The 751.2 is used in conjunction with the Park Tool HTR-1 Head Tube Reaming and Facing Set to ream head tubes that use the 1" external cup headset standard.
Features
Sized for 1" external cup headsets
SHIS: sized for EC30 head tubes. Can also be used to convert EC29 head tubes to EC30
30.1 mm outer diameter
NOTE: Always use cutting fluid such as Park Tool CF-2 when machining.