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Presuming that the measurements are for the purpose of selling an instrument, are there any "standard" guidelines for measuring the bore on an instrument? Do you measure right at the entrance to the leadpipe? If the instrument has a graduated bore, where do you measure for each of the valve tubings? Do you pull out the inner tuning slide, and measure on that (where the bore would be the smallest)?

Not having any equipment, is is accurate enough just to use a measuring tape?

Man, that has started some arguments in the past.

I think you can best measure the bore size at the inside diameter on any of the valve slides. If there is a graduated bore in the valves, you'll find it there. I have measured a Miraphone 186 had valve slides inside diameter at .770" which is the stated bore size.

The leadpipe is not the normal place to determine bore size. Measuring at the inside bore of the main tuning slide might be deceptive because there usually has been some tapering of the bore by then.

If you consult consult a catalogue, you will know what the factory specs were and that might help guide you.

Thanks for the information. The instrument I'm wanting to measure is long out of production, and I do not believe any factory specs still exist (if they even ever did).

Since graduated bore usually kicks in after the first 3 valves, the convention usually is to measure the bore as the inside diameter of the 2nd vale tuning slide.

"Not having any equipment, is is accurate enough just to use a measuring tape?"

My first choice would be to use calipers.
http://www.use-enco.com/CGI/INPDFF?PMPA ... M=890-9374

If you don't have access to calipers, a SS ruler might get you within 0.002" if your eyes are good.
http://www.use-enco.com/CGI/INPDFF?PMPAGE=241&PMCTLG=00

I went through the same situation when I tried to sell my little Conn last year. Here's the picture I took of the 2nd valve slide. For some people this still wasn't enough...