Crankshaft

History

The stroke length was the same for all crankshafts, although I have found reference to a short stroke (80.4mm) used on later machines; I would assume that this was for racing outfits from the factory. On all Commandos up to 1973, four studs, two bolts, locking plates, and nuts were fitted. This was altered in 1973 to six studs and nuts. The 1975 was altered again with the extension of the dowel to a threaded dowel complete with nuts and (washers for the dowel only). The oilway plug remained throughout, except the part number changed. I believe this was due to the change of the factory location from Plumstead to Wolverhampton.

The parts books indicate that the crank assembly could be purchased as separate items up to the 1973 model year. After that only the crank as a whole is listed. If for some reason your crank would need replacing, I would recommend that you replace it as a whole as the balance factor, although poor, is still factory tuned.

General Notes

When dismantling, note that the crankshaft will probably contain some oil in the sludge collection area in the centre. Loosen the nuts on the timing side, some of which may prove difficult as they are usually drift pinned to prevent them from coming loose during running. I always use new nut retaining plates during reassembly as the old ones may suffer from metal fatigue due to crimping and uncrimping during assembly and disassembly. For the cost of new ones I feel it is worth the price. If a piece should dislodge during operation it could become trapped within the bottom end or oil pump and the results could prove fatal to your machine.

Inspect all parts very carefully as this is the life blood of the machine (pistons excepted) and any failure here is sure to result in explosive discharge. New nuts and bolts may not prove necessary although the individual who builds the machine should decide. Upon reassembly I always make sure all parts are perfectly clean, tools are clean and wiped, and I lay it all out on a clean cotton cloth. Be extremely careful around the crankpins with tools and such. After a regrind or upon reassembly one wouldn't want a scar here.

J.B.Nicholson Modern Motorcycle Mechanics

The crankpins should be measured for wear with a micrometer. Standard dimensions on the 650-750 motors are 1.750" -1.7505". Regrinding and fitting of undersize shell bearings is recommended when maximum wear exceeds 0.003" and ovality is in excess of 0.002".

Crankshaft Dismantling:-
When reconditioning, it is advisable to remove the crankshaft halves from the flywheel to enable the large centre oilway which forms a sludge trap in the crankshaft halves, to be cleaned out. Before dismantling, make suitable register marks on one side of the flywheel with the crankshaft half to ensure reassembling in the original position, Before reassembling, make sure that joint surfaces are perfectly cleaned and tighten bolts progressively in a diagonal pattern. Lock bolts and nuts with a centre punch.

Crankshaft