Speedy,
The gear measures 3 7/8" x 1/4". It is actually too big in outside diameter for this particular crane but it works. I think perfect would be just big enough to fit around the steel bearing ring on the bottom of the crane house. Also, I custom made the spacer hub for the center to fit the steel center post and cone shaped the outside diameter to center up into the middle of the plastic gear.
The picture doesn't show the top of the rule, but it measures 3 7/8"
In this picture you can see the wear mark on the truck bed where the bearing ring rides. That shows how much too big the new plastic gear is. It doesn't stick out and doesn't look bad so it works for me.
Jim