Steve Jowett www1.kingston.net/~fortierg/mainb.htm

Crankcase

Several significant changes were effected in this time period. Most notable was the change from distributor ignition where the magneto used to be, to points ignition in the timing cover. This initiated changes to the tachometer drive, which was relocated from the timing cover to the crankcase on the timing side now driven from the end of the camshaft via a worm gear. Further changes to the timing side included closing the old distributor opening on the aft-most position of the case. This was later opened again when Norton thought that an electric start might be installed.

From late 1971 through to the end of the 750 production in 1973, the 850 changed to new cases without the opening. This modification was never done until the 1975 model and the action was done in the primary chaincase instead. The breather assembly attachment to the rear for the 1972 model affected the cases which added the opening and threads for this. Another modification was strengthening added around the main bearing bosses. I believe this was done for the introduction of the 850. At the same time the holes and threads were added for the Allen bolts which add additional support for the 850 barrels to the cases.

Some notes on disassembly and reassembly as follows. Make sure you have removed all bolts and studs from the cases before you try to separate them. I myself have forgotten about that nasty one which is only a screw in the near bottom back and after several good raps on the bench have discovered this one still intact. Make sure the conrods are well supported and held away from the case. I highly recommend cutting a template from wood that has holes for a few studs so it can be clamped in place (only clamp the template to one case when separating the cases). This should hold the rods in situ so they don't bang around when working on the cases or timing gear cluster. Another trick is to use an old inner tube and elastic bands. Whatever you do, don't allow the rods to bang against the cases as nicks and dents at this point could case possible breakage of the rods. This support also makes it easier to remove and fit the pistons to the small end eye.

When all is well and clean after disassembly, inspect all the studs, bolts, nuts, and washers and make sure threads are good and all fits together like it should (the NOC cautions against making sure that you clean the thread holes carefully. The use of degreaser will dry out the threads and cause the studs and screws to become locked in place). I would recommend that you put a drop of machine oil in each thread bore upon reassembly. Clean the crankcase jointing faces well and make sure they fit tight together before installing the crank and rods. I have ridden some 30,000 miles on a set of cases without a drop of oil out of them, and no silicon either. Again cleanliness is the key to reassembly.

If you have an unlimited budget and feel it necessary, buy stainless nuts, bolts and washers for the cases. The nice thing is you hardly ever touch them so they should last forever. With a lot of elbow grease and lots of polishing you can make the cases look like they were chromed.

I have heard of several different scenarios for sealing the crankcases so they don't leak oil. Yamabond, available from your local Yamaha dealer, is probably your best bet for the case that doesn't want to seal or for one in which there are deep scratches from some backyard mechanic who thought the best way to separate the cases was with the hammer and chisel method. Also silicon has been used but I would caution against this method as when the cases are pulled together the silicon squeezes out and could become dislodged on the inside and flow to the oil passages where it could block them and create disastrous problems. Again I say clean the case surfaces well with a razor knife or blade so the mating surfaces are clean and without deep scratches. When nipped up tight no oil leaks should be apparent. After running in for several hours check all fastenings on the crankcases to ensure they are still tight and to proper torque specs.

I have left out various parts from the parts list which go into the crankcase, some examples would be cam bushes and main bearings, and only listed parts which are for the case assembly.

Crankcase