1972 Engine Changes

In his book, Norton Twin Restoration, under Crankcase Types

"The whole assembly was altered for 1972 when the breather went to the base of the cases and internal webs were added.At the same time the threads changed to the Unified form. For the 750, in 1973, the distributor-drive hole was filled in, the blanking plate and fixing holes being deleted."

Unfortunately, he does not quote the relevant engine#.

Well it probably happened before 206XXX as my cases in that range are solid.

The Atlantic Green Norton page is down but that shows that change and others made during 72 production such as removal of a stress riser at the rear of the cases. There will be a 3 digit number stamped on the bottom of both case halves, if they match then the cases were machined as one set. Mine are matching so the 206XXX is valid for both halves.

Roy Bacon's books are written from Press Releases and so mid year changes already made are announced at model year changes to increase the perceived improvements of the model year change. Dave Comeau's Atlantic Green site information is based on actual engines.