144 people have read your question so you deserve an answer, Mark.

I don't know that much about clocks, but I do know that the ratio of hours to minutes to seconds gearing is critical (although I have a clock where the 'seconds' hand goes round in 45 seconds). The ratio of these gears to the escapement is also critical, but the initial drive wheel (great wheel) as you suggest only connects the string or the spring to the rest of the works. A smaller great wheel will (may) need a slightly heavier weight, and will run down a bit quicker, so as long as you mount the clock fairly high up, it should work ok. Or change the size of the winding drum to compensate (if weight driven).

Thinking more about it, any clock with a fusee has a variable drive ratio (as though its great wheel changed in size as the spring ran down).

Why doesn't a web site for Model Engineers recognise the word fusee, I wonder?

Cheers, Tim

If you have a dividing head 108 should be easy enough to do? If it is a 40:1 head then every 10 holes on a 27 hole plate will do it. If 60:1 then every 10 holes on an 18 hole plate. If 90:1 then 15 holes on an 18 hole.plate.

Standard dictionary – you need to add it to your local one.

Neil