Actually it was two drill bits and one tap...
They were reverse twist Cobalt bits and a matching tap that I used for repairing broken crankshaft bolts on C201 Isuzzu diesel engines on Thermo King reefer units.
One year TK came out with a new model reefer unit that had a large cast iron drive pulley on the front of the engine. It weighed about 25 lbs.
It was bolted on the crankshaft with a single
3-1/2" hardened bolt and was hub mounted, meaning that the pulley didn't slide over the crank, but rather mounted to an adapter hub that DID slide over the crank. The pulley simply mounted against the hub and was driven by two "ears" on the hub,, the whole arrangement was bolted on by the single hardened thru bolt.
The problem was, the pulley drove the main belt that was 120 inches long and it drove the water pump and the main fan for the reefer. The factory found out that people were over tensioning the belt and creating a fulcrum effect on the large pulley and being hub mounted,, it would cause a rocking motion as the crankshaft rotated.
The bolt would eventually fail, parts would go flying and then my phone would ring at 2 am.
Some trucker out there with a load of frozen melting... So, I would meet them at the shop to make the repair. If just the bolt head sheared off clean, then a lot of them I could mig weld a small bolt onto the crank bolt and it would spin right out... BTW the crank bolt was about 3-1/2 inches long, but the last 3/4" was the only threaded part.
If the bolt snapped off inside of the crankshaft,
that's when the drills and tap came into play...
We used reverse drill bits because sometimes they would bite into the bolt while drilling and spin the broken stub right out. Under the worst circumstances where the bolt snapped off at an angle or deep inside the crank, I would drill the nose of the crank as deep as I could, and then tap new larger threads.. The taps and two bits were made by some company in Germany and recommended by TK.
Our company bought two sets,, one for my service truck and one for the shop. I remember it was about 1200 dollars for everything. The sizes were like 3/4" and 7/8" and the upper matching metric thread size - it was a very fine thread tap.
This was over twenty five years ago... but somehow I ended up with the tools.
Once I got good threads in the crank, TK had an aluminum pulley and steel hub - bolt together pulley to retrofit the original cast iron one. Thermo King issued a recall and if we got to the unit before it blew apart, it was a half hour job tops to retrofit the unit.
One night I had a callout, that one broke off a chunk of the crankshaft as it came apart... there was a load of frozen on the trailer, and I had to do something. I ground the crank down the best I could, and then drove the new steel hub into place with a mini sledgehammer and arc welded it to the crankshaft,, then bolted on the retrofit kit, and it worked like a champ...
We got an authorization from TK to replace the entire engine after the trailer got unloaded ..
Was a fun time to be a reefer mechanic !!