I'm reassembling my 2004 XR250 engine. It has a 300cc Thumper Racing oversize sleeve in it. I want to know if it is necessary to put the thin, black O-ring on over the base of the sleeve. I forgot to have the groove cut out to allow it go go back in. I don't think it would hurt anything leaving it out, as there is still the base gasket.

Any feedback would be appreciated.

Thanks,
Doug

Ok Duggy, 14 views and no reply, best someone calm ya a bit.
I believe that you are right, the base gasket should do the job seeing as there is near no crankcase pressure.
If it does leak, you aren't up for much more work when u realise it as opposed to what u are looking at now, u may as well give it a go..
My 2 bobs worth on a Friday night...:crazy:

When I installed my 440 Big Bore, I installed the O-ring. It came with the 440 kit so I installed it. I have about 30 hours on the install, no problems so far. Good Luck.:thumbsup:

rideorberidden said:

i tend to agree, if it were me, i would leave it out and get the bike built so i could ride asap.