This is a non Genuine clutch slave cylinder. The Mini Pre Verto clutch slave cylinder is a vital link in the proper operation of the Mini clutch. The master cylinder feeds the slave cylinder to exert the mechanical pressure to dis-engage the clutch. The simple sounding process is only complicated by a worn cylinder. As the wear continues it first will probably bypass, this will mean that the clutch will not stay dis-engaged when holding the pedal down. The next more obvious sign is when you see actual fluid leaking. The cylinder bore size is 7/8" diameter.