Commando base gasket fitting

In my early Norton days, I used to coat the crankcase edge with a thin layer of Red Hermitite then lay the gasket on top followed by a smear of grease. The Hermitie was to help fill the gaps left by the badly cut gasket that came in the engine sets.

I notice these days that the base gaskets are much thicker and a more accurate shape. So now I smear a very thin layer of silicone on the crankcase edge (to fill any dings and scratches) then fit the gasket and as before top it with a touch of grease to act as a releasing agent. Just in case I have to pull all the bits apart again.

I do believe that the 850 crowd don't have a gaskets in this area any more and just use silicone. Lucky sods.

If you are having base gasket failures then IMO you might have case pressurisation issues. Mine would gradually blow out between the front corner and left side studs. However, that was a Combat spec but with Maney bolt thru' barrels and the front oil pickup. Not enough oil was being returned to the tank which I remedied with well known case mods. Yours will have the rear oil pickup so that should not be an issue but it is worth investigating a non return valve in the breather line. On the actual sealing front I don't see any reason why you can't use a thin smear of high modulus silicone in place of the gasket except pistons will rise higher and top rings may clash with any bore wear ridge present. Compression will go up by approx. 0.3:1. I now have 850 cases and 850 style 750 bore barrels with a compression plate sealed with silicon.

I use Hylomar universal blue for everything. See pic. It is a non-setting sealant so it stays soft for life. I am an aircraft engineer and this is what is used on gas turbine engines. It was developed for Rolls Royce for their gas turbine engines and is used by most manufactures now. It is also ethanol resistant so can be used in fuel systems. The big thing is how to apply it.

What I do is get a 1/2 inch paint brush and cut the bristles about an inch long. Then use this to "dab" the Hylomar on giving a stipple effect. Then turn it over and sit it on a plastic bag and repeat on the other side. It takes a lot longer to apply but there is no risk of putting on too much like when it is smeared on. The stippling also fills any imperfections. then just assemble as normal.

the other good thing about it is because it is non-setting I you can take your time when assembling and you can re-torque an item after the gasket has "settled" which I recommend for thick gaskets, even leave it for 24-48hrs and the sealant will stil be soft and still do it's job... Sealing.

the only downside is it is very sticky so once it has stuck you can't remove the gasket without it tearing.... But with gaskets only a couple of bucks each I think a new gasket is worth it for no leaks.

BTW I hate silicon sealant.... Except in my bathroom...