Using Heli-Coils on Drive Gears (click on first photo - use arrows)

Superb OC-4 Final Drive Gear
Wouldn't it be nice if you opened up your machine's final drive to find this?
Drive Gear Hubs on Workbench
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
Drilling out broken studs is easy enough with patience — there's lots of online guides on how to do that (search YouTube)
Heli-Coil Thread Inserts
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.
Inserting Heli-Coils
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.
Heli-Coil hub close-up
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.
A hub from hell!
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.
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