I use an annealed copper washer on my bikes. However if your finding that it won’t compress and it’s leaking then check that the threads haven’t failed on the cover. I had this on one of the covers that have a cast finish.
Now let’s all have a beer
68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)