The 900M-T-H Soldering iron tip can be used on the FX-8801 hand piece however in order for the tip to fit onto the handpiece properly the use of the B1786 tip enclosure is necessary . 

This is due to the bend at the end of the soldering tip and the smaller diameter of the standard tip enclosure used on the FX-8801.  The B1786 has a larger diameter and therefore the bend of the tip fits through the opening of the enclosure and allows for the tip to be used.