SHIS terminology: The 790 is used on ZS56 or EC56 upper or lower headtubes.
55.9mm Pilot for 1-1/2″ ZS Headset Standard
The 790 Pilot takes the place of the 789 Reamer for head tube facing with the HTR-1 Headtube Reamer. The 790 is used with the 690-XXL Facing Cutter and #750.2 Centering Cone Adaptor. The tool allows facing head tubes with 55.95 – 55.90mm I.D. (Internal 1-1/2 ZS standard).