Thanks for the link Guys, I checked yambits and yes oos
it was off the tank for 10 weeks amd shrank in that time about 2.5mm in diameter!! Crazy. I was wondering if it was to soak in a chemical it would expand back to correct size, would like to tey something before waiting for a new one to get back in stock, but i suppose it would take weeks to absorb anything

Have a look at Yamadude's solution in thread #10, worth a try

Yes, it seems a bit odd but I have had to replace the cap on 3 of 4 of my bikes that I have restored. Was kind of stumped on the first on as the threads looked fine on both the cap and the tank. My RT1b was so bad the cap could be removed and replaced without turning it.
Cheers

The one I replaced I had thought maybe it went on a smaller bike like a 100 or 80, I still have it so maybe I'll try the heating and inserting a slightly larger socket into it, can't hurt at this point I guess!

Mine is the same flopps in n out
I had success today though, I fixt it in less than 1 minute
I clamped it in a vise and heated it with a heat gun for 10 seconds then pushed a 15mm socket in, it went in easy and i could see the threads expanding, i heated it again then pushed in a 16mm socket
I then tried it on the tank and its now a nice tight fit on the tank threads. Result, thanks to the Guy who originally did this and saved me a few quid
cheers
Gary

An alcohol and wintergreen (3:1) mix will soften rubber and will induce some minor swelling. Never tried on an oil tank cap, but this mixture worked on my RT-1 carb/air cleaner rubber plenum. Wintergreen is not cheap, but for a rare part might be worth a try. At the time I could not find a plenum so this procedure made my original usable.