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I was getting ready to apply some of the techniques in this excellent bulletin and was looking into using a mag drill to try and control the bit wandering Greg Long addressed with his drilling jig solution. In researching the mag drill a cutting tool called a Rotabroach was mentioned. It is basically a cutting tool that cuts an annular hole and leaves a core. It was available in 1/4″ so I ordered a set of three. To mount it I used a 3/8-24 stud.
Rather than risk irreparable damage to a Pierce block, I used a lesser block out of a ’28 Chrysler 72. I had 22 of the 28, 1/4″ jackets bolts broken off in the block.
I decided to use the water jacket cover as the jig for the Rotabroach. The broach OD was contained by the jacket hole and allowed a centered hole to be cut, no wander. It seemed like the steel shank helped guide the broach and the cast iron machined smoothly
Cored bolt
The broach cut a slightly over sized hole, but undersized for a helicoil bit.
I drilled and tapped for a 1/4-20 helicoil. Since the helicoil is stainless, I figured in 2073 when they need to take the cover off there won’ be a repeat of snapped off bolts. I went ahead and ordered a 5/16″ and 3/8″ rotabroach to see what dimensions they are.
With the water jacket template and the rotabroach, it made a tedious task tolerable. In fact it went pretty quick with threading for the helicoils being the slowest task. Now I have lost a reason to get a new mag drill, guess I will have to keep looking for task for one.
I developed another option. I used a 1/8″ Drill Stop Collar. I reamed the water jacket holes to be slightly undersized and press fit the collar into the hole in the plate. Bolted the jacket back on and center drilled the broken stud. I recommend using new 1/8″ bits
The installed collar is on the left bottom corner.
It looks like the 8 cylinder cars will need a 3/8″ rotabroach and the holes will have to be enlarged to 3/8″. The 5/16 is too small