It sounds to me as though the minor dia. of the internal threads has swollen.
You didn't say but my guess is that you can barely get the pilot of the pin in the hole.
This may be an easy fix and you may not need the tap.
IIRC, the minor of a true DPK pin is .310/.312". Measure the dia. of the pilot on the pin to be sure.
Take the shaft back to the machine shop and have them re-drill the hole @ .3125 (5/16").
The shaft should be centered in the lathe to do this but I'm assuming a competent machinist would know this.
Once the hole is re-drilled, the pin should at least start into the hole. It may be very tight to turn.
Don't force it. Liberally coat the pin with candle wax and try again. Again, don't force it.
You may have to do this 1/2 doz times but each time it will get easier.
The threads are still there. They just got a little distorted from the water.
What you are doing now is to re-form them. I expect they will now fit like brand-new.
Don't forget the wax. That's part of the reforming process.
HTHs, KJ
