I purchased my o-rings from a hydraulic supply company but they look like standard o-rings you can get from ACE. Yes the o-ring can be stretched over the end of the ram. After reassembling the rams, here is how I purged the system of all air. First I pulled the ram fully out on the bench. Then I attached the nipple that comes on the bottles of differential oil to the bottle of hydraulic oil. Then I put this nipple over the top hose connection on the ram and pushed the ram down. Next I attached the top hoses and bolted the rams in place. Then I took a large veternary syringe and pushed all the oil and air over to the other ram (I had this tube elevated). With all the air out I carefully attached the bottom hoses. When I got done there was no/minimal air in the system.