Posted by Rick Denney on May 01, 2001 at 14:48:47:
In Reply to: Valve venting? posted by Dan M on May 01, 2001 at 14:15:35:
Venting valves provides access to free air from valve tubing when it is unused. With piston valves, this requires machining a small hole in the piston opposite the port opening into the valve tubing. When the piston is up, the valve tubing is vented to the outside air through the barrel of the piston.
With rotary valves, a small hole is drilled in the casing between the two ports opening into the valve tubing. This is the side of the casing that points at your valve hand. Most (all?) rotors have an opening between the two ports that exposes this hole when the valve is not in use.
Pros:
-Slides can be manipulated (for tuning) or pulled (for draining) without building pressure or vacuum.
-Pressure caused by normal playing can be relieved without pressing the valves.
-Water drains from vented tubing more effectively when the water key is opened.
Cons:
-Poorly plated pistons might delaminate from the bored hole. This isn't a problem for pistons of solid material such as monel or stainless steel.
-Dirt has an entry into the valve tubing.
-Vent holes in rotary casings can get really close to the ports when the valve tubing is being used, resulting in leaky valves.
-Drill bits and fragile pistons are no combination for the unpracticed hand.
As to how much it will cost, I'll let you know. I'm having Matt Walters vent the valves on my York Master in a couple of weeks. I did it myself on my Yamaha for the cost of a Dremel cutoff wheel (I made a slot instead of a hole) and the experience not to screw it up. The latter will cost either in experience, or in nervous anticipation--and if neither of those costs apply--in a new piston.
Rick "who believes the pros outweigh the cons" Denney