Primary chaincase seal for Garden gate Inter

Maybe it has gone hard?  Wipe it over with olive oil and pop in a plastic bag overnight. Wash it off in hot water and soap before you fit.  One edge should be radiused which fits into the radius of the joddle of the outer case.  

Personally I use Evostick on the vertical face of the inner case. Use normal practice of sanding the rubber, coating both surfaces and let it dry. Try and keep it warm.  Hold it in place by temporarily fitting the outer to evenly press the rubber against the inner case. Hold it in place with long zip ties or ratchet clamps. Strips of rubber or carpet under protects the finish.

Next day, remove the outer and use heavy grease in the joddle of the outer.  Warm the rubber/inner face with a hair drier. Then fit the outer, using the zip ties/clamps to ease the cases together until you can get the nut and washer on, then between the three complete the closing.
 Leave it another day before removing the temp fixings.

Minor imperfections in the case joint should be managed by the softness of the rubber. And when it comes to removal, it should part at the grease joint and not the glued side.  

j

I have a similar problem with a 16H.  I had to mount the fixing nut on a mandrel and turn half its thickness away on the lathe.  I even wondered if the inner case is set too far out with spacing washers from the crankcase but it can't be that because it's very difficult to remove the magneto cap as it is.
On another bike I use foam 'backer bar' strip.  Sold for filling behind silicone sealant in movement joints in buildings.  Fills uneven joints but messy to use because it's soft and slippery when oily.

Closed cell neoprene strip, can be bought in the width and thickness you need from many rubber suppliers. 

... it is probably stock "rubber".  The chamfer is important to get the fit in the "joddle" of the cover.  I think my approach would be to cut the rubber, lay it out on a bench and sand the edge to the profile of the cover.  It needs to be pretty consistent profile, (disk sander and a distance piece to control the shape would help).  Once complete, superglue the ends back together.

If you then increase the plasticity by one of the methods above and get to the point you can get the nut on.  Dry Fit it with the nut on an see how many threads you can get now. assist the nut by using clamps or straps.  If you are happy glue and grease as before. Once on leave it a day leave it a day or so then go again putting another turn on the nut. 

if all else fails fails go with Ashley's suggestion of closed cell neoprene. Much more compressible. But use silicone grease to seal the outer as neoprene will break down with mineral oil based grease.
cheers
 

The early models had a narrower width seal than the slimline models. The width is 16mm with a 1 in 20 taper from one edge to the other.

I have used this seal on my plunger framed bikes (Model 7 and ES2) and also on my WD bikes and the outer chaincase fits easily and the nut tightens to show about 2 threads on the spacer.

Howard, I have just done a trial assembly on my 1949 Inter with similar results.

I use the closed cell neoprene strip, it works well, but important to get an oil resistant one.
Dan