when does a rotary valve need to be rebuilt

[dave]

I have an Alex bariton that has worn first and second rotors. i.e. if I pull the tuning slide an inch, then pop the valve, there is no pop, the air has already leaked in around the rotor (I've checked for leaks by blowing thru the leadpipe with one side of the main tuning slide plugged and valves 1 & 2 depressed). Third and fourth valves OTOH give a good pop, even after waiting a few seconds, so they are sealing. In all cases, the rotors are adjusted, the horn has been played several hours so there is plenty of reason to expect a good seal. Intonation suffers using first and second valves in the upper register, more than other Alex bariton's I have played.

Is popping the valve a good test for a rotary valve's seal, or is it pretty meaningless if you don't get a good pop after pulling the slide?

If I was to try and confirm the clearance using plastigage or a micrometer, what is the ideal clearance between rotor and casing for a rotary valve? .001"?

-Dave

[dave]

What shops are equipped to rebuild rotary valves? Is the casing lapped with a precision expanding lap that uses the guide bearings? Reason I asked is that one shop that I asked said they would get the rotors plated then lap them in place. Seems to me that the right way to do it would be to lap the casing using the guide bearings (after rebushing the guide bearings if needed), turn the plated rotor on the lathe to get the clearance right, then lap the last bit. I suspect the casings are worn more on one side than the other, because the top guide bearings are worn on the side closest to the linkage.

Dave

[tubamirum]

It would take a superb machinist to do a rotary valve on a lathe, I'm not positive on Alexanders but usually they are tapered and the plating won't be that thick. Better to lap it to the casing.