Oil - again

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Re: Oil - again

Postby 1608 » Mon Jun 01, 2020 12:55 pm

Adding additional breathers seems to be too readily adopted when there seems to be oil leaks. I know AMC didn't get everything right
( especially lubrication on the twins ) but if it was as simple as just adding or enlarging the existing breather system then they would have done so. I remember reading somewhere that there is an optimum balance required on a normally aspirated engine and it is possible to overdo crankcase breathing. I have never felt the need to alter the standard breathing on any bikes I have owned. Excess crankcase pressure is caused by wear .

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Re: Oil - again

Postby Oggers » Mon Jun 01, 2020 1:40 pm

It is indeed very difficut to tell anything! I have since whipped off the oil pumps and plate. I cannot see the additional hole as per Neill, but then again, the idler pinion is in the way - and I am not removing that just to see. Pig of a job it seems.

I am still leaning to fitting an additional breather at the top of the timing case to the rear - where it is higher. It cannot do any harm so why not. May reduce it to 1/4 inch as originally suggested to get a better fit on the slighly curved case.

I am also thinking of removing the felt filter in the oil tank to get the scavenge pump working as best as I can get it in an attempt to lower the oil level in the timing case. Any other ideas as to how to lower this oil level are welcome......

I agree that usually blow by is caused by wear of the rings and bores. The only (somewhat tenuous) response is that mine is newly built (less than 1000 miles) and the rings have not quite bedded in yet.

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Re: Oil - again

Postby Mick D » Mon Jun 01, 2020 1:45 pm

Hi

The primary chain case on my alternator single breathes through the annular gap between the sleeve bearing and the 'seal':

Chaincase Seal.png


And the hole drilled below the oil shield:

Breather.png


And if that's not enough, up the alternator loom grommet.

I know the singles have a different arrangement to the twins but if twins breathing was an inherent issue I think it would have been dealt with historically - I'd be looking why your's isn't working rather than modifying it to work.

Regards Mick
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Re: Oil - again

Postby Oggers » Mon Jun 01, 2020 3:27 pm

Cannot say I've noticed any of these breathing holes on my twin - but then again I wasn't looking. I realize an addtional breather may be considered something of a bodge, but in the light of exhausting everything practical I can think of without tearing the bike down, I think it is worth a try.

Any clues as to how to reduce the oil level in the timing case appreciated - drilling holes in crankcases excepted! Felt filter in the oil tank will be removed - as a trial to see if that increases efficiency of the scavenge and thus reduces the level that way,

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Re: Oil - again

Postby 1608 » Mon Jun 01, 2020 5:14 pm

I take it you have researched the Tech articles re twin problems re over oiling. It explains the drilling of the hole in the timing side cover I believe you refered to. I think that if you went down this road it would mean dismantling the c/cases as you wouldn't want any swarf in the engine. I believe the G9 set up is the same as my G12 so removing the idle pinnion shouldn't be a problem as it simply lifts off its stub axle. If you do this make sure the engine is in neutral relax the tappets and mark the corresponding teeth with Tipex or similar.

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Re: Oil - again

Postby Duncan » Mon Jun 01, 2020 6:06 pm

Oggers wrote:I am still leaning to fitting an additional breather at the top of the timing case to the rear - where it is higher.

If you are convinced that you want to fit an additional breather consider putting it in a relatively inexpensive replaceable item like a rocker box or remove the dynamo and plate the hole off and make a temporary breather there see what improvement is available before damaging a serviceable timing case. You should also fit a one way valve like a Bunn? valve.
Remember that the timing gear will also throw oil up to the top of the timing case when running,
With the number of leaks that you appear to have I would start to question how well the jointing surfaces were prepared prior to assembly and the methods used.

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Re: Oil - again

Postby Oggers » Mon Jun 01, 2020 7:16 pm

This is a very worthwhile discussion so thank you chaps.

In short - I am not convinced by anything! Additional breather is a possibility and no more, and I would not like to spoil a perfectly decent timing case so alternative locations are much appreciated. I also take the point that this is bit of bodge and not solving the root cause. I wasn't aware the idler gear just pulled off. This will make things easier if I wish to drill the hole, and yes, I am very aware of getting swarf into the engine.

Thinking a bit more and I realize I am somewhat indecisive here as I do value all comments, I am going to adopt a more cautious approach.

Stage 1

Remove felt filter in oil tank to possibly improve scavenger pump efficiency and possibly reduce timing case oil level.
Use 20/50 rather than straight 50 to encourage better oil flow through all passageways, reduce viscosity and possibly reduce any blow by/crankcase pressure. I'm not convinced at all by the latter, but you never know.....

Test by riding.

Stage 2

Drill the hole as per Neill - if there isn't one already....

Test by riding

Stage 3

Check out breather on drive side. If all is well then introduce another breather - somewhere....

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