Single Breather

[robert.clark]

I have my 1935 race 500 single apart at the moment. It has the open valve gear head which relies on the oil mist lubrication for the valve guides. I have modified mine to include both inlet and exhaust valves.

The drive crankcase does not have the 2.oclock breather tapping. so the question is do I modify it?

I know I have zero crankcase pressure at present as measured from a tapping in the timing crankcase located between the pushrods. I also use this as a breather to atmosphere but it is a vertical exit pipe which could suggest it's not venting at all.

In doing the 2 o,clock modification I feel it is very likely the breather will vent the existing oil mist to atmosphere.

My research suggests the introduction of an in-line PCV one way valve could be the answer.

The forum threads on this subject refers to the use of a Yamaha single breather. I'm assuming this the XS650 but don't know. It may be another model.
It's function (XS650) though appears to opposite to my needs suggesting it creates a negative crankcase vacuum sucking away the oil mist from within the crankcases.

Other research has shown USA companies Bunns Breathers and JS motorsports made them but it appears that was in the past.

Any comments appreciated on this subject.

[Roger Gwynn]

I had a Bunn's breather, not that impressed, don't know where it went or even which bike I fitted it to, probably my KH. As I understand it the idea of a breather is to reduce the pressure in the crankcase in order to reduce oil leaks and blow by so a one way valve that lets air out but not in will give negative pressure. Without it the air will be pushed out and sucked back in but it has some effect as the volume of the crankcase is increased somewhat. You say that a reed valve is "opposite to my needs suggesting it creates a negative crankcase vacuum sucking away the oil mist from within the crankcases." So do you want positive crankcase pressure?

[Vincent.vanGinneke]

I am -also- puzzeld by a 'negative vacuum' ehhhh, No vacuum ?
The yam reed valve lets air out on the downstroke and closes on the upstroke.

But, Ariel only changed something to their engines if it was benificial.
So one can assume the 2 Oclock hole is there for a reason.
If a reed valve would add something....dunno, just try it.

[robert.clark]

Roger and Vincent, thanks for your comments.
The reference to the negative crankcase vacuum is what I read in several articles and reviews on the Yam 650 breather operation. I just picked up on the comments made. I was surprised and the gain was apparently a small improvement in performance. I can't verify it's just what I read.

Ariel design change to introduce the 2.0 clock breather. I reckon that the early open valve models when subjected to high mileages and excessive wear of rings / valve guides would have generated excessive oil leakage / crankcase pressure in many other areas due to the lack of a crankcase breather.
Introduction of the enclosed head plus rocker boxes allowed for a direct oil feed to the valve guides so to me this makes sense. The crankcase breather provides an obvious vent to dispose of the excessive oil leakage to atmosphere. It also removes the need for the earlier oil mist lubrication to the valve guides.
From earlier tests I have seen that the exhaust valve guide mist feed has a tendency to gum up at the guide feed after a couple of thousand of miles.

On my early build I fitted an exceptionally worn old piston and rings, just to get a run out of the engine. I measured 26 psi crankcase pressure from the timing side tapping. The oil leakages were self evident and what a mess. A new piston / rings resolved all the issues.

Like Vincent mentions Ive nothing to loose. It's bugged me for 2 years that I didn't do it at that time. Ive modified a spare drive side crankcase recently so I know it can be done. I'll experiment with a reed valve and see what happens. still researching on the supply on that one.
Worst case scenario I can always blank it off.

As a belt and braces I,ve got some sheep wool on order with the intention of wrapping this inside the valve springs and on top of the valve guides to provide oil soaked pads to lubricate the valve stems, thereby by-passing the oil mist altogether. Both valve guides are made from Colsibro, which are pretty hardy anyway so it can't seeing any harm. It should give some added protection in that area.

[nevhunter]

It's good to have a one way valve on the breather pipe to prevent dust going in there if you are idling on a dirt road. Unless your rings are doing a very good job you will get crankcase pressure with the original sized holes on the breather. You can always run with one (or both)of the rocker caps off. That was commonly done on Triumph twins in the 50's. Nev

[robert.clark]

Thanks Nev, Open head valve gear in my case, but yes I have seen many Vincents with rocker box breather connections. I modified a Tiger 100 in the same fashion many years ago.