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Is there any?

Looking down the barrel and probing with a bent hook there is a slight step from the receiver into the lead pipe. It's not very big, and not very even all round, in fact on one side it is a step down rather then a step up.

What's it all about?

I've seen some info in the web relating to trumpets (sopranissimo tubas?) and the gap between the end of the MP and the start of the LP, but nothing on tubas.

Receiver and Shank Sizes, TubeNet November 2017
Talk to me about AGR, TubeNet April 2018
Gap experts, I need your help, TubeNet May 2018

There's plenty more where that came from.

viewtopic.php?f=2&t=49125&hilit=gap" target="_blank

Just to help me in the thread...

Thanks Don for the useful input.

I'm assuming the following is the "normal" setup for a BBb?



Shown is standard Euro Type MP 13.3mm diameter at the small end and a hole of 11.5mm at that point. The MP is in the receiver 28mm and there is a gap of 12mm to the start of the lead pipe. There is a step to bigger where the MP finishes (13.3-11.5/2) of 0.9mm each side into the remains of the receiver and then a step to smaller into the lead pipe of x, probably less than 0.9mm. The distance between the steps is the gap supposedly 0.50" (12.7mm) being an ideal for BBb.

Obviously there are a myriad of variations, but let's establish a base case for my simple brain.

Is that how yours is? You mention that on one side, if I got it right, it might be more like a step to larger into the leadpipe. If the two were re-fit to be perfectly concentric, possibly they'd be closer in diameter?

I'm just going to assert for an alternative hypothesis, that the design intent here is typically to have the leadpipe and receiver match, at the same interior diameter where they're joined, and any "step" is more accidental than intentional.

They don't match with respect to the other parameter you were asking about last week, though, the taper. The leadpipe narrows to the receiver, and the receiver narrows to the leadpipe. To the extent there's any dependable geometry in this location, that's apparently the key factor - the location and size of this waist.

The picture was not of mine, but I was trying to establish what normal is (based on some of the trumpet stuff and what I read in the links you provided).

On mine the receiver tapers, as you would expect, to approx 13.05mm and the lead pipe tapers in the opposite direction with the join at the narrowest point or "waist" where there is a questionable lip thing which is not the same all round, but the hole approximates to 12.95mm.