On my trip to the SRS the plug on the front down pipe parted company on the M90. A trip to Halfrauds and a piece of aluminium and a jubilee clip fixed the blowing. I have been trying to figure out the thread and took out the rear one. After a bit of measuring and a trawl through t'interweb I bought a brass replacement.
Thread is, wait for it, 1/8" BSP...

Easily obtained as used on lots of things...
I'll buy a few as spares!