This is the gasket fitted to tappet covers on all OHV models (i.e. Model 18, ES2, Model 19) between 1938 - 1947. Manufactured in cork gasket material.
In pre-war catalogs it was shown as Part Number 8899 and postwar I believe it was called A3/95 (although post war Spare Parts Catalogs showing this type of engine were rare, as it was replaced with the later OHV design in 1948.
Although the tin plate 'Norton' inscribed tappet cover looks the same on both models - the gasket for this earlier engine is subtly different - it is an oval only, with the inner cork segment surrounding the cheesehead screw holes. This difference is important, as if the later gasket was used - once worn, the inner parts could break off and enter the rockerbox.
This gasket is manufactured for us in good quality oil resistent cork gasket material by a well established UK gasket manufacturer - but please note, as it would be uneconomical to have just this (quite uncommon) gasket made on its own - we take the other gasket and then carefully trim away the excess to turn it into the earlier version in-house.
These are one of the most common leakage areas on this type of engine (probably because of the engine breathing or the pressed steel cover). Although a new cork gasket like this mostly alleviates the leakage .. . when I was riding one of these models on the road regularly, I also use to smear a light layer of silicone gasket liquid over the cork gasket as well - which seem to make them more oiltight. It was also common to find the pressed steel cover had its screw holes indented through overtightening the screws. Turning the cover over on a flat hard surface and carefully punching the holes back flat with an alloy drift would greatly help the issue.