Lapping Pistons at Home (Caution)

[Rick Denney]

Here's the problem with power drills: They are heavy, and the valve stem is brass and easily deformed. If you hold the drill at even a small angle, it will fatigue the stem quickly and could also strip the threads where it screws into the pistons. If the drill gets away from you at all, it will break the stem out of the piston, which may also deform the top of the piston. The risk seems to me quite significant, even with me doing it, and I am not short of experience with tools and the use of my hands.

But there's a bigger problem. In order for lapping the piston with rotation to work, the casing must be perfectly round. If it isn't, then you will be making the piston exactly round, but at a diameter that fits within the space made by the casing. Only freshly plated pistons in freshly bored valve casings are lapped, for this reason. I have lapped new Yamaha valves that were too tight and got sticky in hot weather because of thermal expansion, but the instrument was still quite new and the casings dependably round and straight. I used Lava soap, and worked in very small increments. I also worked under the supervision of a qualified repair tech.

That said, I would NEVER lap valves by spinning the valve. I lap them by pushing them in and out--that's the path they take in use. If the casing isn't perfectly round, then I want the piston to be not perfectly round, too, but to still fit in the casing. The point of lapping is to mate the piston to the casing, and this is done by moving the valve during lapping the same way it will be moved in use.

Dan Oberloh made an important point: You should know what's wrong before attempting a repair. Is the valve sticky because it's not straight? Not round? Dirty? Is it the casing or the piston? If it's the casing, is it oblique wear or external stress? Have you measured the piston diameter in enough places to know whether the piston is round? Have you measured the casing likewise (this is much harder to do)? Have you checked that the piston and casing are straight? If you are going to bore the casings, and replate the pistons and fit them fresh, then maybe it doesn't matter unless something is really damaged. But these faults cannot be fixed by lapping, without increasing the size of the gaps where the pistons and casings DON'T interfere. And doing that can lead to leaks.

I agree with Dan Schultz--if you want to experiment on a $200 beater, then there's no harm if you accidentally fling the piston against the wall and bend it. But on an instrument one actually plays? The way you presented this suggests that it is easier and less risky than it really is, and that is why you received a lot of debate on the matter.

Rick "not seeing power drills lying around good repair shops--for a reason" Denney

[Kenton]

The trick is in the diagnosis - and knowing how to resolve the problem. If you know the order of possibilities from the least intrusive and most common to the most intrusive and least common; and what the remedy is for each, then a repair should be fairly safely done.

The problem comes in when only some of the causes are known and only some of the repair techniques are known. Then the chance for error are increased markedly.

[iiipopes]

The trick is in the diagnosis - and knowing how to resolve the problem. If you know the order of possibilities from the least intrusive and most common to the most intrusive and least common; and what the remedy is for each, then a repair should be fairly safely done.

The problem comes in when only some of the causes are known and only some of the repair techniques are known. Then the chance for error are increased markedly.

Bingo. The same reason we go to physicians for accurate diagnosis of medical conditions. A fever can mean anything from the flu, to strange illnesses that can kill you in a couple of hours without the proper diagnosis. Likewise, a sticky valve can be dried beer from the Oktoberfest gig that just needs cleaning, to serious deformities that may not be apparent to the untrained, or only partially trained or relatively inexperienced eye.

[windshieldbug]

Likewise, a sticky valve can be dried beer from the Oktoberfest gig that just needs cleaning

That beer can dry there!? You mean I've been spinning my leaky valves all these years with a power drill for no reason!?!?

[Dan Schultz]

Likewise, a sticky valve can be dried beer from the Oktoberfest gig that just needs cleaning

That beer can dry there!? You mean I've been spinning my leaky valves all these years with a power drill for no reason!?!?

Beer residue is a minor problem compared to bratwurst and sauerkraut. Rotary valves are A LOT more forgiving when it comes to passing kraut!

soooo.... Pistons for Dixieland music. Rotors for German music!

[iiipopes]

I knew you guys would get a kick out of that one. Personal experience from a picnic gig last August.

[sungfw]

Rick "not seeing power drills lying around good repair shops--for a reason" Denney

Hmm ...

[windshieldbug]

"not seeing power drills lying around good repair shops--for a reason"

...ah, because they're all in use!?

[Rick Denney]

...but I lapped mine successfully and didn’t commit any of the disastrous damage that has been warned of by others in several of the preceeding posts.

Here's the problem with risky practices: Sometimes they work, and sometimes they don't. That's why they are described as risky, and not as flat-out wrong. Your sample of one may represent a low-risk method, a high-risk method with appropriate common sense, or a high-risk method with a good dose of luck. A sample of one does not reveal which of these possibilities ruled the day.

I am not a repair technician. But being self-taught with a range of tools has resulted in making many of the possible mistakes one can make that don't result in loss of body parts. For (entertaining) example, I tried to drill a hole through carpet recently. The drill was turning at a very low speed, and I was being a careful as possible. In about one quarter of a second, without me realizing it, the drill bit pulled a thread and unraveled about a 3-inch-round patch of the carpet and wrapped it into a ball around the drill bit. Most people with common sense (but lacking specific experience) wouldn't have seen that coming--I didn't.

A power drill at low speed has a lot of torque. If the valve binds (and it will as soon as the toothpaste dries, which it will perhaps even before one realizes it), the drill has the power to pry itself out of the user's hand (at which point it's heavy enough to bend or break the valve stem), or, if the user has big and strong hands like I do, to tear the stem off the valve. Just because it didn't happen to you doesn't mean that common sense is enough to prevent it, or that it isn't a significant risk. Frankly, what you describe violated my own common sense, and who's to say your common sense, or the common sense you assume of your (often quite inexperienced) readers, is better than mine?

Most people don't realize that valves and valve casings lose their roundness as they wear. If they don't know that, then they won't know that spinning the valve in the case is not always a good idea. When Dan or any other qualified technician laps in a valve by spinning it, I assure you that they have already tested the roundness, and that they have also already considered their reasons for taking that approach.

Do not break the rules before learning them.

I am a proponent of home repairs, having, for example, long advocated the use of Du-Bro ball links for rotary valve linkages. I even created a web page about it. The most powerful power tool I used was a Dremel tool with a cut-off wheel or a small drill bit. Most times when I drill a hole in a tuba, though, I use a pin vise and do it by hand. The pros use a lathe, but owning and knowing how to use a lathe are far beyond the skills of the type of person to whom you made your recommendation.

If you want to spin a valve in the casing using a safer method (after you are sure that spinning it is a sensible thing to do), then insert a tight-fitting rubber hose over the stem, and spin it between your hands as you rub them back and forth. If it binds, the hose will slip or wind up instead of tearing something apart. And you won't be able to spin it fast enough to make anything hot. That's how I was trained to lap in intake valves on an engine, for example. We never used power drills for those, either, and those are made of steel.

Actually, my repair tech did indeed suggest the use of Lava soap if I needed to lap in my tuning slides if they proved too stiff for my taste. But that recommendation was made on the basis of two facts: 1.) The tech knows me and knows my capabilities (and their limitations), and 2.) the tech KNOWS that the slides are round and perfectly aligned, which are an essential requirement before lapping can do anything but harm. He DID NOT suggest that I lap the valves, though he did warn me that his valve jobs were tight and would need frequent oiling and good cleanliness to avoid sticking. I observe his warning and they do not stick.

Rick "who has learned many lessons the hard way" Denney

[Rick Denney]

As this has been going on for several days in a row now, I am beginning to tire from the process, but before completing my case for home repairs, I’d like to point out more of what I believe to be inaccuracies that are still being presented for the reader to consider.

I just erased my long response, which I should not have written at all.

If you want to chuck up a soft brass valve stem in a power drill, be my guest--you know your own skills and limitations best. Others have seen my cautions, and can judge for themselves. I think it was apparent, for example, that I wasn't basing my caution on the use of the trigger lock. By the way, a small sample doesn't cause chance, but may reflect it. That's the problem with small samples. But you knew that.

Keep contributing, but please don't take it personally when people express more caution than you concerning what may be hidden risks.

Rick "wondering where the accusation of emotional attack came from" Denney

[sungfw]

As this has been going on for several days in a row now, I am beginning to tire from the process, but before completing my case for home repairs, I’d like to point out more of what I believe to be inaccuracies that are still being presented for the reader to consider.

Sorry, but the ONLY inaccuracy I've found in this thread is your minimizing or outright dismissing the risks inherent in the procedure. Every risk identified by other posters is a real risk. Granted, there are ways of minimizing or mitigating the risks, but the risks remain real.

Yes, I did, in fact, lap a single valve on my euph using toothpaste and a power drill, and GOT AWAY WITH IT, but:

1) I took a calculated risk, recognizing full well that I stood a very good chance of screwing up the piston, the valve casing, or both;

2) both techs who had previously worked on the valve assured me that it was not bent or damaged, and that the casing was not out of round to a meaningful degree;

3) I have access, through a friend, to a metalworking shop that has the equipment to ensure that—barring catastrophic failure of the equipment—the drill could not pitch, yaw, or roll, that piston was precisely aligned with the valve casing, and that it remained aligned throughout the lapping procedure;

4) the fix is "good enough" for the purposes for which I use the horn (Christmas Tubas, Red Kettle Band, etc.);

had the horn been my only horn or my primary horn, or had (2) or (3) not been the case, or had I not known that I could get a new replacement valve piston and valve block for a reasonable price, NOTHING could have induced me to take the risk.

[J.c. Sherman]

To all of this, I will once again say:

Brass is soft - that's the reason they make brass instruments out of it.

Anything you do to it and around it changes it. It's alive, it moves, it changes. Any repair is a risk. You can take chances if you want. But I (and others) don't think the risk is a good one. Some of us have experience to back that up.

Brass is soft. Take whatever risks you wish or think you can afford. I and others are here to take you money when things don't work out.



J.c.S.

[windshieldbug]