Cut/planed from hard wearing oil resistent rubber (Nitrile), to the same width and thickness as originals, these are fitted both above and below the rocker arm central boss in all SOHC cambox’s. Along with the cork side washers, it is these being worn that normally results in these cambox’s leaking badly.
An original single piece rubber (new old stock from the 1940’s I believe, I bought some many years ago from that great Norton exponent Phil Heath) was used as a template to ensure the correct width. This original type was supplied longer because each one was cut into two, i.e.one rubber for each rocker arm.
As shown in the 1948 catalog, I supply two separate ones of similar length, both of which are slightly longer than required, which can be carefully trimmed to your required depth. (They should be trimmed so they protrude just proud of the cambox slot, so the rocker arm has to be forced – carefully, into the cambox). The smooth end should be abutting the rocker boss and your trimmed end opposite.