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 boss...no
mag tensioner.
20M3S late...06-1537 residue of mag mount bosses..top 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
