Norton 850 Mk 1 1974, Service Notes, Page: 20 / 48

GROUP 8:- Primary case and clutch
PRIMARY CASE: There is a commonly held opinion that all Commando primary chain cases leak oil. In
many cases, oil drops from the lowest part of the case--just where the joint is. Sometimes this oil collects
from the rear chain --oil flings from the chain onto the cast cover over the gearbox sprocket-then down the
back of the case. The chain case can leak, though, especially if the wrong number of washers is fitted on
the centre bolt before the inner case is fitted. Or washers left out, the case then buckles anti cracks. leaks
nil especially at the front, the alternator is then off concentricity, touches overheats, burns out etc. 1 know.
, . , So check correct washers when you next take the inner case off, allowing for the gasket as well.
When the new washers or shims are set so that the case is not distorted-you can check on assembly by
offering up the case and trying to rock it, If it rocks about a horizontal axis you have too many washers on
the bolt-if it rocks about a vertical axis you need another washer or two. The washers come in two
thicknesses, thick and thin, and you can got it near enough by combinations-or any old flat }in washer.
Our continental friends can use 10mm washers as long as Ogri isn't looking. If the outer cover of the
chain case is difficult to fit try pulling one of the steel dowels out-but not both as this would enable the
outer to swivel round and mess up the timing scale location. Leaks, if they are (here. can be cured with
our old favorite Silicone clag--R TV. Now for what goes on inside-
The position and adjustment of the shims behind the clutch and alternator is all in the workshop manual,
but they don't tell you about the exploding alternator rotor. Norton dealer Jean Souper at Mantgeron (near
Paris---Jean being French, is a, feller!) has a row of exploded rotors in his workshop,;-there is over a yard
of them stuck over the dour. He is only one of many dealers who could tell the same story, but perhaps
they haven't all got magnetic lintels., What happens is this-the engine as it rotates produces considerable
torsional vibration-not just due to the firing pulses but also the fact that every revolution the pistons come
to a dead halt together and then rush along at about 100 mph together, twice. This is not very good for
alternators 'I he centre of the rotor is made of hexagonal bar, the f magnets are placed round it and then a
grey substance made of melted down carburetors is poured round them. When you shake such an
assembly 14,000 times a minute (twice every rev, see) and whirl it round at the same time, the magnets
can start to fly outwards. The first sign can he a tendency of the strobe mark to wobble about when
strobing the machine. Another symptom is a smear of grey-black dust outwards on the rotor from the back
of the centre washer, If you examine the rotor carefully in this condition you will detect that the magnets
are slightly proud of the aluminum between them----only just enough to feel with a finger nail. Next step is
for the magnets to move further out, and then the rotor makes a noise very similar to failed main bearings.
Just in case any reader has no experience of mains failure (his is a low pitched rumbling noise, the same
under load as on the over-run, and in bad cases audible on kicking-over the motor. Once the rotor has got
this bad it will rapidly use up all the clearance inside the stator, and then seize. This can cause any of the
following:
1. The hexagonal centre can spin free in the rotor, leaving the rest stuck around the inside of the stator by
magnetism. -the red light in the headlight will come on (no charge) but you can get home on what's left in
the battery. Perhaps the real purpose of that red tight is to tell you your rotor has exploded.
2. The rotor can seize in the stator and bring the engine to a sudden halt. You can then only restart by
removing the seized alternator and substitute a spacer to hold the engine sprocket on-but it's worth a try
without the engine nut if the tapers are O.K. and you take it steady.
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