I dont remember what the size was, but I took a junk deep socket and cut a groove up the side with an angle grinder. After putting the socket on the sensor, put a hose clamp around the bottom to prevent spreading.
Anything that goes through a roof has the potential to leak, eventually. It can leak for a long time before you even notice it. Sealing from inside of the attic will not stop the water from getting under the shingles on the roof. The best way would be to remove the chimney, plug the hole, and...
FS: KD 2011 Inner & Outer Snap Ring Pliers Tool. This tool will work on both inner and outer rings, and will hold the snap ring in the desired position while removing. Cash, check or Paypal.
My first timing light did not need any power source. It was a super cheap RAC. The light had two spark plug wires coming out of it. You pulled the wire off the spark plug, then one end went on the plug and the other end went on the wire.