Jack,
The belt diameter is roughly 3 inches in diameter depends on the thickness of the material you use to make the replacement door/cover. I found it easy to work out the diameter when all the bits where attached to the Lhires,then used length of string to wrap round collimator body and cog of motor. Have attached another photo showing the size of the teeth on the belt (these are small,don’t need them too large) and the bits and pieces before assembly. As I described in my opening post the belt was originally in an Epson printer. Being a large loop I just cut it open and then just cut suitable lengths off to use for both the micrometer shaft and this mod,to close the loop I used Araldite and heat shrink tube.
Just to clarify the motor I have on order is the one I’ve added the text to a few posts back.
Best Regards
Alun