Original classic Mini clutch master cylinders were always .75" bore and used a cup type main seal on the upper end of the piston. This design was improved with the introduction of the later cylinder with ring main seal. This conversion kit includes a new ring seal piston and seals to rebuild and upgrade your early cylinder; all components from OEM source(s). Ref: GRK3007, HMK150, GRK3007-KIT.