Author Topic: engine breather (again)  (Read 2027 times)

Offline duTch

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Re: engine breather (again)
« Reply #15 on: 29.11. 2016 19:27 »

 
 How about a possible bad seal of the small internal joint (in front of and slightly above the camshaft) that the breather gallery passes through?

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Offline KeithJ

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Re: engine breather (again)
« Reply #16 on: 29.11. 2016 19:30 »
Interesting thought.  I will have a look at another set of cases to see exactly what you mean.
Thanks
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Online chaterlea25

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Re: engine breather (again)
« Reply #17 on: 29.11. 2016 22:33 »
Hi Keith,
Quote
I don't understand why?  Everyone says so but as long as there is no end float, why does it have to be compressed so much?

Because it has to be air tight!

John
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Re: engine breather (again)
« Reply #18 on: 30.11. 2016 04:38 »
Hi Keith,
Quote
I don't understand why?  Everyone says so but as long as there is no end float, why does it have to be compressed so much?

Because it has to be air tight!

John

I'd be interested to know why it has to be airtight? I thought the cork was just to provide an adjustment to stop the drive pin from rattling around? E.g. A bit of a bodged design by BSA.......
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Offline KeithJ

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Re: engine breather (again)
« Reply #19 on: 30.11. 2016 08:56 »
"Because it has to be air tight!"

But it would only be on one side?  If being airtight was critical, surely it would need to be airtight on both sides?

Keith
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Offline olev

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Re: engine breather (again)
« Reply #20 on: 30.11. 2016 10:04 »
Its an urban myth.
Has anyone actually seen with their own eyes that replacing the cork washer fixes oil coming out the breather?
When the pressurised air gets into the breather and the timed port opens, it has to rise 50mm to get into the breather drain.
Surely the only way oil can get in here is as oil mist.
cheers
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Offline duTch

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Re: engine breather (again)
« Reply #21 on: 30.11. 2016 10:48 »

Quote
...But it would only be on one side?  If being airtight was critical, surely it would need to be airtight on both sides?

  If it's not airtight on one side,surely it's prone to be half-not airtight on both sides... *dunno*

 Having said that ^^^, I have to admit, I've quandered this also to an extent *beer* *eek*, but isn't the flow between the outer-case 'tophat' centre hole (not Topdad) and the 'tophat' breather holes??
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Online chaterlea25

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Re: engine breather (again)
« Reply #22 on: 30.11. 2016 17:41 »
Hi All
Doubting Thomas's or What  *????* *????*

The top hat flange seals against the inner timing cover on one side and against the cork washer and cam gear on the other
The breather holes are on the cylindrical part and align with the case holes at the appropriate time
The cylindrical  part is a good fit in the case and the oil film should prevent the passage of air
If a very thick cork washer is needed be careful that theres enough of the driving peg protruding to drive the breather  *warn*

Yes I have experienced the difference that a tight fitting cork makes
A few years ago I rebuild an A10, after some mileage it was puffing a bit on the breather and a bit of smoke in the exhaust
A new cork washer sorted it *smile*


John
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Offline KeithJ

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Re: engine breather (again)
« Reply #23 on: 30.11. 2016 17:48 »
"Yes I have experienced the difference that a tight fitting cork makes
A few years ago I rebuild an A10, after some mileage it was puffing a bit on the breather and a bit of smoke in the exhaust
A new cork washer sorted it"

Still does not explain why?  How about if there is end float, it allows the drive pin to move in the hole in the breather.  It also allows the breath hole to move in and out.  This makes the amount of "breather hole" be reduced and less breathing? 

What would happen if the hole was made into a slot?
ATB

Keith
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Online chaterlea25

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Re: engine breather (again)
« Reply #24 on: 30.11. 2016 18:03 »
Keith, AKA doubting Thomas

(Basically) The breather is closed during the piston up stroke creating a partial vacuum in the crankcase
this vacuum helps hold the oil inside the engine
If the breather is loose the engine puffs in and out as it rotates = oil leaks

There are some people who modify the breather sleeve holes to increase the open period
Its not an answer to other engine issues that increase crankcase pressure (worn bores ,rings and valve guides)

The hole on the drive flange of the breather is only for the driving peg, its not part of the air passage
Its effectively sealed  when the engine is assembled

John
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Offline KeithJ

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Re: engine breather (again)
« Reply #25 on: 30.11. 2016 19:12 »
I understand the movement of the pistons up causes a vacuum which should minimise oil leaks.  So if the engine has good pistons, bores and valves etc, then, the pressure to be released on the down stroke needs to be released, hence the timed breather.  I guess this is set to just after the power stroke to just after BDC non the upstroke.

Still don't understand why the cork washer is so critical?  Surely, all it does it to keep the breather and driving peg in a fixed position relative to the breather drilling?  The clearance between the breather and the inner timing cover would also impact the vacuum
 
Perhaps mere mortals such as me are not meant to comprehend such matters?

ATB

Keith
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Re: engine breather (again)
« Reply #26 on: 30.11. 2016 19:39 »

Perhaps mere mortals such as me are not meant to comprehend such matters?

ATB

Keith

The timed breather does have a little more to it than the obvious  *doh* *doh* (there are high speed pressure fluctuations going on, with "inertia" of gases being required to move quickly etc, akin to the mysteries of megaphone design) and I think it best to ensure the cork thickness is sufficient to prevent end float of the breather as pretty clearly BSA in tended that to be done, as to whether a breather with endfloat (and leaking) does not work as well as one without? I doubt anyone can conclusively say why that would be the case  *dunno* there may be theories about the outer case having different pressure to the inner case area, but that's what they would be....theories  *fight*

On my bike I had to use such a thick piece of cork I had to extend the breather drive pin length as beforehand it was slipping out of engagement, I guess the case had distorted over time  causing this *dunno* anyway I doubt this is unique to my bike and indicates that having a good look at the breather and making sure it is set up right may very well solve an issue with excessive gases coming out of the breather tube, but a friend of mine spent numerous hours solving his issues in this area (excessive case pressure causing the crankshaft oil seal to pop out), what ultimately fixed is a bit inconclusive, maybe the new valve guides? Rebore? Fitting a modified BSA breather (elongated holes in the breather to change its timing) instead of the bunn? On that last mod, this may be worth you trying..........
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Re: engine breather (again)
« Reply #27 on: 30.11. 2016 19:59 »
Hi
Quote
The clearance between the breather and the inner timing cover would also impact the vacuum

When the case is bolted up and compresses the cork washer there is zero clearance between the breather flange and inner case !

Kiwi,
If the case was bolted up without aligning the peg and breather it may have damaged the case ???
Worse if it was run like that, or with a too tight preload and worn the alloy case ????

John
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Offline olev

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Re: engine breather (again)
« Reply #28 on: 30.11. 2016 21:30 »
hmmm, I hadn't thought of the oil coming into the breather arrangement past the tophat bearing surface.
I assumed oil somehow got inside the top hat and was slung or pushed up the timed output hole.
or the oil arrived as a vapour and condensed inside the tophat and the outlet drain.
Maybe its possible the breather system is working within specs but just overcome by extra oil being slung into the area by the timing gears.
Maybe once oil gets into the outlet it increases the resistance to air flow and compounds the problem.
Maybe some modern oils create a denser oil mist when stirred up by the timing gears and this gets into the breather system.
Maybe I'll just stop thinking about this and go and get on the p!ss.
cheers
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Online RichardL

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Re: engine breather (again)
« Reply #29 on: 30.11. 2016 21:52 »
I think, maybe, we don't need to talk about any relationship of close timing between up and down piston movement and hole alignment in the breather. The hole is only aligned every other crank rotation. I can't believe it's been almost ten years since I mocked this up to see what was going on, so my memory may be weak, but I believe the breather is open at 90 deg ATDC. I have guessed, right or wrong, that the breather is trying to keep crankcase pressure as close to one atmosphere as possible so that oil can be pumped in and sucked out. This may disagree with those who believe (or know) that there is some performance benefit based on where in the stroke the breathing occurs. By my theory, such benefits (or detractions) would be purely coincidental with respect to BSA intentions. (I can be wrong and often have been.)

Then there is the matter of why a lot of oil spills out when there is substantial wet sumping. Seems to me it is way too much to get in from the back side past the flange. That means it would need to enter past the tapered end in the portion contained by the outer cover. There is an oil drain hole that could allow this. So, it seems that the wet sumping would have to fill the crankcase about up to the height of the breather for oil to get in through the breather's open end. (Waiting chastisement for silly or "Richard, are you just now realizing this?" Well, not quite just now.)

Richard L.
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