Chuck Crack Repair

Personally, I would retire the chuck. Why risk injury when you can buy a safe chuck? One visit to the ER would cost as much or more than a new chuck.
 
Drill both ends of the crack and braze it with bronze or sliver solder, grind flush.
 
Duct tape is fine if you are turning the chuck slowly enough. Hmm, there is a centrifugal force calculator on the web. If you plug in 1 lb., .3 ft, 5 ft/s for 200 rpm, you will get 2.6 lbf. Yup, duct tape will hold this. If the piece weighs 10 lb, you might be in trouble, but the tape might still hold if you gave it several wraps.
 
That thickness looks good enough to put in one or two threads across the crack. I would mechanically secure the left side (in the photo) of the break to the right/bottom side of the crack and fill the angled countersunk holes with JBweld - after drilling the end of the crack of course. Then you can get on with using it wihout having to worry about a piece flying out at 1000rpm...
 
I believe I would not want to be in the same room with a JB welded cast iron chuck running 1000 rpm. I have been wanting to buy a plain back chuck for my rotary table and my paranoid self would never be happy spinning a welded chuck. It will probably be just fine?
 
There are too many 8" 4-jaw light and medium duty chucks around to warrant the time/effort, let alone the risk, of attempting to somehow "fix" it. If you have a rotary table application, so much the better. But regardless, if the back is threaded for 1-1/2"-8, I would probably bore the threads out so that no one else can "Po-boy" it later.
 
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