Mk.lll cylinder base sealing

I think on a standard engine the pistons will protrude above the barrel by the thickness of the base gasket, if left off. It would be worth checking piston to cylinder head clearance at TDC, as well as valve to piston clearance at full cam lift.

If your going to use plastic gasket, then make sure not to use too much, especially around the small, cylinder head/barrel, oil drain hole.

Martin

I used the base gasket, to no ill effects, when refitting the barrels on my Mk3, might be stating the obvious but be sure the small drain hole lines up! (From memory I'm not sure if it can be put on upside down or not.)

I believe that the old NOC Service Notes referred to the gasket being re-introduced on the Mk3 850 as owners had been filling the drain with plastic gasket ! There is no mention in the official workshop manual. It is listed in the Mk3 parts book but not the earlier 850 listing.

The problem with fitting a base gasket on an 850 Commando is the amount of work required to re-torque the cylinder through-bolts which are not accessible without removing the head (and probably binning a composition head gasket !)

In my experience, the front and rear base nuts will move a certain amount when re-torquing after 100 miles or so and if the head is not removed to access the socket headed through-bolts, they will effectively not be providing any clamping effect.

I go through the procedure of removing the head after 100 miles and re-torque the through-bolts (the head will then need re-torquing after another 100 miles).

If re-boring, you could eliminate the gasket but it would not be a good idea in a worn cylinder as the top ring will contact the wear lip. Mk3 cylinders and rods have the same dimensions as earlier 850 parts so clearance should not be a problem and compression will be increased slightly.

I've never re-torqued the barrel socket head bolts, with no problems. The eye-letted gasket does require a re-torque of the head bolts.

Especially with no gasket at the cylinder base, there's no need to re-torque as there is nothing to get compressed.

When any bolt is correctly torqued down it puts the bolt approx. at the start of the materials elastic phase, so that the bolt can stretch as req, in use, due to component expansion and/or strain, and still retain the original clamping force when the engine is cold and no strain.