These uncommon weld-on hubs are wider than the standard width "X" series weld-on hubs.
The "XX" series is 3" wide versus the 2" width found on the standard "X" hubs.
Welding tip: When attaching any weld-on hub to a plate sprocket, place 3 small tack welds in a triangular pattern and let cool. Check for accuracy, then weld the remainder in the same pattern so the sprocket does not torque from the heat, causing wobble on the shaft after installation Power-Link Catalog