26x12x12 tire mount secret???

[CliffordK]

My trick for setting a bead is to take a bicycle inner tube and put in on the rim between the tire and rim. Inflate enough to make contact with the side of the rim and tire, and then start inflating the tire. A little tricky. Try not to pinch your fingers.

I've mainly done car tires, so I'm not certain about your tractor tire, but it can't be too bad.

I'm not sure about the size for a 12" rim. Bicycle tires are sized by the outer diameter. So a 16" tube might be close to fitting over your 12" rim.