[sammyg2]

OK, here's the details:
This was a vertical deep well pump with a 4 piece ridgid coupling.
The motor sits on top of the pump and the solid coupling not only transfers the torsional power, but also provides the lift to keep the pump rotor centered axially in it's stationary parts. The motor acts as a radial bearing and also a thrust bearing.

The coupling takes hex head cap screws (bolts) to hold it's parts together.
The female holes drilled and tapped in the coupling parts are 7/16" NC.
The millwright didn't have any 7/16" bolts handy (they are not that common in this industry but I had them in my shop) and instead of asking his supervision to get some (which might have taken 10 minutes to round up) he grabbed some 3/8" NC bolts and tried them. He wanted to get the job done so he could co home on time.
The 3/8" bolts have a different thread per inch count so they grabbed.
He tightened them up and they seemed to work OK so he said to himself that'll work.
This was a small 12 stage centrifugal pump designed to provide high discharge pressure but relatively low flow. it was in critical service.
It had a spare pump but the spare was out of service waiting for parts to repair it. The OEM didn't support the model anymore so we had outside vendors replicating the parts.

We started the process unit up after the maintenance was done and the 3/8" bolts held for a while, but eventually broke and damaged this pump.
Without the spare pump to go to, we lost production. $$$$$$$
If he had asked his supervision for the right bolts this would not have happened.

Even a really bad mickey-mouse mechanic knows better. This guy was making $34 an hour. No excuses.