TubaForum.net

Ok, I've got the trueing tool, but have always wondered if I'm doing it correctly. Because let's be honest, I'm not really dropping my mouthpieces, just occasionally buying one with a ding in the shank.

To fix it, do you just kind of jam it in and twist by hand? Or do you need a small mallet to tap it in, and to tap the outside of the shank to get it round again? Or "whatever works"?

I should know the answer already, I know... But again, I'm careful with my stuff and just never need to do it.

(I bought a used mpc with a dinged shank, so now I need to know. I've been off of buying used mouthpieces for a while now, but it's a Laskey, so... )

Jamming it in (too far or too hard) can cause the end of the shank to "bell" or even split. I push the truing tool in as far as I feel safe, then use a hard mallet around the sides. Eventually it fits snugly without spreading the shank. Takes a while but gets a good result. Some of the dings on my mouthpieces over time were "bought"; some were self-inflicted.

What JPerry1466 said: insert the truing tool until snug, tap the tight spot on the shank with a small leather mallet while applying a little bit of firm support to the truing tool; repeat until round. Don't overdo it.

I've never been able to get a dinged mouthpiece absolutely round. I can get pretty close, but at some point, continued tapping doesn't provide any improvement.

Maybe I'm close enough that insertion into the receiver will round out any remaining lack of concentricity. But there's no way for me to know if this is true or not . . . or, in fact, if it matters.

Hup

I focus more on tapping down the high spots than pushing out the dent. Like the other guys said, you don't want to push it in too far and possibly over-expand or crack the shank.

I don't mean to contradict, but my experiences are slightly different.

Since the truing tool's taper is so gentle, it's somewhat difficult to over-expand a mouthpiece exit using it.

Being lazy, I've repaired hundreds of mouthpieces without going to get that thing, so (sure) I had to be careful using "whatever" (including phillips screwdrivers, dent balls, needle-nosed pliers, etc...The method isn't it...It's the results, yes?)

Sometimes a mallet might help to tap on DOWN on the end of a mouthpiece against an inserted truing tool, as there might be a particularly sharp crease (worse-than-normal damage) encountered.
To avoid marking up mouthpieces where that seems to be called for, I use hard plastic mallets (though it takes longer than when using a regular steel dent hammer.

...so this:

The more acute the angle (say: the bottom of a plumb-bob) OR cylindrical (back to the phillips screwdriver) the angle of the insertion tool, the more the risk of over-expansion, and (as the angle of those truing tools is ideal) the risk of over-expansion is low.

bloke "...or not"

Most of my 35 year career as a band director, I was in a couple very rural schools that may see a music store rep every 3-4 weeks, so I had to do redneck engineering many times. I found that a trombone mouthpiece was good to true a tuba mouthpiece, a trumpet mouthpiece was good to true a trombone mpc, and a french horn mpc was good to true a trumpet mpc. I'd use whatever I had to make a FH mpc better. Just a fix from the trenches.

Bring it over, I'll fix it for you.

I just use a French horn mouthpiece.