Remember that these bikes came with a standard set up for the model, and over time this may have been changed by previous owners. Gearbox sprockets were 19 teeth standard, but 17, 18 were available for sidecar use, and now 20 and 21 teeth are available for higher overall gearing. Rear sprocket was 42 teeth on a solo, 49 for sidecar, but 45 and 46 were also available.
TT has the answer, it's suck it and see. Roy Bacon's books states 100 links (19/42) or 104 links (19/49). In an ideal world chains and sprockets were changed together, or at least two chains used to spread the wear, one in use, the other ready to go and the two swapped from time to time.
Fitting a new chain to worn sprockets will shag the chain in next to no time, sadly changing a Plunger Gearbox Sprocket is an engine out job.
Swarfy
