Aluminium Decompressor Plug
These little beauties are made especially for Totally TTRs from an aluminium blank on a lathe. They are fitted with a Viton O ring to ensure an oil tight and long-lasting seal
The original rubber plug is prone to leaking and seems to have a limited life. I can't guarantee it but these should last a whole lot longer - if not for the life of the bike.
Simply remove the old rubber plug and clean up the socket. Unscrew the retaining bolt. Grease the new plug and just push it fully into the socket by hand - no force needed. Screw the retaining bolt back in (don't forget its copper sealing washer!) and job done. New retaining bolt and washer set available from this shop.
Pictorial fitting guide here