Drive-shaft pin
 

I have been having a problem with breaking the pin in the driveshaft at the engine coupler. I can see that the hole has become oblonged and I suspect that is why I keep breaking the pin.
This is a 127 hydrostatic by the way.
If I replace the driveshaft and coupler, is this going to solve my problems or is this a common problem with hydros???

You can weld up the worn spots in the coupler and grind it back to the proper shape. I did this in one of my Cub 147 restoration youtube videos.

You will keep shearing pins until that coupler is back to its original form. Same problem occurred on my 129 until I replaced the coupler. You will need to fix that coupler or replace it to get back in action.

Cub Cadet 123

As SaturnV stated, yes you can weld and reshape the coupler, I do it all the time. I didn't know that a worn coupler would break the pins.

What kind of pin are you using in the shaft?

Agreed. The pin at that specific location should be a solid pin, not a roll or spiral. Still need to fix or replace as stated above but they can get super worn out and normally not break?

My experience has been you can run both until the pin breaks.
The pin will likely be worn down half thru or so before breaking.
Then if you install a new pin with the worn coupling, the new pin with snap really soon after.
Why is that?

After losing or breaking the pin on my 125 a couple of times I did away with my coupler and pin and did a rag-joint conversion. I found a thread on here somewhere that showed how to do it.
I had a 1250 parts machine and used the drive shaft coupling and rag-joint off of it to add to the front of my shaft. I had to shorten the drive shaft and drill a hole for the pin. I don't remember all the specifics, but it wasn't too hard.