The Convert winter rebuild is going along nicely but a little problem has raised its head, the original metal seat base is in reasonable condition, slight rust but painted with Hammerite, the problem is the grab rail is mounted with 4 big self tappers, 6.3mm/8mm x 19mm. Have taken out the rusty originals and replaced with stainless ones, but the 2 rear ones now just spin in the holes and do not grab. Anyone had this problem and solved it? No bigger screws seem to be available.
We use jack-nuts for putting threads in thin sheet metal at work. There is a proper tool to close them up but can be done with a nut and bolt.
Have you taken the foam off the steel base?
If you have, could you tack weld some nuts on the inside of the base to screw some 6mm bolts into?
Tack welded a plate to the base, then welded the bracket to that, worked a treat 
i use rivet nuts drill hole put them in and tighten up the tube distorts like pop rivet job don then fit with 6mm bolts
Thats posh Ray 
Jacknut is the same, just a different name.
Foam and cover still on seat, I want to keep the patina, like the sound of jacknuts, off to look on E-Bay…
can send you my kit if you want just return afterwards
Thanks, Ray, have ordered a pack already.
Cheers, Gerry.
if just got inserts best bet place in hole (Needs to be tight) screw in a bolt with loose nut on it then use the loose nut to tighten up and deform the nut till tight in the hole that way it stops the insert turning as it would with just a bolt
Clever!
Exactly how I did it, Ray, grab rail now solid enough to haul bike onto stand, (which I have a habit of doing as rear crash bars are a bit low).
Cheers, Gerry.
brilliant news
Thanks for the wrinkle folks!