[Dirk Van Driver]

I fitted an N55 mid pipe this last weekend, luckily I'd purchased 4 new flanged nuts.
The lower two nuts were pretty much seized after 11K miles and 14 months.
We ended up chiseling them off. The thread looked to be M8 X 1.0 or 1.25 (didn't need a die so didn't establish exactly what it was)
The top two nuts which hadn't been exposed to road salt and water, came off with no problems.

The studs will be a press fit into the downpipe itself (and hence aren't listed as separate part no) possibly with a straight knurl to make them an interference fit.