Squre Four Engine Breather

[graham brookbanks]

I am struggling to understand the logic behind the engine breather arrangement on my Mk2 Square Four. On a single cylinder engine, or a 360 degree parallel twin where the pistons go up and down together, I can see how the crankcase pressure will increase on the downward stroke of the piston(s) and hence the need for a crankcase breather to relieve that pressure

However, with the Square Four, two pistons are going up as the other two are going down, hence the pressure in the crankcase should remain constant, unless there is any piston 'blow-by'.

If crankcase pressure due to 'blow-by' is a problem, surely there should be a breather directly from the crankcase?

The only engine breather is from the timing chest, and the only route for gases from the crankcase to the timing chest is via the two oil drain holes beneath the main bearing, which presumably will be full of oil from the OPRV on the oil pump.

My questions are as follows:

1) Why is there no breather on the crankcase? or :
2) Why is there no large breather hole between the timing chest and the crankcase? And:
3) Why does the timing chest need a breather as there are only rotating parts and nothing that could cause pressure variations in the timing chest?

[john.bebb]

Hi Graham
1/ There is a breather in the crankcase - it expires through the timing chest... just as any fluid will "find it's own level" and equalize, similarly gases behave in exactly the same way with respect to pressure & volume - if you put your tyre to say 30psi - that is per square inch (actually 'tis better described as per cubic inch but that's a separate apparent anomaly) - that lump of air you put in did not lay in the bottom of your tyre, nor did it float to the top so as to unbalance your wheel, no ... it has equalized!
2/ You answered this one yourself in your paras #1&2
3/ The crank breather is fitted where it is for convenience... fit a longish flexi tube & plum outlet so as to vent over the drive chain.
You might just as well ask why you do not have a hole in you chest for your lungs to breathe...
Cheers, John

[david.anderson]

Graham
Yes the breather direct on the crankcase is in theory more correct, however there is a massive amount of oil thrown about from the bigends. A direct crankcase breather may well expel more oil than air. By putting the breather on the timing chest that problem has been eliminated. A direct crankcase breather is normally baffled to prevent excess oil loss and in this case the timing chest is an effective baffle.
It is possible as you have said to blank off ventilation to a timing chest (which is regularly done to Norton Commandos) but the oil pressure relief valve would need to be altered as it dumps excess oil in the timing chest (mk2s)
Also as you have said in the case of the 4 the crankcase pressure is mostly by blow by. Put your finger over the end of the breather when the engine is running and you may be surprised just how much air is being expelled, even with a new engine.
Generally large breathers increase rather than decrease crankcase pressure, as more air is sucked in. That is unless of course a reed pcv is fitted. David
David

[Richard Kal]

David,
Thanks for your words of wisdom (again); much appreciated.
I am building up an iron 4G engine; would you recommend installing a reed pcv? I will be installing a Morgo pump, and a camshaft oiling (Longman) kit.
Richard

[Bob.Murphy]

I fitted a copper pipe to the inside of the breather on my 4G, with a soldered tab mounted on the upper oil pump stud.

This was to prevent oil that was running down the walls from being blown out.

After a long run there was a surprising amount of vapour wafting out of the breather - and I had new sleeves and standard pistons so minimal blow-by.

It may seem counter-intuitive with four pistons running in pairs at 180 degree phasing, but there is a surprising amount of 'blow' from the breather.

Bob.

[graham brookbanks]

Thanks, that was useful. I still can't really understand why the crankcase breathing into the timing chest is via the two small oil drain holes under the main bearing, rather than a larger hole higher up, with a baffle if necessary to prevent oil being thrown from the crankcase into the timing chest. Especially with a Morgo pump (which of course the designers didn't know about) dumping large quantities of oil back through the drain holes/breather holes.

[allan.walker]

Hi Graham
...... if you put your tyre to say 30psi - that is per square inch (actually 'tis better described as per cubic inch but that's a separate apparent anomaly) - that lump of air you put in did not lay in the bottom of your tyre, nor did it float to the top so as to unbalance your wheel, no ... it has equalized!

I understand the logic in this, in a static situation. However, in an engine, we have a dynamic scenario and it is entirely possible, IMHO, that the rate of pressure increase under a descending piston is greater than a small vent hole to the timing case can cope with - too much gas trying to escape through too small an opening in too short a time.

I have a Mk2 engine in my bike, which was totally rebuilt less than 1000 miles ago. It has a Morgo oil pump fitted. (I bought the bike as such.)There is a massive steady pressure from the breather pipe with the engine running. With a long clear plastic tube attached, rising upwards from the breather, there is a lot of emulsified oil/water vapour droplets (a.k.a. 'mayonnaise') which forms in the tubing after about 200 miles. It then has to be cleaned out.

I suspect that the Morgo pump adds to the problem due to the amount of additional oil flow. Another symptom of this, it would seem, is a difficulty in sealing the flexible oil return pipe on to the brass pipe on the engine. There's a persistent drip. The vent cap from the oil tank also 'weeps' a fair amount.
(The oil tank is a standard FH tank due to the engine being fitted in an FH frame.)

I wonder if a breather vent from a rocker cover would help...does anyone have any experience of this?

[david.anderson]

Richard
I have fitted a reed PCV valve to the end of the existing breather pipe on my mk2. I used a short length of flexible hose to connect the pipe to the inlet on the PCV.
https://www.mikesxs.net/yamaha-xs650-re ... e-pcv.html
With the 4g it is worth considering the AAR rocker boxes (copies of the boxes fitted by the SA police bikes). They solve the oil leak problem of the 4G.
https://www.australian-ariel-register.com/ $225 for the casting and then you arrange machining, templates available.

David

[allan.walker]

Hi David,
I was thinking about the 'Mike's XS' reed valve (actually I believe it is an emissions valve as found on a variety of 125 Yamahas.) These have been proven to work in many applications.

Have you located yours under the engine/frame somewhere? Do you have any problems with 'mayonnaise' forming and causing blockages in the valve?

Allan.

[david.anderson]

Alan
mine is about 10mm below the bottom of the breather pipe and is connected to it with a short piece of plastic hose and a couple of hose clamps.
No mayonnaise and no blocking of the valve.
David