ClevTrev might be right in principle but read on. I fitted one of our car alternator kits recently to a local chap who is an MS sufferer and uses a sidecar to keep everything upright. I normally do not like to fit these kits but have done several recently and found horrific variations from bike to bike. On this particular Comet it was impossible to find a position for the large idler which allowed the drive for the dynamo/alternator to be anywhere near central. I am not talking thous here but nearly a tenth of an inch. Fortunately the bearings used in the replacement for the dynamos are sealed for life ones and this, in conjunction with a liberal application of sealing compound, acts as an oil seal. So, by all means fit the dynamo first, but be prepared to have to move it if other items do not mesh correctly. Remember that different sized half time pinions are available if needed.