Posted by John Swensen on March 21, 1999 at 20:04:34:
In Reply to: Rotary Valve Bumpers posted by Joseph Felton on March 21, 1999 at 11:58:35:
The easiest way to get synthetic bumper material is to go to an industrial O-ring supplier and ask for an O-ring with the appropriate cross section (it should be slightly larger than the hole in the bumper plate). When the person behind the counter asks what diameter, just say, "oh, three or four inches; it doesn't matter much". You may get one for free (I did). Just cut it to length with a sharp, single-edge razor blade and force it into the hole of the stop block with a screwdriver. Trim the contact area with a razor blade until
the marks on the rotor shafts line up.
As for materials, the standard is neoprene, but silicone is also available, as is Buna N
and a few other materials. Another variable is the firmness (durometer). Normal O-rings are about 70 durometer, but you can get as low as 30 or 40 and as high as 90. 70 seems about right for the down direction, 40 for the up, but you could try several to see what works for you.
For cork, I have heard from horn players that champagne cork is the best; carve it to shape, then gently chew it to moisten and compress it before stuffing into the hole.
Ferees also sells cork in various diameters.