Author Topic: Oil breather - the truth ?.  (Read 1106 times)

Offline Josh Cox

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Oil breather - the truth ?.
« on: 16.07. 2009 11:15 »
Some very very interesting reading between the two of these web pages.

Theory: install a PCV on rocker cover, run the breather to the rocker cover, no more drips or wet starts.

Sure one of you gents will play Devils Advocate for me ?.
Black 1953 Golden Flash Plunger

Offline MikeN

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Re: Oil breather - the truth ?.
« Reply #1 on: 16.07. 2009 12:32 »
Thats interesting ,although Im surprised to read that most crankcase pressure is caused by piston blow-by. I would have thought that in an engine in good condition , most pressure would be caused by 2 (in our case) pistons descending into the crankcase followed by a depression as they rise again .which is why our bikes have a timed breather to minimise positive pressure on the down-stroke and maintain low pressure on the up-stroke.
  I always think it a bit strange when people add extra PLAIN tube breathers to (usually) a rocker cover,as this must surely reduce any crank-case depression on an engine fitted with a timed breather and actually increase crank-case pressure.
   The Webco/beezaBill rocker covers with PCV's look like a good idea though.
 I can understand it being the case in, say, a 4 cyl car engine that has a 2 up/2 down crank .As 2 pistons fall the other 2 rise and the puming action will be cancelled out .
Maybe Ive misunderstood how it all works.

Offline chaterlea25

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Re: Oil breather - the truth ?.
« Reply #2 on: 17.07. 2009 00:10 »
Hi Josh/Mike
I have no issues with the standard BSA  breather, properly set up it works very well, It is vital to have the cork disc the right thickness or it wont work.
in one of the articles he goes on about reducing leaks from the top end by having a rocker mounted breather
The pressure or vacuum is the same in all parts of the engine!
Rockerbox to head,  gasket leaks are usually caused by damaged faces, bolt holes filled with crap or I believe poor head steady mounts, I think the head steady mounts can cause movement between the rockerbox and head causing leaks
To this end I have fitted an isolastic type bush between the headsteady and the rockerbox, no problems so far
John O R
1961 Super Rocket
1963 RGS (ongoing)

Offline trevinoz

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Re: Oil breather - the truth ?.
« Reply #3 on: 17.07. 2009 06:03 »
I have seen this done on an AJS twin and it cured the problem of oil being forced into the generator.

Online groily

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Re: Oil breather - the truth ?.
« Reply #4 on: 17.07. 2009 09:39 »
and boy can that oil-in-the-dynamo (and everywhere else sometimes!) be a problem on those machines trevinoz. I have one with and one without (a humungous) breather mounted on a milled flat on the timing cover - and the difference is equally huge. With my BSA at least, nothing like the same hassle although there's evidence on one rocker cover (- as in a plugged 'ole) of someone having done the mod in the past with or without PCV. However, the standard breather will allow a bit of any excess oil that gets into the cases after a week or two standing to make a small mess. No matter - round here a bit of oil more or less makes diddly-squat's worth of difference. It's only when I tread in it and traipse it round the house that the trouble starts *smile*