Oil from heavyweight crankcase breather

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Oil from heavyweight crankcase breather

Postby LynP » Fri Aug 11, 2017 7:15 pm

Hi, I have just started my AJS M16 (1960 G3 Engine fitted), following an engine rebuild which included a re-bore. I find that there is oil dripping from the crankcase breather, I have looked at previous post and see that others have had the same problem after a re-bore and some have fitted a catch bottle and found the drips get less as the rings settle to the bore.
The reason for this post that my bike has no centre stand fitted and so is left parked and also ticking over on the side stand, is it possible that more oil will be directed towards the drive side crankcase half and so run down into the breather hole drilled in that crankcase and so out of the breather?
I carefully fitted the breather and its flap into the case and it seemed in good condition.
Finally, as there is no seal between the engine crankcase main bearing housing and the primary chain-case and therefore there is pressure released through into the chain-case, how efficient would the breather and flap be?
Any thoughts gratefully received.

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Re: Oil from heavyweight crankcase breather

Postby SPRIDDLER » Sat Aug 12, 2017 11:20 am

LynP wrote: I find that there is oil dripping from the crankcase breather

Oily mist will always exit the breather and drip, it rather depends upon how much. There could be excess crankcase pressure due to piston blow-by until the rings bed in.
The reason for this post that my bike has no centre stand fitted and so is left parked and also ticking over on the side stand,

I never leave mine parked for very long or ticking over for more than a minute when on the side stand but it would encourage oil to flow to the breather (left) side and the scavenged level of the oil would be higher than if the bike was vertical since the scavenge drilling is on the right (oil pump side) of the crankcase. If it's parked on the side stand and wet sumps neat oil would/could flow out of the breather and/or into the chaincase.
Finally, as there is no seal between the engine crankcase main bearing housing and the primary chain-case .....

Not quite true. The large washer between the main bearings was intended to act as (partial) barrier to oil although it's efficacy is debatable. Only the oil (mist?) getting past this washer would exit via the breather.
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Re: Oil from heavyweight crankcase breather

Postby LynP » Sat Aug 12, 2017 5:51 pm

Thank you SPRIDDLER for your helpful post.
The washer between the drive side main bearings has been an issue as I see there are two different ones listed in the club spares depending on the replacement bearings used. When I replaced the bearings in this engine I managed to get SKF ones as were found fitted when I stripped the motor and so I re-used the washer between them that was in there. I don't know what the difference is in the two types of washer, can anyone give the relation measurement for each and why they differed?

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Re: Oil from heavyweight crankcase breather

Postby SPRIDDLER » Sat Aug 12, 2017 6:07 pm

LynP wrote:Thank you SPRIDDLER for your helpful post.
The washer between the drive side main bearings has been an issue as I see there are two different ones listed in the club spares depending on the replacement bearings used. When I replaced the bearings in this engine I managed to get SKF ones as were found fitted when I stripped the motor and so I re-used the washer between them that was in there. I don't know what the difference is in the two types of washer, can anyone give the relation measurement for each and why they differed?


The smaller one is an additional spacer for when Hoffman ball bearings were used. The larger (inner) one, which from memory is the same or almost the same dia as the bearings' outer races is the oil barrier/baffle. I know not why the spacer washer was necessary on the Hoffman but not other (SKF) bearings. It's in the workshop manual somewhere........(I think it's something to do with separating the bearings on the shaft to prevent the outer bearing's balls' cages touching the larger washer and interfering with each other. Perhaps the SKF ball cages were narrower than those on the Hoffmans....) I've just had a look at some old SKF main bearings in t' shed and their cages are significantly narrower than and within the outer races width. (I'm rambling, hope you're keeping up).

Here's an exploded drawing of a '54 H/W engine which if you zoom in clearly shows the smaller and larger washers:
http://archives.jampot.dk/technical/Cut ... d_View.jpg
Last edited by SPRIDDLER on Sat Aug 12, 2017 8:06 pm, edited 1 time in total.
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Re: Oil from heavyweight crankcase breather

Postby LynP » Sat Aug 12, 2017 8:06 pm

Thank you again SPRIDDLER for your time and the detail you have given, certainly not a 'waffle' the more detail the better. You have put my mind at rest because from what you say is seems I have correct large washer fitted with the SKF's and no additional small one.

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