Trials Australia Forums

Hi all, I am new here, and have just bought a Bultaco M92 Sherpa t 350, which I have apart to give a freshen up at the moment, and have come across a question.

When I pulled the head and barrel off, the studs were rusted and gritty with general rubbish, which can fall down into the crank case when the barrel is removed.
The source of the rubbish is the gap between the top of the barrel fining and the underside of the head, as the head seals on the spigot of the sleeve, not the top of the barrel as such.
There is nothing to stop this rubbish getting in, as it stands, and I have a "cunning plan", which is the purpose of the question.

Have anybody tried fitting a thin silicon rubber (high temperature) o ring into this gap to stop the rubbish getting in there in the first place, carefully sized of coarse to just seal when the head is tight against the barrel spigot.
One problem I foresee already is if the head to barrel seal leaks slightly, the pressure will push the o ring out until the gas can vent, but as soon as the gas stops the o ring will seal back against the head to barrel joint again, which is the desired outcome after all

Top of barrel, before pulling barrel, you can see the rubbish around the studs.


After barrel pulled, lots of rubbish and rust.


Head, showing the stud and bolt holes outside the sealing surface.


Has anybody had this problem?
Has anybody tried my proposed fix?
Can anybody see a flaw in my reasoning and or fix?

Its the same problem as with an OSSA motor. One solution is to stand the bike up on its rear wheel when you are ready to lift the barrell so the dirt and rust fall outside the crankcase mouth.

I am finally in a position to do the planned fix for this problem.

The local bearing place could only supply viton (high temp) cord in 2mm.


Just a bit over 400 mm is required for about a 25mm overlap.



It wouldn't stay where it was supposed to, so I loosely fitted the head and used a small screw driver to tuck the ends in place.

This is the view from the back, where the overlap is.


This is the front, where most of the cr@p I am trying to stop will be coming from.


I have just realized that he carburettor that the bike came with is too small, an Amal 625 off a 250, not the correct Amal 627, so I now to figure out what to do here before I can proceed further.