Using Heli-Coils on Drive Gears (click on first photo - use arrows)
Wouldn't it be nice if you opened up your machine's final drive to find this?
Unfortunately, it's not unusual to find some hub damage — broken studs, damaged holes, welds etc. In many cases, there is a remedy.
Drilling out broken studs is easy enough with patience — there's lots of online guides on how to do that (search YouTube)
Heli-Coil inserts are coils of extremely hard diamond-shaped 304 stainless steel wire. When installed into Heli-Coil tapped holes, the inserts provide permanent conventional 60° internal screw threads that accommodate any standard bolt or screw.
Here we have an OC-4 drive gear with damaged 9/16ths hub threads — indeed, some of the original studs have been welded in — and not straight! The machinist is removing them, re-drilling and tapping them to 5/8ths on a stand and inserting Heli-Coils to bring them back to 9/16ths. Note the casting date on this hub — Jan. 22nd 1960.
Here's a close-up of the new 5/8ths to 9/16ths Heli-Coil inserted into the drive gear hub. This is stronger than the original 9/16ths thread.
It wasn't a bad idea but it was very poorly executed! Clearly the hub has been rotated a few degrees and a new set of ten 9/16ths stud pockets were tapped. Unfortunately, the spacing wasn't perfect, the studs were not straight (perpendicular) AND a bad wear plate was welded on top! My machinist is Heli-Coiling them all properly with a drill stand jig. As long as the old holes are welded up, it'll be strong and fully functional again soon.