How would you check the o-ring. Just pull off the valve? I do have extra o-rings and valve seals. I took the block to the machine shop and had them do work on it when I first built it. I also put a straight edge on it and check it. I hate to have to tear it back down again to check the rings on that cylinder. I will try to check the compression this afternoon. It runs good though untill the plug gets covered in oil.