Cheers guys, but rather than wanting to reduce bore size, I need to increase it. I'll try to explain:
Downpipe is double skinned, so can't do much with the i/d, the o/d = 41.4mm.
The connecting tube needs to fit OVER the downpipe, so needs to be, say
42mm i/d, which, in turn means the o/d is the i/d + metal thickness x 2. Assuming it's 1.5mm,
that'd make the o/d of an i/d 42mm tube 45mm.
On the silencer end, the o/d is around 44mm and the i/d is 41mm.
So, how do I get a a i/d 42mm, o/d 45mm pipe fit over or into an i/d 41mm, o/d 44mm silencer.
There are some 'expanded end' items out there which means the i/d of the expanded end is the same as the o/d of the rest of the tube. If I could get a double-ended expanded pipe of 45mm, that should fit over the downpipe and silencer, but I'd have to use a reducer at the downpipe end.
Good bedtime reading is this, just don't show it to Bergs