Older motorbikes used copper crush washers on their exhaust headers. IIRC they had some sort of fibre inside to make them a bit more crushable(probably asbestos in those days!!).
Maybe something like that inside (although having it disintegrate and go through the turbo would be a concern).
A groove and compression ring (or 2 rings so the gaps didn't align) like some turbo housings have would be great, but finding the right size rings or getting some made up would likely be a pain.
A small groove with a wrap of annealed copper wire might be enough to do the trick. You could wind a circle of wire then flatten it with a hammer/roller to make more of a strip than a round profile. Ideally taper the ends so they overlap. Taper the start of the bore of the turbo housing slightly so the copper compresses as you press the horn in.
How about building the horns up with bronze or a nickel arc rod and machining to a tighter fit. Years ago you used to be able to get shafts built up with flame spraying but I've no idea if that sort of process is still around.
Got to say though, sounds like an issue with your turbo housing since you've tried other horns with same result.
Regardless of the manufacturer saying they've never had the issue and can't help, I'd be asking them to provide the dimension and tolerance of that bore. If yours doesn't meet that dimension I'd say its pretty cut and dried that its a manufacturing defect.
Steve
1985 County - Isuzu 4bd1 with HX30W turbo, LT95, 255/85-16 KM2's
1988 120 with rust and potential
1999 300tdi 130 single cab - "stock as bro"
2003 D2a Td5 - the boss's daily drive
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