This should be 14mm. I just did it and the 15mm was loose. 14mm is a perfect fit.Now its time to remove the fork cap. Clean all the fluid and dirt you can off of the damper, fork spring, spring collar and fork cap. You will need a 15mm wrench and a 3/4in wrench for this step. If you have a spring compressor or access to one it will make this step easier but its not required. You may need a second person to help here.
Place the damper on a firm surface like a rag on the floor or top of your work bench. (Now look at pic2 of the damper without the fork cap. Your 15mm wrench goes on the body not the lock nut at the bottom.) Using a towel for grip in one hand pull down hard on the spring collar compressing the spring until you can slip your 15mm wrench on the damper. Now using the open end of a 3/4in wrench you should easily be able to remove the lock nut on the top of the fork cap. The cap will be tight on the damper. Remove your 15mm wrench. Now turn the fork cap counterclockwise until it stops then lift up. It should come off the damper. Remove the white plastic spring collar and fork spring. You are done with disassembly of the internals. Clean all parts and put them aside.