak off groove,  so almost all A series leak past those threads,
The usual problem with SRM's prv is that they are too tight in the threads needing the use of their special tap to clean up the case threads.. I would ask SRM to search through their stock and get the biggest one they have.... or get them to make a custom one?
The leakage has been the subject of many topics on "Britbike forum" usually involving Triumphs... I think Dave Madigan has some modified   prv's??

A few weeks ago I made up a test rig for the prv's using a variable air supply,   the leakage past the threads was significant before the prv opened at 60 psi.

Mike,  are the cams and followers worn on your engine? These are usually the first items that suffer on low oil pressure,
I would suspect dirt being the problem if they are OK

John

[Swarfcut]

Mike Yes, the thread in the crankcase to the high pressure side of the PRV needs to be oiltight, otherwise pressure will leak down those threads to the groove connecting to the blow off vent. This is why a close  fit  with the thread in the crankcase is  desirable. The design here relies on the minimum of oil penetration down the threads, and a normal thread could not be considered to be hermetically sealed without some additional type of sealant, none is mentioned in the service sheets, only the sealing washer to the crankcase.  Locking compound  on those first few threads here is a better choice than silicone.

 So once more CJ's analysis delves a little deeper into the problem. It's not such a simple answer as I first thought, so apologies to all. I think you have to be unlucky to have problems with the crankcase threads as normally they are not disturbed very often in comparison to others. In the grand scheme of things there must be many bikes running with leakage down these threads with no apparent ill effects and it is all down to how marginal the particular parts are. Reasonable...OK. Badly fitting...disaster.

 Swarfy.

[sean]

have you tried teflon tape on the threads , we used to count back 3 threads on tapered pipe fitting and start the tape so no tape would end up in the system as far as loctite  I use the loctite 565 paste in a tube .
just use something like brake cleaner to remove any oil first ....good luck .

[chaterlea25]

Hi All
I would be reluctant to apply any sort of sealer or PTFE to the threads The likelihood of some of it getting into the works of the PRV or into the engine oil feed is too risky in my opinion

John

[mikeb]

Thanks for all the comments.
John – (re your first reply) yes I’m talking about the leakage between the lower PRV thread and the cases. Your air test is interesting (and should concern most of us). I have a new SRM PRV in hand and being 12,000 miles away don’t’ rate my chances of getting them to find the largest dia on the shelf (and assume SRM would say they are all machined to exacting tolerances…).

The case threads are poor. Perhaps from a PO's maintenance? I recently learned that it says in the owner’s manual to

‘remove the [PR] valve periodically and thoroughly clean the ball and its seat’ (p12)

I can’t think of another likely source of this wear. As these bikes age perhaps more of us will face this problem?

re dirt vs oiling. The cam followers were a bit marked, but unfortunately I’ve had them linished already (when this job was just about the top end). But they weren’t bad. Cams look ok – I have been running a high zinc oil (Penrite HPR30). The oil (due for a change) was quite dirty and there was about ½ teaspoon of sludge in the sludge trap. But with my routine oil changes, I can’t see how the problem is due to dirty oil except maybe blow-by on the seize and the generally poor JP rings. I’ve had a number of clever folk look at the shells and the current theory is probably oiling. Either way I need to secure that thread. Not sure what else to do.

John – (re your second reply). I share your hesitation tho some people claim it achievable. If not a sealer (applied to the bottom female case thread) then I’m getting short on options.

Sean – I can’t use thread tape as its fitting to the bottom of the two thread sections, hence a sealant? I haven’t used Loctite 565 before tho understand it’s the lower temp (150 degrees C)/faster curing version of 567 (204 degrees C/72 hour cure/low strength) that the Henkel guy recommended when I spoke this morning. He said these (565, 567, 577) were better as ‘gunkier’ for course threads compared to the hydraulic pipe sealers that are for fine threads. (tho 542 likely OK). AND use a primer (7649) due to the surfaces.

I’m also going to drill a gallery (to just in behind the PRV) for a gauge/pressure switch so I can keep an eye on this. If I observe pressure problems (yes I know they may be little pressure at hot idle) then maybe a custom PRV, or some other solution.

All suggestions and comment welcome
Thanks

[KiwiGF]

pics would help understand the issue!
Does oil that passes through the prv, including the thread, then go to the cam/followers? If so a leaky thread might actually help with follower wear?

Largest dia thread of the three is 0.867”

[KiwiGF]

Sorry I will answer my own question and yes the oil that passes through the prv either via a loose thread or over pressure comes out of a hole in the camshaft trough

I put an air line on case to test this to ensure my memory was correct

Its s tortious route from the prv to the trough and I wonder if blocking it off would be beneficial if the prv thread is poor? If, as is thought by many, the prv only releases pressure at start up when the oil is cold and thick, and one is using multi grade, maybe it does not serve a useful purpose?

[berger]

you could do what i did and put your mind at rest, butcher an old prv and put a pressure gauge on it. then test the bike with engine running and you get a reading of pressure that is being controled by the prv and leaking threads. then put a suitable O ring as in my case a square section one over the holes on the prv which blocks off the flow to the camshaft and see what readings you get, excuse the picture i must start to master my other phone. i have now got a newish computer thing so i am half way there

[Triton Thrasher]

I wonder if the threads could be copper plated to a beneficial thickness.

[mikeb]

an interesting thought TT. The SRM one is stainless and I've read its possible to copper plate stainless tho never done so. I'll look into that.

for the record, I just talked to an SRM workshop guy and he said either get new cases  or see if I can fit an o-ring below the prv somehow. He hadn't done it tho had heard of it being done. it would need retaining. he was against any kind of sealant incase some dislodges. he said in production, BSA sometimes left the tap in too long (?I don't understand that - its not tapered?) and this could result in oversized case threads, with a gap of upto 0.5mm between the PRV. hence SRM's PRV is often a bit too tight (if the case threads are tapped to spec) and may need extra fitting. He said all of theirs are the same exact size - no variance to select from.

I'm learning this is an unusual problem so I'll have to give it some thought and see if i can measure the gap between the threads and maybe test it with some air. another option is do nothing and assume the 18thou wider SRM PRV thread will be sufficient. there must be a lot of A10s and A7 riding around with this situation.

thanks for the suggestions (and for any more that you may have)

[Swarfcut]

   Just had the delivery of a job lot, including a marginal PRV with a chipped face at the gauze end.

 This got me thinking as a taper seal to the crankcase would solve this thread leak syndrome. Measuring the threads on a crankcase to hand, the depth is around 20mm, the PRV measures around 16mm, so this is not how it works, but the gap gives scope for a support bush for an O ring, which would then seal against the face of the PRV and be pushed into the start of the  crankcase threads.

 Waddya reckon? Sensible solution or pipe dream unnecessary effort for marginal gain?

 I'd add that in the light of experience the valve body is best left in place, unless you REALLY want to check the gauze screen. Ripped threads always a risk on these old castings.

 Swarfy.

[orabanda]

I  machine a sealing washer out of teflon, which is 0.010" taller (longer) than the space between the bottom of the RV, and the bottom of the cavity.
Richard