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Triumph TR6 - TR6 Oil Filler Cap

Anyone know whether the oil filler cap on a 73 TR6 is vented. Mine has a small hole in the center that goes all the way thru. IF I plug it the engine seems to run better. Was the TR6 a presurized oil lub system. Thanks in advance for all answers.


I would hazzard a guess that there is no such thing as a vented oil filler or old. Well maybe you need to go back to Henry Ford era. Oil filler caps should be a "sealed" fit....that is why we have the pollution controll stuff on cars.

Maybe the DPO thought this would help to oil the bonnett area.

All engines are a "presurized" oil is simply the nature of the beast. You have a pump pumping oil. I am sure the engine gurus can 'splain it better.
Plug the hole or get yourself a new cap.
Rick C
Rick Crawford

Jerry, rather than start a new threatd, I'm going to piggy back on your thread and Rick's comment to revisit an old issue that's been addressed in the archives.

My car runs oil pressure about 75psi on acceleration and 25psi at idle. I think that's normal. Today, while warming up, it ran almost 100psi and 50 psi. 5 minutes down the road it dropped back to normal levels. From what I've read in the archives, the oil pressure relief valve may be at fault. My questions is: would everyone agree with my diagnosis, and under what circumstance would you replace with the "heavy" spring which Moss says "provides some increase in oil pressure".
Mati Holland


It sounds like your pressure relief valve may be sticking. It's a simple matter to remove it to check for wear/gunk/muck that may be causing the problem. Also, when the valves' spring wears, you can have the opposite effect like I had. An old time Triumph mechanic told me to place a few shims behind the spring to see if the pressure came up a bit. It did which told me to replace the spring. While I was at it, I also replaced the valve and crush washer. I've heard of valves sticking open more (resulting in constant low pressure) vs. sticking closed.

I don't know why you'd need the heavier spring. If the engine is old and tired, I wouldn't think a heavier spring on the relief valve would help. I don't think you'd get any added benefit from pressures over 50 lbs. On the contrary, wouldn't this put more work on the pump?

Bottom line - check for wear and replace if necessary. It's a cheap, simple and quick fix.

Don from Jersey
D Hasara

From Mati:

My questions is: would everyone agree with my diagnosis, and under what circumstance would you replace with the "heavy" spring which Moss says "provides some increase in oil pressure".

First for Part 1, yes I agree there is noticible difference in the cold and the warm oil pressure on the TR6 motor. Time for us to preceed to Part B.

Two situations come readily to mind. One is if the engine bearings are worn. The increased bearing clearance would cause oil pressure to be lower. When I was using the TR6 as a daily driver and as an autocrosser, I used lower oil pressure as the cue to pop in some new bearings. I probably put in about four or five sets over the years. Since I refreshed them before there was ever a real problem, the crank in that engine still checks out at "standard" spec.

The other would be in a very cold climate where a thin oil was run. The heavier spring would help keep pressure up even as the oil warmed and thinned from the cold molasses viscosity to something fairly fluid.

There should be no vent in the oil cap. Rivet likely just rusted through? You have a PCV valve thats pulling vaccum. If its sucking air through the hole it will run rougher at idle.


Its been cold overnight. Almost 0C or 32F lately. Mattis not far away for the rest of you guys reading. Depending on what oil your running thats fairly normal for viscosity change I would think on summer oil.

Bill Brayford

I'm using 20w50 and the nights have been cool. From the info in archives, I figured if the psi was wacky, the valve could stick open or closed. Maybe this is faulty logic. I just don't have enough motor knowledge to really undestand the valve.

Can anyone explain to this kid exactly how the valve works? What pressure is being exerted on the spring and I assume that the plunger is opening up a passage and re-routing oil flow? And is it the change in oil temp that reduces the pressure on the spring?
Mati Holland

Bill, I don't think its a rusted rivet. The hole looks like it was always there(smooth hole). I'm now thinking thats its not the original cap. Anyways, I've plugged the hole. It definitely runs better at idle with it plugged. Thanks to all who answered.

The pressure releif valve (PRV) does exactly what its' name says it does....releives pressure. If pressure is too high the valve opens. The PRV does NOT function (open if you like) at low pressure. When the PRV opens oil is sent past (bypasses) the oil filter. A safety feature if you like. There is a possibility of 3 problems. 1: plugged oil filter...not likely the case as we tend to lean towards overkill in pampering our toys. 2: An oil filter type that does NOT have an integral "filter releif valve" (check yours for this). 3: very thick oil at start up due to low temperature. The nail gets hit on the head. I suggest #3 is your problem Mati.
I asked the same question as you a few years back...what is the pressure that the valve opens at? Got no answer.Maybe the question should be: a what pressure will a head gasket blow?

The following link does a good explanation.

There is a problem in the drawings though. The drawings show the oil going back into the pan on PRV "activation". This is not the case. There is a passage from the "out" side of the PRV to the "out" side of the oil filter. If that drawing was true...well we would have a few less TRs on the road today. Talk about cold start problems! GEESSS the engine would never warm up:) would self destruct first!

Jerry, I was slightly wrong in saying Henry Ford era. Cars use to have a screen filtered oil cap. The screen was used to prevent the oil from exiting the cap into the engine bay. The cap became a sealed unit when pollution control stuff was added to cars. Bill has probably got it right..a rusted through rivet.

This summer at a car show watched the local fire department drain the oil out of a Chevy V6 and sell tickets for the time to self destruct. It took about 8 minutes..just got louder and louder...then STOP!

Rick C
Rick Crawford

Mati, my 73 TR6 also runs at about 75lbs while driving and drops to about 25 at idle. I run castrol 20w-50. I too am assuming this is normal.

Referring to the crankcase vent system (?) on the older TR's: It's very simple. There's a big tube running from the crankcase to close to the bottom of the car, cut at an angle to suck out the fumes at speed. The oil filler cap has a screen filter to keep dust out of the crankcase as outside air comes in to replace what went out the bottom. Hi tech, eh?


Positive crankcase ventilation is required as there is a small amount of 'blow-by' of unburnt gasses during the compression stroke (ie fuel/air mix which slips past the rings.) This will contaminate the oil if not removed. The crankcase is open to the rocker cover via the pushrod apertures. Inlet manifold suction draws the fumes back into the combustion chamber to re-burn the vapour as required by emission control regs. There is no need for ventilation in or out of the rocker cover at the oil filler cap. If it's 'vented', you are more likely to get oily air being forced out. High pressure or pulsing of the air out of the rocker cover could be an indication of worn / cracked compression rings.
Roger H

Good description, Roger. And I should have been more clear than just saying "older TR's". I was talking about TR2's, 3's and early 4's. Sorry if that's not much help to the original poster, doesn't apply to a TR6.

Rick, Bill anyone else still following.....

I have a supplementary stupid question / comment!

a) I read that the oil pump can pump more oil/pressure than the engine requires under heavy load, therefore excess is released at some point (100 psi?) which corresponds with the strength of a normal healthy spring. As it weakens, pressure will be released soon (ie 90psi?) Therefore a weak spring should be replaced because "you can't get it up" so to speak.

b)a stronger spring would simply increase the max pressure a little (110psi) which I am guessing might be desirable if racing?

c) on another thread some have suggested putting a washer on the securing nut, but that would weaken the spring, and reduce maximum psi and have no impact on normal operating psi. However, the treads suggest the reverse. Am I understanding this PRV correctly?

My transient high oil pressure therefore, as Bill suggests, must be attributable to cold starting. It's been parked for a few days, I finally got around to re-shimming the rear end to squeeze the door gaps a little! I'll see what happens this weekend when I fire her up again.


Hi Mati

Temps have been dropping to 0c. or 32 F. 20W/50 at that temp is a very very heavy oil until warm. Don't know how you store the car but if you just keep in the garage and start every so often over the winter. I would suggest an oil change. 10/30 lets the oil flow a lot better. Or go to the FCI Quaker oil for diesels 15w/40. Stuff flows better cold due to additives.

For the rest of this season let her warm up at idle for 3 or 4 minutes before revving or loading it up! Remember also the tr has a mechanical oil gauge. Until everything warms up your oil pressure show high.

The pressure relief valve is just a spring and piston that work as a bypass. They dont mean to add a washer to the outside were the threads and seal are. There looking at the back of the spring.

You do not want to increase or decrease that valve!!!!!

Its only reason for being there is to save the oil pump in case of a major failure. Like the filter plugging at 45 hundred rpm or wrong install etc. If it does cut in under regular conditions it just allows a little to run by then closes. Not a big deal whatsoever.

As far as racing goes?
On that particular engine if the bearings are correct and all is well racing or no racing there should be no need to adjust the valve pressure. I would suggest they should have spent more time on there machine work and balancing? Most of the Rice burners have a max oil pressure of 40 PSI. Trying to pump oil at 90 psi through improper restrictions wastes a lot of horsepower.

Bill Brayford

This thread was discussed between 09/10/2004 and 14/10/2004

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