Author Topic: breather bore  (Read 1055 times)

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Offline poorbiker

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breather bore
« on: December 10, 2013, 01:47:21 PM »
At what point is some scoring in the breather cavity in the case to much, some, a bit more than some or breather valve not smooth when turned, when do you know you should ream it and install a .30 over gear valve....
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Offline Ultrashovel

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Re: breather bore
« Reply #1 on: December 26, 2013, 07:17:11 PM »
At what point is some scoring in the breather cavity in the case to much, some, a bit more than some or breather valve not smooth when turned, when do you know you should ream it and install a .30 over gear valve....

Usually the scoring will be at 90 degrees to the center line of the breather unit. That is, all around the cavity in a circle, usually in one or two places only. You could measure the O.D. of the breather and also the I.D. of the cavity and subtract to get your clearance. It could be done with a dial caliper and an inside spring measuring caliper.

I looked in my manual for Shovelhead which is similar, but only found the spec. for end clearance, .001 to .016. There was no value given for breather gear clearance in it's opening. As a matter of fact, the manual doesn't show any repair for this area of the breather case.  This in only a guess but clearance of more than .003-.005 would probably be too much.

If you have it reamed in a machine shop for an oversized breather gear, they will set it up so that the reamer will be on dead center to the existing cavity. If done incorrectly, there will be improper clearance in the breather gear to cam gear area and it will be noisy. That job definitely needs to be done on a milling machine with a proper reamer and all dialed in to the center.

I'm sure that someone else will know more about this than I do. I've had them score but only in one area and it didn't cause a problem.
« Last Edit: December 26, 2013, 07:19:23 PM by Ultrashovel »