I have a 1994 XJS 2+2 with the AJ6 4.0 liter engine with 51,000 miles. The original cam cover gasket developed a leak just above the exhaust manifold. I ordered a new gasket that on arrival was about an inch short when fitting it to the cover. i replaced the half moon plugs and was able to stretch the gasket and reinstall the cover. after about 50 miles the gasket blew out near the cam chain area. i ordered another gasket (still needed stretching) and put the cover back on torqueing the screws to the required inch pounds. Again it blew out near the same area. What am I doing wrong? The surfaces are clean and i'm not using any gasket sealer or cement. Oil pressure is normal.

 

It's a rubber gasket, so you shouldn't need any sealant. Could it be the supplier? I have changed that set twice without any memorable difficulties or resulting problems. It's not a cheap set though, I remember paying about $50 for it. You're putting the gasket on the cover first, right?
You said it blew out . Was there pressure causing it to squeeze out of place? Or is that just a turn of phrase? There should be a slight vacuum inside the motor....

 

Are you using aftermarket or OEM. I've had trouble with aftermarket camcover gaskets on the AJ16 engine, slightly different than yours. Eurospare, or some other un-marked part is also too large. Paid for the OEM product and the job never went so smooth!