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Steve asks:
I am a mechanical engineer- I specialize in machine design… close-tolerance design is my specialty. I am looking to buy a used double French horn, but I know that sometimes used means wear… especially on valves. Can a valve be chrome-plated to decrease “slop”? What are normal clearances/tolerances between valve OD and hole ID?
Dave replies:
Steve, the short answer is yes, a valve can be plated to decrease a loss of proper tolerance.
The method used by most technicians does not involve chrome plating. Many different elemental metals and techniques have been used over the years. Valve rebuilds from about a century ago usually used silver plating. These days, valve plating is almost always nickel over copper.
Generally, a fine valve “plate and fit” valve job includes the following steps:
- Repair any damage and fit the rotors closely in the bearings
- Introduce a wide tolerance with a “heavy” lapping compound
- Plate the rotors with a “build up” layer of copper
- Fit the plated rotors by lapping with a “fine” compound
- Finish rotors with a plating of nickel
- Introduce a finish tolerance with “ultra-fine” lapping compound
Exact fineness is dependant on the job and the desired tolerance. Valve rotors are generally considered “tight” if the tolerance between the rotor face and the valve casing is 0.001″ diameter, and if the tolerance between the rotor bearings and the valve casing bushings is half that, or .0005″.
