Dan: The hardest part of fixing a sheared drive pin is getting the old pin out (but breaking it makes it easier). If the pin is broken there is often a reason why it broke aside from age and general wear-and-tear. Be sure to address any "play" you find in the drive train.
It also makes a difference if the pin breaks at the transmission or at the engine. I think the drive coupling at the engine on your 106 is the same as the slotted coupling on my 149 and my son's 125, since the 106 falls between the 104/125 and the 108/149. Even though the old pin and coupling might have lasted years, the new pin in the old coupling often doesn't last 10 minutes. Worse, a new pin in a new (expensive) coupling often falls out just as fast. I'm told it will even eat its way through a stainless steel clamp intended to hold it in place.
Long story short, consider upgrading your driveline to the so-called "rag joint" as found in the Quiet Lines and 82 Series tractors. It isn't perfect either, but there aren't any pins to fall out (the Spirol pins for the couplings can still shear, though).
If the rear pin is broken, then I would just try replacing it, but, again, be sure to check for play. The QL and 82 Series employed swivel bearings at either end of the drive shaft, and some implementations even had a ball bearing at the rear (I'm thinking it was in the QL era, probably doesn't apply to your 106).
Parts Lookup for the 106 Model is not especially helpful on this point, that is, the drive shaft connections. Unless I'm missing something, there doesn't seem to be any "pin" detail on either the clutch, the engine, or the transmission assemblies. Perhaps there is more detail on some of the other Models in the same series, or one of our sponsors could enlighten you. Someone may turn up with all the answers and colored pictures with arrows and circles.
I'm betting (with Nic) that it is the drive pin at the front. Based on my experience, if new parts (beyond the pin) are needed, I would give serious consideration to the QL/82 driveline setup, it has fewer problems.
Note: I am indebted to forum member Paul Bell (in previous discussions) for this insight and solution. Thanks, Paul. Hope you're doing well (or at least better than Bryan!
