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I have come across 2 BMB F tubas on which the alignment marks on the bearing plate of the rotor are wrong. On both of them, the mark only ever lines up with one of the spindle marks when the valve is pressed. I have no idea how accurate it is (need to pull out the borescope.

Check it out: https://www.dropbox.com/sh/flhxkocc3088 ... 024-a?dl=0" target="_blank

Has anybody else seen this? Can people with these check? Is it all of the Fs? The CCs too??

Must not be worded well. Check out the pics - when the valve is open, neither of the two spindle marks line up with the alignment mark on the bearing plate. So the only way to trim that bumper to the right size would be to use a borescope (unless somebody else knows some third way?)

Oh yeah, I see how that could be interpreted now. But I mean that when not pressing the valve, the mark that would be used to set that bumper is 90 degrees from the alignment mark.

Kind of difficult to digest the images but could those back plates be switched?

That being said... I've seen all sorts of crap on the cheap imports. That's why I rely only on a borescope and often grind the factory marks off and make them right.

Hi Dan. I have both the F and CC BMBs and no problem with the general orientation of the valves/valve marks. But for maximum allignment I would not trust them without an optical tool to corroborate how close they actually are. I suppose once you do get a precise allignment you could scribe new marks, but they would be VERY close. Both 5th valve rotors offer a very full sound so I am trusting they were alligned rather well from the get go. I did replace the bumpers so did need to reevaluate things. As you know, basically the rotor should turn exactly 90 degrees, and that should be able to be observed relatively easily, so remarking the back plate, if necessary, should not be to difficult. I have seen some Chinese rotors that were 90 or 180 degrees off. As long as the rotor is aligned, of course, it makes little difference what the scribe marks say.

Rotate you back plate 90 degrees clockwise?... Would anyone more knowledgeable see an issue with rotating the back plate 90 degrees on a presumably new horn then just remark the outer edge of the plate? Assuming that they're aligned currently.
Also, Paul, your signature should be adjusted... you're now a 4 valves! ... someday I'll get that good.

Of all the horns we have sold i have only seen it once or twice so its not very common. Once you get the bumper cord installed and trimmed properly you can easily make the correct hash mark with a small file. I would not recommend rotating the back plate 90 degrees because that is less precise. How would you really know how accurate the rotation is unless you take careful measurements each time you remove it, more of a pain in the butt. If you are going to make a new alignment mark do it now and dont worry about it after that. Pretty sure, about 99.99999%, these are made by JinBao. I have seen this a few times, mostly when a new model is just released but usually corrected pretty quickly, should have been caught during production. Havent seen it on horns that have been in production for any lenght of time so it this is on a more recent horn then, yea, should have been noticed.

If you install 3/16th size bumper cord there is generally no trimming if they use the same stop arm as on most other tubas they make. Using 1/4 cork would need trimming and scoping for this horn and the 1/4 neoprene may be too thick to install.

toakstertuba wrote:Rotate you back plate 90 degrees clockwise?... Would anyone more knowledgeable see an issue with rotating the back plate 90 degrees on a presumably new horn then just remark the outer edge of the plate? Assuming that they're aligned currently.
Also, Paul, your signature should be adjusted... you're now a 4 valves! ... someday I'll get that good.

It would be easier long term to make a new hash mark. All thats needed is either 1 new mark 90 degrees on the valve stem or a new mark 90 degrees on the back plate. My choice would be 90 degrees counter clockwise from the original mark on the back plate so you dont risk putting a burr on the bearing.