Norton 850 Mk 1 1974, Service Notes, Page: 15 / 48

GROUP 4:- Crankcases, breathing and main bearings
CRANKCASES: In comparison with the fairly clever design of the Norton head, the crankcases are well,
not to put too blunt a point on it, unsophisticated. It has been said that they only serve to keep the oil in
and they're not very good at that!
Anyone who has read "Tuning for Speed" will remember that bosses on crankcases ought to go all the
way across, not just halfway, even when the engine is not used as a stress-bearing part of the structure.
On the Commando, there are two such bosses at the front, where the isolastic bolts on, and three at the
back, which hold the gearbox cradle (and an assortment of shorter bosses and stud holes, all of different
lengths and not one going right across!).
The three back bolts simply hold the gearbox cradle, but when you think about it, you will realize that they
take all the pull from the primary chain when driving and on the over-run and they take the load when you
put the back brake on. These forces combine with those from the out-of-balance of the engine (which also
constantly change in magnitude and direction) to give the crankcases and the through bolts a particularly
hard time. This will show in any of several ways.
Early engines before engine 200,000 were mostly fitted with studs and nuts all made of mild steel and
infernal serrated washers. (Yes, infernal-as well as internal!) These washers are jokingly referred to as
"shakeproof", but l can think of no instance in engineering where serrated washers couldn't be replaced
by something better. Here where the load is constantly varied, they serve only to dig into the gearbox
cradle and the back of the nut, until they have loosed one from the other when disaster follows. At the
same time, the serrations serve to let salt water into the hole, and this can seize the studs into the case.
Plain washers, lock-nuts and high tensile bolts were introduced later, to eliminate some of these faults,
and 1 would strongly advise any one still with the early type to change. Use Nyloc nuts, though, not the
all-metal type used on production, as you can chock and tighten them much more easily and they don't
gall and seize on the bolt when you want the engine out. The top rear bolt seems to be the one which is
the most stressed and even if the bolts themselves do not loosen the crankcases may well crack around
the LH end of the boss. Look at the corner of the machining where the boss meets the crankcase-I have
seen more cases cracked here than anywhere else. The cure is to apply a dollop of weld, just like the
weld on Peter's bike, and for the same reason. If you don't weld the crack up it will only spread very
slowly. Very little oil will ooze out of the grey line produced, and I have seen machines do 10,000 miles in
this condition without disaster occurring. They weren't racing machines, though!
Finally, that oil leak between the top rear bass and the base of the barrels. Any 750 owner will show you
the one I mean, If you take a used pair of crankcases apart-any crankcases, not just the Nortons, you
may see areas of the joint face which are a dark grey color, compared with the original bright machined
finish everywhere else. This effect occurs when the faces in contact have been moving in relation to one
another-a process known as shuttling. You get the same thing with crankshaft assemblies where they
shuffle (and they do!). The only type of gasket compound I know, which will stand this sort of movement is
the silicone rubber type, Hermetite RTV, or Dow Corning RTV (Room Temperature Vulcanising) or in
France. CAF 4 or Loctite plastic gasket 275. When glueing and bolting up the crankcases, incidentally,
always put suitable bolts and spacers into every bolt-hole, to clamp the case up solid while the gasket
compound sets-so that it sets in the condition it will be in when the engine is finally in the frame. This
applies to all cases and all gasket compounds (although if I catch any of you using Red Hermetite
anywhere on
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