trumpets for 40's....

what did they start life as?

*puts hands and fingers in a strange pose* "word"

Yeha they do need a bit of fettling.

Because the tube was lopped off at an angle to get the rise, the angled end obviously then had a bigger diameter

So i used 38mm pipe, and the angled end became 40mm ish once cut, which i then beat out with a hammer and flared it from an oval to a circle again,
so i ended up with a 40mm flare at the base

(does that make sense?)

The only trouble there is, the trumpets are effectively flared at each end to 40mm,
where as the rest is 38mm

I don't know what thats going to do for performance

I think you'll need more of a flair on them for it to be effective, but yours look cooooooool that long!

Did eBay fail you for stuff like that?

I doubt it will affect performance noticeably being 38mm. Don't forget the chokes only go up to 36mm for 40's so you're still bigger than that. The flair is the most important part as I understand it and the most expensive ones are the ones where the flair is rolled back on itself. Try making some as long divergent cones. If you pie-cut some 50mm tube (or bigger!) you can get it to taper from 50mm down to 40mm. To put the flair in you can bash a steel ball into the end. You can get big 60mm solid steel balls off eBay for about £5 and they make perfect flairs in tube.

There is a good section on trumpet design in David Vizard's "Tuning the A-Series engine".