Posted by Rick Denney on March 06, 2003 at 12:53:37:
In Reply to: Lapping posted by Bruce E on March 06, 2003 at 10:55:43:
When a piston is plated, it is plated to be slightly too large for a freshly bored casing. It will fit, but with slight interference that would cause sticking. The repair guy will apply some pumice or fine lapping compound to the piston, and work it in an out of the casing a few times. He'll then thoroughly clean it and test the valve for fit. It may take a number iterations to achieve a perfect fit. Sometimes, new instruments have valves that are a bit tight, and benefit from a bit more hand-lapping. I had to lap the valves on my 621 F tuba twice before they would no longer stick, but Yamaha is known for tight valve tolerances.
Also, the slightly rough surface created by lapping provides better lubrication, because the lubricant has little microscopic crevices to hide in. If it's polished to a shine, the lubricant will not stay put as well.
Sticky valves on a new instrument, however, don't necessarily mean that lapping is needed. A lot of time, it's just the oxide on the inside of the casing, which is initially a little rough and has to be polished in a way that doesn't remove it. The best way to do that is to clean and oil new pistons frequently, and it takes several months at least.
Rick "thinking that the term came from a pitch-lap used to grind and polish mirrors and lenses" Denney