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I don't have anything worthwhile to say about it, but isn't it more likely metric?

Wade.... according to the Allied catalog... that's a 3 x .5mm screw. I would tend to agree but the 'authority' on Allied parts is on vacation for a week.

the elephant wrote:No missing digit here, Dan? 3x.5 (1/5) mm?

Thanks! (What page, by the way? Feeling a bit thick at the moment...)

Page 175 (in the latest catalog) under Miraphone parts. R172 - Cork plate screw (3 x .5mm)

(3mm is the screw diameter. .5mm is the thread pitch.

the elephant wrote:No missing digit here, Dan? 3x.5 (1/5) mm?

3 by 1/2mm. 1/5 would be .2, which would be fine thread indeed.

Rick "whose Miraphone is no handy at the moment to confirm the measurement" Denney

the elephant wrote:Nothing like that at all. These valves must be MUCH older - like very early 1960s. The guy at the screw and bolt place could match neither the screws nor the holes with anything, metric or not. We tried both gauges and screws. Nothing fit at all, so he cannot even get me a tap.

I suppose Miraphone must have used some funky stuff back in the day and moved over to more standardized bits later. They used to advertise that they made all parts in-house including the screws. Maybe they used a non-standard pitch on purpose.

So I will have to fill the holes and then just tap new ones at 4/40. I have nice dome-headed 4-40x.25" screws on hand, but they are brass and not nickel. Guess I will have to torque them down a bit more gently.

THANKS!

It was quite common in Europe up through the 60's to use metric diameters and U.S. customary thread dimensions, because thread-cutting machines were U.S.-bought before and immediately after the war. Screws of that size are usually stamped/formed and not cut, but probably not if Miraphone made them in-house. Miraphone was just a little startup in those days and may well have bought surplus stuff to make their own hardware, which is, after all, a fairly small portion of what they were making.

4-48 is very close to 3mmx0.5mm, by the way. 4-48 is the standard fine thread (UNF) for #4 wire, but it's quite hard to find--I would doubt your local fastener guy would have it.

I think your idea of rethreading makes sense. I would be tempted to enlarge the hole and cut it for a larger thread. If the hole really is something like 4-48, cutting 40 tpi (UNC) threads would possibly leave the threads weak for the cork plate. If it's 3mm in diameter, #4 wire threads might be smallish (at 2.8mm). You might consider enlarging them to 6-40 or 6-32. I've always found that black-oxide-finished steel screws look okay on tubas--I doubt you'll find anything in polishable nickel or stainless.

Rick "who has rethreaded stop arms for 4-40 threads for a loose fit, but only with a screw threaded deeply into the part" Denney

The new Miraphone tubas use a 3-56 threaded screw for that part.