Norton heavy twin case progression
23 Oct 2001-updated 19 Feb 2007
David Comeau

20M3 atlas magneto cases with bosses at 1, 5, and 9 oclock.... mag chain tensioner does not exist

20M3S early...residue of mag mount bosses...lowered cam tensioner mag tensioner.

20M3S late...06-1537  residue of mag mount one above gasket buffed off...bosses a 11, 3, &8 oclock for failed early starter...
tensioner boss????? at 4-5 oclock with 2 tapped holes is for starter not magneto....starter hole is machined but blanked off on production engines.

72 and 73-750's all used the same "combat breather" cases until the end of production. The early cases had the machining for starters done, but as later 20M3S the hole was blocked off and unused. These early cases had been poorly machined with a sharp cutter as well as being cut to deep into the casting itself. This stress riser contributed to many cracked cases as a result of over revving the "combat state of tune" engines. Note how large the red outlines area is and compare to the later cases (below) to see how much less had been cut. The green line above the rear engine mount is where a typical crack might propogate. This style of finish machining stopped in the 206xxx to 208xxx sn region.

This pix shows where a typical 3/8-1/2" aluminum plate would be welded in to both repair a crack and reinforce the rear of the case. Welding along both red lines along with the plate should give a radiused effect to reduce the chance of cracking.

Later combat breather 212332 cases showing the reduced depth of cut (green lined) leaving the blue lined area with a better radiused finish.

This pix show the reduced area/depth of cut by the black outlines area. You might even be able to see the much more generous radius of the cutter adjacent to the top through stud, evidenced by the lack of a reflected bright line that would be present like in the pix above.

73 combat.... magneto bosses all gone....starter bosses still there...BUT no machining at all performed like 72 combat cases would have.
note 75 e-start cam bushes...extra timing cover  hole next to oil pump is mystery

74-850 all e-start bosses STILL there but rear breather boss added....timing chest breather holes added...extra hole next to oil pump not taped.

75 e-start all prototype e-start bosses gone....only breather remains