Problems with Piston Valve Action
When I mentioned that I wanted to get suggestions from readers for future content, I should have figured that there might be some difficult issues to address. I've gotten a request for information about problems with piston valve action and what can be done to improve it. I believe that this is a challenge for every brass repairman and what little information is available to us is very spare on specific issues. The major frustration is that even though most players never seem to have valve action problems, there are cases in which there is no solution beyond plating and refitting, which is expensive, when done right.
I'm generally up to a challenge, so I'll do my best here to share what I've learned the hard way over the years. Perhaps it is best to first describe the ideal conditions for piston valve action: In the manufacture of piston valves, the inside of the casings are honed to a perfect cylinder, without any deep scratches, gaps or other changes in diameter. The pistons are ground and/or honed and then lapped for a perfect fit to the casing.
The state of the art today is pistons made of Monel fit to brass casings. Hard nickel plating is at least as good a piston surface, but as long as the valves are well made and maintained properly, the materials are of less importance although one surface should be harder than the other. The clearance between casing and piston on good quality instruments ranges from something approaching .0005" to about .001". Anything more than this is considered a loose fit, although many players are perfectly happy with double or even triple this clearance as long as the action is quick and sure.
Valve oil of an appropriate consistency must be used to assure good action and longevity. There are now available valve oils that range from extremely thin, for the tightest valves, to quite thick for those that were either fit loosely or have worn to that extent. No matter how tight or loose, the valves will only work well, without "hanging up" or otherwise sticking, if the piston is "floating" or "riding" on a film of oil within the casing. This is the same phenomenon that prevents the engine in your car from seizing when you drive it.
In absence of any lubricant, the metal surfaces will tend to stick to each other and in the extreme case will gall, which is when tiny particles from one surface will adhere to the other. Some galling of a microscopic size is normal and you will be able to see the tiny longitudinal scratches developing over the pistons' surfaces and this is rarely the cause of problems with valve action. When it is, larger scratches form and seem to multiply until the valves no longer function.
If the piston or casing are damaged, even very slightly, they will not function well if at all. It is important to assess exactly where the damage is and address only that. Too often the casing is reamed to fit a bent piston or the piston is sanded down to fit a damaged casing. If a bit more thought was applied to the situation, one would realize that it would be better to address the damage than the symptom.
I'm not going to address the repair of these occurances now, but rather the more common cause of problems with valve action. In these cases, the player rarely oils the valves and instead relies on saliva and condensation as a lubricant. This can work quite well for an extended period of time, and many years ago, before reliable valve oils were available, was considered the best lubricant. It was common practice, at the beginning of a session, to remove each piston and spit on it. If you've had much experience with antique brass instruments, you might now understand better why they usually have very loose and leaky valves.
Not only will the minerals in saliva promote corrosion and other electro-chemical reactions, but as it dries, the minerals solidify and become abrasive. At least as damaging are the chemicals secreted by bacteria and mold that thrive inside your instrument, voraciously feeding on cheek cells and food particles that travel with your saliva. If a brass instrument is kept clean throughout and is well lubricated, it will serve you well for decades and even a lifetime.
Being human, good habits are often forgotten and at least 90 percent of cases of unreliable valve action are caused by lack of maintenance. Even with obsessive cleaning and lubrication, any brass instrument that is getting played will deteriorate over time, but at a tiny fraction of the rate of one that is not well maintained.
At this point, you can grab your own instrument that has more than a year of use and observe what I will next be describing. Take out the pistons and bottom caps and carefully set them aside. If the instrument has accumulation of detritus inside, give it a good cleaning and dry it. Thoroughly swab the inside of the casings until they are dry and free of particles.
Holding the instrument up to the light, sight up one casing from the bottom, moving it until you can see the inside wall lit. If there is any damage, you will be able to see high and/or low spots or areas. More likely, you will see pitting to some degree and towards the bottom there will be more pronounced pitting in a ring around the casing. This is where the bottom of the piston sits when at rest. Below that ring, there will be little or no pitting. In the photo to the left, I photographed an advanced case that is easy to see. This is an instrument that has been used for some time without much maintenance and the inside diameter of the casing is larger above that ring than below it, not from mechanical wear, but from corrosion. Every time that the instrument is put away with saliva on the valves, a tiny bit of brass dissolves from the inside wall of the valve casings as mentioned above. The valves may be tight enough to not affect the acoustics of the instrument, but that ring or ridge in your valve casings might be causing the valve action to be unreliable. The ridge in the casing is disturbing the film of lubricant on the surface of the piston, actually scraping the oil off the bottom of the piston as it passes by. This is also why the problem is more pronounced as the piston is pushed down at a slight angle, forcing the piston against the ridge.
Lapping the piston into the casing usually improves the situations to some degree, but when the problem is extensive, a lot of metal will be removed from both the piston and casing before much improvement is achieved, leaving the valves a bit leaky. The best solution is to plate and fit the pistons to the casings that have been just honed true again or fit oversize new pistons. These processes are very expensive and not worth the expense in most cases.
I've developed a technique that I will do my best to relate here that will satisfactorily improve the valve action in at least 80 percent of these cases. It has to be carried out with great care and awareness of what is resulting from each step. I have had to pay Anderson Silver Plating to plate and refit a couple of sets of valves when I went wrong.
A very handy tool, when used properly, is an expanding brass lap, shown in the photo to the left. These are available from industrial suppliers such as McMaster Carr as well as Allied Supply. Get one that is the same diameter as the pistons or larger and turn it down to that size. Apply 600 grit lapping compound and using a very slow motor or by hand, lap just the bottom of the casings. Be gentle and careful to keep the lap centered and not forced at an angle. Concentrate the lapping action on the bottom of the casing, below the ridge that you had observed, with occasional strokes that cover the entire casing. This will not cause excessive leaking or any problems with the action.
If you force it in too tight, the lap and casing will easily gall and stick solid. It can be very difficult to extract the lap if this happens and should be avoided. Expand the lap as needed and continue lapping until you've increased the inside diameter of the bottom of the casing to approximately that of the eroded upper portion. This will be impractical to measure and you will only know how far to go through slow and methodical work and eventually your experience will help guide the process. This is the point where it can go (and has gone) very wrong by lapping a ring or ridge where there was none before. Only careful practice and experience will guide your progress and knowledge of when to stop.
Assuming success: clean out the compound from the casings, apply 900 grit to the pistons and, by hand, lap them in the casings. It can take longer than you might be accustomed to lapping valves because there is too much clearance for the fine compound to quickly smooth away the coarser scratches caused by the previous process. The job is made easier if you make some piston lapping handles, threaded to fit various makes, as shown to the left.
Next, clean out the casings again and change to 1200 grit and lap some more. This last step can be skipped, but it gives it a final polish, without the need for the valves to "break in". Thoroughly clean the grit from inside of the instrument and on the pistons and assemble the instrument as usual. You should find a noticeable improvement in the valve action.