I have a Bunn Special sn 4480193 it is a non 60 hr. PW
I have read that a 60 can be installed because it will fit.
Is this a common practice? I figured to ask before I go ahead
And get the most recommended version.

James I only use the 60hr mainspring 47309 on 60hr watches models 13,14 and 15. Illinois has a variety of mainsprings specific to different models. If you have a copy of the Illustrated Manual of American Watch Movements by E.J. Swigart Co. it has a nice breakdown of mainsprings for specific models. It also breaks down other major companies---not only dealing with mainsprings, but other parts as well. If you don't have this manual it is a must for watch repair. I would say you could find a copy on ebay.

Rob
www.pocketwatchrepairs.net