While the majority of hobbed bolts on the market are M8 X 60mm the stock bolt from Printrbot is a 5/16" x 2" 18TPI with a 1" shank length and 1" thread length. This is a very close match to a standard M8 x 1.25 50mm bolt. They are not identical, but they are extremely close and should be within tolerances for hobbed bolt use. Nearly every vendor I have found online sells M8 hobbed bolts, though 5/16" are out there.
It's also worth noting that hobbed bolts sold for Greg's extruders may not work for Wade's extruders. The Ultimaker site lists an M8x60mm hobbed bolt but warns:
Please note this bolt's teeth are directional. The Greg's Extruder design is the reverse of the Wade Extruder Design. This bolt is not compatible with a Wade Extruder. It is compatible with a Greg Extruder.
The critical dimension is the distance from the bottom of the bolt head to the center of the hobbing. Hobbed bolts found online come in many variations on this dimension, 29mm seems common. Many vendors allow you to request different distances. Many don't list this critical dimension at all!
The Printrbot bolt is hobbed at 21mm:
I suspect this is an artifact of using a 2" bolt, you really couldn't manage to move it much farther or you'd be hobbing in the threads. On My V1 PB+, I find that I'd really like the hobbing to be slightly farther out, it's not quite centered even though the bolt as tight as I can reasonably make it. (That's probably why so many of the bolts online are 60mm instead of 50mm.)
Has anybody had a satisfactory online purchase of a replacement hobbed bolt?
