The ST 4 is a mill spindle with R-8 taper. It fits all Mill Turn machines from mid 2015 onward. It is designed to accept double sealed double row angul;ar contact ball bearings. On the tool end it uses the 5207 2RS1 and on the pulley end it uses the 5206 2RS1 bearings which are rated for 7000 RPM. The spindle has an 8 MM X 21 MM keyway for the drive pulley key. The end of the spindle is threaded for the 25 MM X 1.5 MM spanner nut. There is a lower bearing retention nut which has 35 MM X 1.5 threads. There is a 6 MM Dog point set screw and jam nut for the R-8 slot. This set screw can be easlily adjusted for proper depth and replaced when worn. There is a 15 MM hole through the spindle for the drawbar and a 40 MM flat milled on the tool end for a wrench to hold the spindle while loosening or tightening the drawbar.