Re: Lapping of Valves


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Posted by Chris B on December 16, 2003 at 15:41:44:

In Reply to: Lapping of Valves posted by Jim on December 11, 2003 at 23:55:22:

When an instrument is made, the valve casings and valves usually are made to be very tight. Lapping is the process where a special compound (approx. 1200 grit) is put on the valve and it is inserted in the casing and the valve is moved the length of its throw repeatedly to remove a very small amount of material and insure a hand fit with very close tolerances. Most top line horns are hand lapped this way. Sometimes a new horn is so tight that it takes a bit more lapping than the factory did. There are lapping compounds that are grease based and there are those that are water based. Usually, the grease based is used. Once the valve has been lapped, the compound is reomoved by wiping and cleaning with a suitable solvent. Often the horn is flushed out as well to be sure to remove all the compound.
The process removes small imperfections and creates a custom fit between each valve and its casing. Once the valve is cleaned and re-oiled, it is tested. If it is still too tight, the process is repeated.
I have seen suggestions about using toothpaste and all sorts of other strange materials to lap valves. It is important to search out the proper stuff. The valve lapping compound found in an auto store is way too coarse and is designed for automobile engine valves, which are also lapped to ensure a tight fit in the head and to make sure there is a seal inside the piston.



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