There is only one locating dowel for the cover (hope that's right!) so keeping the gasket in place whilst fitting a gooey cover will be interesting.
Me again Brian, I've amended my chaincase diagram to indicate the dowel positions by blue circles.
I always make my own gaskets. I buy Flexoid gasket paper, which comes in various thicknesses. Bought some last year 2m x 1/2m x 0.8mm thick for making cover gaskets. It cost £14 including postage.
This is a bit long winded but:-
My method is to lay the cover on the gasket paper and push down on the dowels to mark their positions on the paper. Then draw around the shape of the cover. Cut around the shape, about 1/2" or so away from the drawn outline and then cut or punch the dowel holes out. Lay the paper on the cover and fit the holes over the dowels to fix it in position. With a small ball peen hammer carefully tap on all the screw holes in turn and with a few taps it will cut through the gasket paper. The little hole pieces cut through with the hammer often sink (some don't get completely cut out) into the holes in the cover and this helps to hold the paper firmly in position to do the remainder. Then tap around the outside and inside edges of the cover to cut out the shape of the cover accurately. Nip out any holes not completely cut out. You'll then have a perfect gasket and usually better than the ones you buy.
I then prepare the cover for fitting by carefully spreading a thin layer of Blue Hylomer on the side of the gasket that goes on the cover mating surface and place the gasket on it. Then do the same on the exposed (dry) side of the gasket. Smear a thin layer of grease on the inner chaincase metal mating surface and fit the outer cover and loosely fit all the screws until the joint is closed but not tight. Leave it for a bit (have a cup of tea) to allow the gasket cement to thicken (Blue Hylomer never dries solid) and then tighten the screws. Job done. I never get a leak from the chaincase joint - only a tiny amount from the drain plug sometimes, even when I fit new fibre washers
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The grease allows you to remove the cover easily again with the gasket undamaged and still stuck to the cover
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A mechanic friend of mine showed me how to do this about 40 years ago when I used to rebuild my classic car engines. It was in the days when they often made their own gaskets in garages.