Expert: Kenton Scott - 5/20/2005

Question
Dear Kenton,

Do you work with modern instruments also?  I'm learning to repair instruments on my own, have a Yamaha trumpet I'm practicing (repair work) on.  The valves don't work smoothly.  I've honed the casings and now they're very smooth and shiny, but there are little vertical scratches on the pistons, and tiny dots of rust on them.  How do I deal with that?  Also, can you explain the general procedure of valve lapping to me?  (I just ordered a valve lapping block from Allied Supply.)

I appreciate your input very much.

Joe Tein


Answer
Hi Joe,

>Do you work with modern instruments also?

Yes.

>I'm learning to repair instruments on my own

Well, first you need to decide what the problem is. There is nothing that make you feel quite so foolish as working and working trying to fix problem X, when the real problem is Y.  Or worse, yet by trying to fix X you end up doing more damage.

Usually, poorly working valves are the result of some tolerances being too close. Don't forget to check for the easy things first.  Like make sure the stem isn't rubbing on the valve cap, that there is something defective with the spring, or it is just plain dirty.

Then you need to determine whether the problem is in the casing or the valve.  An easy way to check that on modern horns is to swap valves.  A defective valve will show similar probems in each casing and a defective casing will show the same problem with different valves. Also consider the possibility that both the casing and the valve are damaged.  

Do you have the Ferree's catalog?  In there they have casing mandrels and valve sleeves that can help you get casings and valves undented and round again.

The last thing on the list is lapping.  I assume that the block you ordered from Allied is one of those wooden ones with the hinge.  I have one of those too, but have never actually used it.  I think it is best suited to rough work.  

Allied adjustable valve casing laps are good for smoothing out casings, but for the piston, I think simply working in the casing is the best.  

Smear lapping compound on the piston and start working the valve in its casing.  It will be removing material where the tolerances are too close. Just keep working it till you don't feel the drag any more.  I have a dowel rod I will use to push up from the bottom of the casing and I have a piece of flat bar with a hole in the center to put under the valve button, if it is a little to stiff to do comforably otherwise.  Be careful you don't dent the valve with the dowel rod.  (I broke the bottom out of one one time.) [If the valve is that stiff, you better go back a step or two.]

Good luck