1981-1984 Official Honda ATC250R Shop Manual, Page 203Get this manual
ATC 250R Install the piston with the concave end on the brake pad sideInsert the piston into the caliper cylinderNOTE Install the piston with the end projecting about 10 mm (Ye in) above the caliperWipe excess brake fluid off the caliper with clean cloth
'82 ATC250R ADDENDUM
Install the pin boots and bushing in the caliper and lubricate the inside with silicon grease