TZ350 Yamaha's use a Teflon ring in the same manner as you have described.

I cannot remember if the later model250 used it.

I believe that I still have some in the workshop and will measure them later today for you and post the inside and outside diameters here if it helps anyone.

Be warned that they are seriously not cheap to buy at £10 each back in 1992 when I last bought them for my old TZ350G….. God knows how much they are over 16 years later……lol

Cheers Rob