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I own a Conn helicon dated 1929. It looks similar to a Conn sousaphone of that era (model 14 or 32K?). My problem is the number 2 and 3 upper slide knuckes on this helicon are badly dented. Possible red rot is prevented them from being repaired. Is there any interchangeability of parts from the newer sousaphone models to the older helicons?

Check the bore size (at the second valve), and then compare it to the newer Conn sizes. My own 1924 Conn Eb helicon has a smaller bore than my 1952 Conn Eb sousaphone. I know that Conn actually had several different size helicons and sousaphones, so I think that the answer may depend on the bore. My helicon does not have any model number stamped in it, so if yours is like mine, you'll have to measure to find out.

Does it make a difference if the port on the piston is measured, or the port on the valve casing? Would a digital caliper of the $30-$40 variety do the job?

Alex C wrote:I would take the bore at the second valve as a good representation of the correct bore size.

I usually measure all of the valve slides, just for accuracy. Ports on valves are almost never in one plane or at least are hard to measure in one plane accurately. When I have compared valve tubing bore size with factory specs, I end up with the same numbers consistantly.

That is, take the second valve slide out, and measure the size of the inside of the inner slide section of that slide. (and its a whole lot easier to move around the a helicon...)

I suggest looking at the archives, but a search on "bore" AND "size" AND "measurement" brings up a lot of results...

Chuck(G) wrote:A shotgun bore gauge is inexpensive and works well for most piston tubas (up to about 0.775").

Sounds reasonable, but I've never used one, so I don't know much about it.

There is some explanation of Conn bores and Conn sousaphone models at the Conn Loyalist website.

Any relatively accurate caliper should do, as the difference from bore to bore is larger than any inherent error the caliper may have. And as usual, what you're looking for is a match, not necessarily a specific number, so the numbers are relative anyway.

Thanks everyone!

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