rocker tins, cups, curious holes

[Steve Little]

This response is not totally on subject.
I agree with Jerry on the use of modified Panhead breathers for Knuckles.
I recently rebuilt a Knuckle engine for a friend, and he wanted me to re-use the original (un-shimmed) style breather valve.*
When I was setting up the cam chest I measured the gap from the nub on the breather to the cam cover. There was a gap of 0.027".

When the pistons go up in the bore, they create a vacuum in the crankcase. This vacuum goes through the gallery at the back of the cases to the breather gear. This *vacuum is timed through a hole in the rotary breather to create a vacuum through the ports in the lifter blocks, lifter tubes, rocker boxes, and finally through the tubes to the base of the valve spring cups, to suck oil out.

Aside from the timed effect that pulls oil from the top end, the vacuum created in the crankcase also pulls the breather gear tightly into the bore until the shoulder of the gear rests hard against the case.*
When the pistons change direction and come down in the bore, they create a pressure charge *in the crank case area.*
This pressure charge would then push the breather away from the case and onto the special landing on the cam cover. This back and forward motion through a 0.027" gap, must cause a clattering noise in the cam chest. *
Shimmed breathers would eliminate this noise.

[koanes]

That's an interesting idea Steve, and you know the H-D Spec for breather gear endplay is far less than .027. I have never given any thought to breather travel because of vacuum or pressure, although I suspect proper end play is probably more critical for passage alignment than noise reduction, as the breather is being driven by the cam and I don't think the gear would be free to move back and forth much under normal circumstance. Seldom do you see any wear on the breather recess or the cam cover from banging back and forth.

[poorbiker]

Ya I would tend to agree that vacuum is to weak to overcome the gear mesh to create any significant movement on the breather itself while under a load, however if you have ever watched a gear under some load they do have a tendency to want to walk out or depending on the load applied, walk in. Of coarse gear fit and geometry have allot to due with it also.
i would be interested to know about how much crank case vacuum a properly sealed and running knuckle engine creates, anyone know?
On a kinda side note, typically the breather bore on these dont go bad unless there has been some sort of catastrophic failure, like oil loss and metal getting in and around the breather and bore. If everything is working properly the breather gear should run on a film of oil, keeping it more or less centered. On mine only the breather bore is scored, gouged. I have never reamed a Knuckle breather bore before, if there is enough meat around the bore on a knuckle, like as on mine, I could get away with reaming. I prefer this to welding. I have access to a reaming tool made by "Jims" Any thoughts on knuckle breather bore repairs?

[Steve Little]

Howdy Kyle.
In my head, *what I have written above is a question.*
But I can see that the way I have written it, it has lost the questioning tone.

When I had the (nice clean) left case set up on the bench, with all the gears installed and cam cover temporarily fitted, I could stick a scriber in the mesh of the breather and easily slide it back and forth....creating a clunking noise.
You may be right. The pressure of the interlocking gears, and the viscosity of the oil could hold it in place.