M20 Oil Pressure

Information relating to the Matchless G9 or AJS Model 20 500cc twin
autosprink
Posts: 26
Joined: Thu Jan 01, 1998 12:00 am
Location: AUCKLAND NEW ZEALAND

M20 Oil Pressure

Post by autosprink »

Hi forum members
1953 M 20 - shaft -10 cylinders +60 all on spec, new bearings. Oil pressure at idle 30 ps1 engine cold. oil pressure at say 1500 rpm engine and oil hot - zero Plenty of oil returning to tank. Oil very thin when hot Castrol GP50 Any suggestions welcomed Regards

John Powell NZ


Eric
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Joined: Fri Jan 01, 1993 12:00 am
Location: WEST MIDLANDS UK

M20 Oil Pressure

Post by Eric »

You dont say where the take off point for the pressure gauge is this is important before an opion can be given.

autosprink
Posts: 26
Joined: Thu Jan 01, 1998 12:00 am
Location: AUCKLAND NEW ZEALAND

M20 Oil Pressure

Post by autosprink »

The gauge is connected to the 1/4 bsp plug on the non return valve as shown in F W Neill's manual

Eric
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Posts: 1221
Joined: Fri Jan 01, 1993 12:00 am
Location: WEST MIDLANDS UK

M20 Oil Pressure

Post by Eric »

Its good to know you have got the book. The oil pressures quoted occur because if everything is correct the flow of oil is restricted mainly by the very close fit of the big end & centre main bearings to the crankshaft journals. Some engines will have even higher pressures than those given when the engine is first started from cold. A low oil pressure would normally suggest worn bearings that are allowing oil to leak easily from them. From what you have said this is not the case, so assuming the delivery pump is not worn out there is probably a relatively large unwanted internal leakage of oil somewhere from the pump onwards. Check for leakage from the pump itself is the gasket fitted between the pump and pump plate and everything tightened up correctly. Is the gasket between the pump plate and crankcase fitted correctly and everything tightened up. Leaks here will still allow a plentiful return of oil to the tank from the crankcase but without going through the correct parts of the engine. Some oil is diverted from the delivery pump to the return pump via an internal drilling in the pump plate, this primes the return pump is everything ok in this area or could some special modification be allowing oil to escape? From the pump the oil goes to the filter tunnel, is an oil pressure relief valve normally fitted to your engine? If yes are you are sure it is fitted and working correctly? Again you can have a good return of oil that is going almost nowhere except the filter tunnel, timing gear and the bottom of the crankcase. Forgetting for the moment the oil distributor bush, mainly because I cant quite remember all of its details; the oil goes to the drilling in the centre web plate and through the centre web main bearing into the crankshaft. Is the centre main bearing all within specification including its thrust washers? A problem here can dump almost all of your oil into the crankcase again with good return of oil but no pressure. Oil at pressure should now be in the crankshaft oil way drillings, have the sludge trap screws been removed? If so were they correctly re-fitted and punched to stop them coming out? If only one were missing almost all of the oil and pressure will be lost from here. You have said the big end bearings and crankshaft are all correct or they were when fitted but how long was the engine run before you checked the pressure? Why did you check the pressure? Was there something happening that made you think there was a problem? It will not last very long with no pressure when hot. My memory of what actually happens with the oil distributor bush is vague I think if it was badly worn or missing it may cause your sort of problem with low oil pressure but don’t really know. The condition of almost anything else in the engine is not relevant to your problem because nothing else influences the creation of high oil pressure once the main and big end have been lubricated everything else is done with low or no oil pressure.

Eric
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Joined: Fri Jan 01, 1993 12:00 am
Location: WEST MIDLANDS UK

M20 Oil Pressure

Post by Eric »

Another couple of thoughts. Has the gauge been checked elsewhere for correct pressure? If the bike wet sumps a lot this suggests the unwanted internal leak is before the non return valve at the drive end of the filter tunnel.

lawrence
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Joined: Mon Jan 01, 2001 12:00 am
Location: NORTHAMPTONSHIRE UK

M20 Oil Pressure

Post by lawrence »

Hi John
You say the bearings etc are new. Were the conrods a tight fit on the journals? I recently collected a crankshaft after regrinding having been quoted +10thou but luckily noticed a pencilled mark on the tie-on ticket stating '20'. The engineer had forgotten to tell me that he had reground it more than estimate. When assembled onto the crank the conrod when released should fall slowly and smoothly if it is correct.

Lawrence [}:)]
autosprink
Posts: 26
Joined: Thu Jan 01, 1998 12:00 am
Location: AUCKLAND NEW ZEALAND

M20 Oil Pressure

Post by autosprink »

Thank you for your comprehensive reponses
I bought the engine in a reportedly reconditioned state. I checked the oil pressure on starting just as a matter of interest.
When the pressure indicated low I dismantled the engine and ckecked the crankshaft and bearings. Cleaned out the crankshaft drillings and replaced the main and con rod bearings which were worn but not too badly. The oiling system I modified slightly as far as the overhead supply is concerned by having a 1/32 hole continously supplying this area and doing away with the distributer bush. I made up a jet to supply the overhead and I have checked this by blocking the jet entirely and rechecking the pressure (no different) The valves seem to be getting enough oil but not too much, so I dont think this modification has anything to do with the problem. I then rigged up a bracket to allow me to run the engine with the timing cover removed but with the relief valve spring in place. I thought that far too much oil was being sloshed around when I ran the engine with this in place but I couldnt really determine just where it was coming from. I suspected the relief valve was loose in the bore and perhaps allowing oil to escape so I made a new plunger with a very close fit(about .002 bigger in diameter) No difference.
At this stage I checked another M20 which has had a reconditioned engine fitted about 900 miles ago and it showed the same result 30-40 psi when cold no pressure when hot. even when the motor was turning at about 2000 rpm. (we now have another rather worried owner).
All the pump gaskets are in place and look ok. My next step will probably rig up some device to pressurise the oil system with the engine stationary to see if I can establish just where all that oil is being released. I am going to a Jampot NZ rally next week and if I can persuade some other M20 owners to let me check the pressure on their machines I might know a bit more. ( I suspect they will not want to know) I will be riding my 18S not the twin. So I will not be able to investigate further for a week or two

Regards
John Powell


lawrence
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Joined: Mon Jan 01, 2001 12:00 am
Location: NORTHAMPTONSHIRE UK

M20 Oil Pressure

Post by lawrence »

Hi John
If you have tested another bike, which presumably has done 900 trouble free miles, and got the same reading then is the test flawed? I would have thought the problem lies with the pressure guage not being suitable or 'zero' pressure being a characteristic of the engine? If there was a serious problem with the oiling, an engine would not run 900 miles would it?

Puzzled of England

Lawrence [}:)]
Eric
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Posts: 1221
Joined: Fri Jan 01, 1993 12:00 am
Location: WEST MIDLANDS UK

M20 Oil Pressure

Post by Eric »

Lawrence I too suspect the gauge being used. [;)] As I said many engines can develop much higher pressures than those published when started from cold. [;)] The pressure maybe too high for the range of the gauge I dont know what this would do to the gauge [?] But what would be the chance of it reaching maximum, then going round again as it were, so what you see is gauge max plus what you see. [:)] You must get the gauge tested. [^] Our twin plain bearing engines rely upon oil at pressure the oil is the equivalent of rollers or balls with no pressure the bearing will fail rapidly. [;)] Even skilled and knowledgable engineers at AMC could not get the twin oiling system correct for all applications and they had much more testing technolodgy available to them than one engine and a suspect gauge. [;)] Drastic modifications like leaving out the distributor bush and replacing it with a fixed size leak is about as drastic as you can go. Where did you put the 1/32 bush [?] But why did you think a controlled hole would be better than the distributor bush if AMC thought it would do they would not have added the extra expense involved with the bush. [:(!] What I would really have liked was for you to have published everything you have now, so that I and Lawrence would not have wasted hours of our time trying to help somone we thought had a genuine problem. [V] [V] [V]

lawrence
Posts: 2453
Joined: Mon Jan 01, 2001 12:00 am
Location: NORTHAMPTONSHIRE UK

M20 Oil Pressure

Post by lawrence »

Crikey Eric! Are you having a bad day? [:0][:I]

Time spent on something you enjoy is never time wasted (Oscar Wilde??). It keeps my brain cells active and boy do I need that. Keep the questions coming John. They can't get more stupid than some of mine [:I]

Lawrence [}:)]
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