Making silicone gaskets...

[tkent02]

Here is a way to make some pretty good gaskets for those which are unavailable, it also works great on surfaces with too much damage to seal with stock gaskets. The first step is to put strips of waxed paper on one surface, the side you don't want the gasket to stick to. Takes some time to do, you can stick it down with oil or tape or something but get it flat. The upper side of the paper must be perfectly dry. Put some high temp silicone stuff on the other surface, a smooth layer, thicker than you want the final result. Make sure there are no messed up areas or dried chunks of RTV. I also squirt a similar blob someplace else at the same time so I can tell how dry it is getting without messing up my gasket. Let it set up a little bit, not quite having a skin on it. Put the cover on the engine, use the screws to adjust the thickness of the sealant, get it exactly the thickness of the original gasket or just an RCH more, get the thickness uniform all the way around. Let it dry thoroughly. After it dries, take the cover off, remove the waxed paper and trim the new gasket with an exacto. Leave no strings, flanges, flashes, floppy bits or any other defects that could ever get loose inside the engine, as they will end up in some oil passage and ruin your engine for sure. Pay special attention to the area around any bolt holes, you cannot allow any loose pieces which could come off later. Then put the cover back on, torque the screws correctly, and keep an eye on it. Use locktite if you feel you have to torque the heck out of it to keep the screws tight, as too much clamping force will make it leak.
I would recommend practicing this on something simpler in shape than a valve cover, maybe an ignition cover or clutch cover first.
Been doing this for years to make gaskets that are unavailable on cars, bikes, whatever. Very time consuming but it works.

Also I have seen some old tubes of RTV that won't dry at all, maybe it has been ruined by heat or just age or something? That was in a very hot and dry climate. A month or two later it hadn't dried. Get a new tube rather than use some old stuff you have laying around.