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Oil Pressure Relief Valve - Gone.

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Oil Pressure Relief Valve - Gone.

Posted by john_young1 at July 10. 2017

Just been out for a short ride when I realised that my right foot was getting rather oily - the large domed nut from the oil pressure relief valve in the timing cover had disappeared, along with its internals. I think I noticed pretty quickly and pulled over within a couple of hundred yards. Just hoping for some reassurance that there will be no damage caused.

 

Re: Oil Pressure Relief Valve - Gone.

Posted by les_howard at July 10. 2017

Hi John. Once the valve drops out you would have lost all direct oil pressure instantly. However when running the crankshaft creates a certain amount of suction on the oil supply by the centrifugal fling of the big ends whirling around. This suction may well have at least sucked some oil into the big ends and if the revs and engine load were not too high this rudimentary oil supply might have stopped any direct metal to metal contact and prevented any run on the shells. This is the best case scenario of course.

You can check though by using an oil pressure gauge fitted to the bolt hole directly below the relief valve. You will need the correct drilled bolt and banjo plus of course an oil pressure gauge.

The best indication of wear will be when the oil is hot (50 deg upwards) If you still get a decent pressure at medium revs (40 PSI?) you will still have sufficiently tight big end shells.

If you could do with an oil change, you might want to drain off the old oil and examine the oil closely. Perhaps drain the oil through a funnel lined with a clean open weave cotton or wool rag or such like. After all the oil has passed through, closely examine the residue trapped by the filter rag/cloth. When big end shells "run" you will have lots of white silvery fragments of alloy showing. Their absence will definitely mean you have had a lucky escape. ...I hope you have...Smile....Les

Re: Oil Pressure Relief Valve - Gone.

Posted by john_young1 at July 10. 2017

That's great, thanks. Is this the correct oil gauge?

https://www.rgmnorton.co.uk/buy/oil-pressure-test-gauge-with-pipe-take-off-for-rocker-oil-feed_1260.htm?bskt=38347417-b620-44ef-a355-8859aa6f78e0

Just dropped the oil from the crankcase and its clean - no bearing debris.

Re: Oil Pressure Relief Valve - Gone.

Posted by les_howard at July 10. 2017

That's the correct set up...but be careful as I'm not absolutely sure the thread size is the same for all models of timing case. Roger or Lawrence at RGM can advise and swap it if need be....(much of RGM's stuff is suited to Commandos)

Re: Oil from crankcase....you will have to check the oil in the tank by draining it too as the particles could have been scavenged back into it, they are quite small usually ( sometimes just tiny specks) soft and light. However if the oil is new and clean you can always pour it back in...Looks like you might have go away with it though....Les

Re: Oil Pressure Relief Valve - Gone.

Posted by Gordon Johnston at July 11. 2017

It's surprising haw long an engine will run without oil. Youtube has some interesting videos. It looks like you discovered the loss well before any damage was done. Phew!

Re: Oil Pressure Relief Valve - Gone.

Posted by les_howard at July 12. 2017

Hi Gordon...yes, some interesting videos on the "No oil" theme.

The real determining factor of how long a plain bearing will last with pressurised oil or one that is just oiled is the side load it has to deal with. Plain bearings when running create their own centralising effect by creating  a "hydrodynamic" wedge of oil which aquaplanes the shaft and floats it away from the bearing surface.. Big end bearing however centrifuge the oil out which is why it needs to be pumped in, but you still get the hydrodynamic effect going on which will protect the bearing if the side loading is not too great and some oil remains inside the bearing. Running at medium revs and lightly loaded will let the bearing survive for some time. It's high revs that hammer big end bearings where at TDC where the G force of stopping the piston creates tons of vertical force.

Best aural test for big end wear is to listen for any engine rattle when on the overrun situation at medium to high revs. IE: when you come off the throttle after the engine has been pulling hard or when descending a hill with a closed throttle. You will get a metallic rattle. This is caused by the bearing being loaded at the bottom  when the piston is being pushed upwards against compression IE: the clearance gap is at the top, and then when the load is released after TDC the crankshaft (in effect) pulls the piston down which transfers the load to the top of the big end bearing which moves the clearance gap to the bottom side. This alternating movement is the rattle that can be heard distinctly.

This might give more assurance to John that everything is OK if this sound cannot be heard.

Les

Re: Oil Pressure Relief Valve - Gone.

Posted by david_evans at July 12. 2017

Have a look at the you tube clip "chinook tested to destruction" whilst several of the pundits say that this was ground resonance, it is almost certainly lack of lubrication in the rear head. Ground resonance is something completely different.

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