If the thread is completley goosed, then you could consider drilling out to 18.5mm and tapping the hole to M20 x 1.5 (M20 electrical conduit tap) then use an electrical M20 to M16 conduit reducer. The 18.5mm drill may not be part of your armoury, so you could just consider filing it out carefully with a round file.
Lapp cables make a nice reducer complete with a sealing o-ring or you could just use a regular brass one and loctite it in permanantly.
You should be able to find the tap and reducer on Fleabay or your local electrical stockist.
