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Am I right in thinking the factory bore of Holton 345 Bb is 19mm through 1-3 and 20mm in the 4th; same as the current B&S/Meinl Weston 'big' blocks?

I've heard of folks shimming the 4th slides to bring the bore down presumably to 19mm or so. Supposedly improving the 4th register response right?

Question is, is there a concensus on preferred bore through the 4th valve on these big tubas? I'm putting one together and have a choice in 4th valves...

Thanks for any advice!
MP

Peach wrote:Am I right in thinking the factory bore of Holton 345 Bb is 19mm through 1-3 and 20mm in the 4th; same as the current B&S/Meinl Weston 'big' blocks?

It's .750" in the first three valves and .810 in the fourth valve, though the valve itself is .750" bore. That's more like 20.5 mm in the fourth valve.

As to whether it's a good idea, one well-known pro sleeved the fourth valve of his Holton to make it a bit easier to blow. But many others do not. I would put that in the "matter of opinion and personal preference" category. Mine is not sleeved, but I've never tried it.

Rick "thinking nobody at Holton could spell 'millimeter'" Denney