oil pressure relief

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Dec 30, 2003
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How do you check that this is working properly? Recently I had the exhaust valve guides replaced at Raber's. Bob told me the rocker spindles had some wear looking like not enough oil pressure to the top end and to check the oil pressure relief. I'm going to have to use an air impact to get the valve off the timing cover which is fine since I'm going to tackle the wet sumping -again - this winter.

The crankshaft seal on the drive side keeps allowing oil into the primary as well and I have to keep removing the oil level screw to let out the excess - otherwise the primary drips on the floor after a ride. I've replaced the seal three time and it doesn't seem to help.
Hi David,

Have you considered fitting an oil pressure gauge? My gauge showed the pressure soaring to 90 psi + on startup, then coming down after the valve finally opened. When I took the valve apart I found out why - the relief valve piston was heavily scored from all the metal debris that had gone through the engine. Apparently it was sticking. That was my first clue to the damage in my motor...

seal replacement

Hi Illf8ed,
Regarding your leaking left crank seal, there was a fellow that posted back in february claiming the hot set up was to install a 30mm double lipped seal. Unfortunately some maching required, but, you have replaced it three times and winters coming.
He also had a slick looking anti sump valve he sells on his website
www.britishclassicbikes.de (yes the thought of a one-way valve in the oil line makes me wince too).
justa thought,
Hi all,
When tearing my bike apart I found the valve was seized solid - In a posting I advised anyone who hasn't checked, should do so - The manual talks about shimming to adjust the pressure - but what is the reading??

For testing I was going to rig the valve up to my air compressor and slowly increase the pressure until it releases... but what is the correct reading?!

If anyone knows the pressure reading- post away! I think we could sleep better! :wink:

Here's how I check it:

I actually do it with the engine running while still cold. I have made an adapter that replaces one of the rocker feed banjo bolts that allows the connection of a gauge on a short piece of nylon tubing.

With the gauge connected and the engine running, rev the motor until you achieve a peak pressure reading (usually 3500 rpm is sufficient). This pressure should be 60-70 psi. At idle, you should see about 35-40 psi. Add or remove shims until you see this pressure range. Once warmed up, you should see a peak oil pressure of 35-45 psi at 3500 rpm and 15-20 psi at idle. I am running Amsoil 20w50 synthetic oil.

Of course, this assumes that you engine is healthy. Shimming the relief valve to get oil pressure is not a good way to compensate for a worn out engine.

To achieve a good starting point, a thorough inspecton of the valve should be done. Make sure that the bore and piston are free of galling and burrs. The passage holes in the piston sould also be free of burrs. The piston should slide in the bore freely. On initial assembly, add enough shims to take up the free play in the spring. If you put it together and you can hear the spring rattling around inside when you shake it, add more shims.



I get the impression that the crankshaft seal is “blowing” despite your regime of draining the sump prior to starting the engine. If so, there’re some mysterious gremlins at work here.

What about the crankshaft seal diameter - is it “grooved” from rotating against the lip seal? If so, perhaps it’s now undersized and no longer capable of affecting a seal. Or, is there a nick or gouge on the OD of the shaft that is damaging the lip seal?

The anti-drain valve is tempting, but it's sure scary. Has anyone had any experience with the valve posted valve?

oil pressure relief valve


I might spring for an oil guage. Your comment in an earlier string got me thinking more about my relief valve. Eight years ago when I bought this combat it had not run in 20 years due to a left piston colliding with the exhaust valve. There was enough metal fragments to have taken out the oil pump which was replaced. I didn't consider looking into the oil pressure relief, though and have never taken it apart. It's going to take my air impact to remove it which means the timing cover has to come off first...this winter. Do you have a recommendation on the oil pressure guage/


Yes, I drain the excess crankcase oil off before riding. I'll look again, but I don't believe there is any worn portion on the drive side of the crankshaft that would allow blow by. Appears to be the gremlins.

I bought the OldBritts gauge kit mainly because I really like the way he did the mounting. It's a bit pricey but very nice workmanship.

Is the seal actually blowing out of its bore? If so, a couple of remedies I've heard about are staking the seal in place :shock: or using some sort of epoxy (maybe araldite?) over the outside. That had been done on my engine. I like that idea better than pounding on the case with a punch - ugh.

Might sound like a dumb question..but what type of oil are you using? The only time I had oil end up in the primary case from the main sump...was when I used Mobil one. It was so thin that the seal allowed the excessive oil which leaked through the oil pump and into the sump, to simply flow on through to the primary case, when the sump filled up to the level of the seal, or upon startup with a full sump....which I admit to having done for a good thirty years now. If so much oil is passing the pump and getting to the level of the seal on the end of the crank, it will make little difference if you drain the oil before each ride, the amount of oil which has already passed the seal and entered the primary case will gradualy get to the point where you will notice problems with the clutch and co. If the level of oil in the sump is kept below the level of the seal by fixing th oil pump.... can't much get by the seal and fill the primary case. The primary case is...or should be, a fairly well sealed unit, and what is sealed, can't give much of a route for blowby/oil...or? The pressure might be increased in there, due to blowby, but just so far..after that, the pressure itself will counteract oil getting in there, I'd think, especially if you go ahead and fix the reason for the "oil in sump"problem.
Why do you need a air wrench to get at the relief valve? A 28mm box will take it off the timing cover..or use a smaller wrench to just remove the cap and use a thin magnet to pull the piston out. Note though, that if you pull that stuff off with a full oil tank, it won't be full for long...it will run out pretty fast....I know because I had about a quart in a puddle under the bike this evening :lol:
oil pressure relief valve


Thankyou, you gave me an idea about why the crankshaft seal would leak. I'm currently using Valvoline 50wt oil, so that's not it. I do have weak piston rings and probably pressurizing the sump causing the oil to be forced through the seal...anyone want to comment? The engine does have the 850 type breather modification and a PVC inline.


Thanks for the Old Brits recommendation on the oil pressure guage. I didn't really want to mount one of these on the handlebars, but it's probably a good idea.
Straight 50....wouldn't think THAT would leak past anything...including a really worn out oil pump , sure you put the little sealing donut on the oil pump? That stuff shouldn't pass the pump at such a rate as to make it nessesary to drain the sump before each ride...unless you only ride once every six months or so...or? Is straight 50 good for the motor, in that it is so thick that if the temp drops below 40 or so...it will be like honey. Darned hard to kick over and get started, and, does it reach those places up in the head in time to lube them at startup?...Ask the guys about this oil...everyone will have a view, I'm sure.

Wouldn't that stuff stick the PVC closed/block it...? I see you have the 850 breather convertion, which I don't have but what If you don't have the breather/PCV free, at least in the "out" direction....where does all that pressure go? Out the seal to the primary? There will be a lot of sump pressure upon startup, which causes all sorts of leaks at all sorts of places around the engine...no matter which oil you use, but with that PCV stopped up....that would be like a water ballon waiting to hit the pavement...somethings got to go somewhere.

I have the feeling that the conbination of a really thick oil, a PCV and maybe a reason for so much oil ending up in the sump, may be causing all this grief. When I used Mobil one, and it dumped all that oil into the primary, I just replaced the seal and changed back to 20W50 and the problem went away. I don't have a PCV because I am lazy about draining the sump, and I just let it all come up the breather tube into the tank and I don't know how a PCV would handle all this quart or so of oil that pumps up the tube at startup......it that got blocked, and took even a minute too long to get up the tube and clear the tube so it can do it's job of providing a route for the air the pistons displace each time they rise and fall...I would have all sorts of leaks...all over the motor...and fast. Try bringing the engine back to basics...20W50 and no PCV and see what happens.....you can always put the stuff back later if it doesn't seem to make a difference......and check that donut...sure it's there?
something you might want to try as an experiment-or leave it permanently installed as i have-is to make up an adjustable relief valve.i silver soldered[make sure it is absolutely clean of oil or the silver solder wont stick-maybe better to tig weld] a 1/4 inch nut to the ouside centre of the relief valve cap and put a long bolt through it that would bear on the relief valve piston. be sure to put a lock nut on it and teflon pipe sealer and run an oil pressure gauge to monitor what you are doing.pressure will be high when the bike is cold so make sure you have a gauge that has lots of range because the first couple of blips of the throttle can destroy the accuracy of it until you have the valve dialed in. good luck barry
oil pressure relief valve


50wt oil isn't that viscous, you make it sound like grease. My PCV is in the same position as the original aluminum diameter reduction piece inline with the breather which means not that close to the timing case. It is working well. As far as back to engine basics I've been there and it doesn't work. I had just as much trouble with 20w50 oil. I've also run without the PCV, no difference.
Then where are you going to go with this? There has to be some understandable reason for oil getting from the sump into the primary case...or? The only thing that can cause that to happen is either TOO much oil in the sump.......or so much pressure in there that even a new seal wouldn't keep it out...Pressure would be a result of a blocked breather/PCV or bad bad rings. Any amount of extra oil should be either going to the pump to be put back in the tank where it belongs, or going up the breather pipe to the tank. I can't imagine, even with really bad rings and a really bad seal, that if the oil pump was in order, there could end up extra oil, to that extent, the the primary...how could it if there was not a hugh amount of oil laying around waiting to be pushed in there. No matter how you look at it, something basic isn't correct. I have to assume that the pump isn't so bad that it doesn't pump the sump clear and it just fills up to the point of running into the primary case, or? Could be. How much oil are you draning out before you go for a ride? A couple quarts? Or just a couple cups? Quarts would be pointing towards a really bad pump...or a missing donut. If you are sure the oil viscosity and the PCV are not at fault, what else does it leave?

Bite the bullet, pull the pump and make sure it gets put back in good order, change back to 20W50 just for good measure, replace the seal on the crank and schedule a ring job this winter, and if the problem continues...I'd be surprised.
oil pressure relief valve

For now I'm going with worn piston rings as the cause. I'll live with having to drain the primary and crankcases until the winter teardown. I'm still going to look at the pressure relief valve in case it is stuck wide open and delivering too much pressure.

If the rings were THAT bad...it wouldn't run. If my logic is incorrect....oh well...but I stick with the conclusion that if the pump was not letting a LOT of oil into the sump...your problem would be history...unless,.....sure the oil in the primary is motor oil? Trans isn't making strange noises...or?
oil pressure relief valve


After a week of sitting I usually get around 250ml drained from the crankcase before starting up the engine. After the engine rebuild 8 years and 15000 miles ago it didn't wet sump at all. Since I'm draining the crankcase there shouldn't be any extra pressure on the crankshaft seal due to wet sumping. After running the engine between 4000-5000rpms on back roads it drips oil from the primary coming from the felt seal around the gearbox mainshaft. The gearbox is in good condition and I have one of Dave Commeau's clutch seals. The oil I drain off from the primary is engine oil and I check the level before going on a ride.

I'm not certain of any reason for the failure of the crankshaft seal, but I know the piston rings need some attention.
As time goes on...we get more specific info here......so the felt seal is leaking and you have to drain the primary( from the level screw) to get it down to the level it should be...and you're SURE it's motor oil? The place yours leaks...leaks on mine too...with a bit of driving, the oil will get thrown up there from the clutch/chain and go out the felt and drip on the floor...not a worrysome leak, just replace the felt seal (Soak overnight in oil before putting the new seal in).

The place I get lost is the fact you say the primary has to be drained. The amount you drain out of the sump is not so exciting/worrysome , mine must have more than that in it after a week...I just don't bother to drain it. So you don't have enough in the sump to say it just runs through the seal and fills the primary, like mine did with Mobil One. I'm lost then...there is no reason it should be happening, even bad rings can't push oil from the bottom of the sump and up through the seal in such amounts that it is nessesary to drain the primary after a short ride. The leak you can fix..pull the clutch basket,rotor and chain, the rear half of the primary case and replace the felt seal...maybe with a bit a care, the eal can be fitted over the trns shaft without pulling the rear primary case, but I don't think so. If the oil isn't from the tranny...I see really no explanation for the primary filling up. Is the main bearing fitted with some sort of cover that has come loose and acts as a sort of funnel to route oil towards the primary...don't think it has a cover on it, that job is done by the seal you say you have replaced a couple of times, already. No fancy routing of the breather into the primary or? Past owners have made some pretty imaginative alterations to some bikes. Have you tried running it a bit with maybe the timing mark viewing hole open? Mind your fingers, naturally, but if when you goose the throttle, you get a lot of pressure out of it(Blowby), you might be right about the rings...but I still say that if that is true, the pressure should be going up the breather pipe through the pcv and into the oil tank. If the PCV, or tank for that matter is so well sealed that blowby forces can't find a relief there, then blowby might have to find it's way through the seal into the primary and out the bad felt seal that is dripping...but all this is really rather an outlandish chain of events and seems too unlikely to me. The tube from the oil tank into the back of the air cleaner isn't removed, blocked or such? If the tank has no exit for the pressure from the sump...that might cause a lbuildup of pressure in the sump...are all your gaskets leaking too...the valve covers etc? Signs of a lot of crankcase pressure,other than the oil level problem in the primary? Got to be a reason for all this.

All I can suggest at this point is to check for extreme blowby/pressure and figure in that rebuild soon...fall is almost here anyway. I just did my rings...it's not that hard...and the results were very noticable...
On your problem, call me stumped. Maybe someone else will step in and give you the tip you need. Let us all know when you have rebuilt and whether it took care of the problem...ride safe!
Quick thought.
If the seal on the crank is mounted in backwards, it will cause the same problem. Did a rear main seal on a Chevy Corvair years ago because it had a drip and the clutch was getting oil on it. Same problem after I replaced the seal...so I sold it, for ten dollars to a friend. He pulled the motor again, turned the seal around a drove it for 5 years....stupid me.

Another very simple possibility. The three small bolts that hold the rear primary case half to the bottom end on the motor...if one of the two bottom ones is loose, has fallen out, leaks....the oil will pour into the primary during that week between rides.
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