exhaust header size

[Froos]

Is there any difference in the e30 m42, e36 m42 and z3 m44 1.9(?) header tube sizes? Are they interchangable?

[Cristian G in Oz]

They will all bolt up to the head.
As far as interchangeable goes, I would guess it would be what differences there are between models for teh first exhaust joint.

I can't help you with tube sizes though.  I do know the ports on my M44 head were smaller than the ports on my M42 (early E30) head though. I don't know if this has any bearing on the header size though.

Cristian