Warning! The threads on the nipples have usually distorted from the rivet and backing out will quite often harm the very important threads in the head! After carefully removing the rivet, cut the old nipple with a small grinder so that it may be collapsed and pulled away rather than un-screwed. Beware also that Colony nipples must often be trimmed shorter to completely bottom out on the lip of the head. (Save the o-ring for something else. And dont use their oversize rivet unless necessary: a trimmed sprocket rivet often does better). Setting the rivet requires making an anvil to fit inside to hold the rivet-head, and I suggest using Seal-Lock goober on the threads and rivet, as well as pressure testing before final assembly. I do NOT consider this an easy operation, but it is critical to the motor.