I assume you want this information to set up the timing gears. While there was an exhaustive series of articles as mentioned above, the main requirement is to have a simple practical method of differentiating the sizes of your collection of pinions. I assume you are setting up a Twin, and that your spindles are parallel, tight and unworn.I further hope you have a ROUND steel large idler and the camshaft pinions are ditto. Set up your one-piece or steel large idler boss and large idler against each cam pinion in turn to check for tight spots. If none is found, dress the key E81 to be a tidy fit in the half-time pinion slot. If you want to be rigorous, make new E87 studs 1/8" longer and use Aerotight or Nyloc nuts instead of punchlocking. Offer up the large idler and mesh it with the cam pinions. Slide in an apparently sloppy half-time pinion. Have a look at the backlash. Take out the sloppy pinion and put the shank portion of two 1/8" drill bits diametrically opposite each other. Measure this, and all the others across with a micrometer. Line them up in a graduated row and "suck it and see" as my apple-cheeked old grandmother used to say! It may help to make a sloppy E81 for starters and a tighter one for final assembly so as to aid frequent extraction and insertion. If you are using an alloy large idler or a two piece alloy/steel idler boss, God help you, 'cos after 60 years they are past breaking time. If you are setting up a Single WITH a timed breather and a Lucas or Miller generator, set the generator central in the seal first as if it were a cam pinion. If you are using an ALTON and an atmospheric breather on a Single any old (new) half-time pinion will do, but please use a steel small idler.