I used .039" diameter copper wire. I bought it at the local hardware store. It was labeled as 28 ga copper craft wire. Readily available nowdays on Amazon, etc. I ground a tool from HSS to cut the groove. It was a tight fit for the wire, needing it to be tapped into the groove with a small hammer. I cut the wire close to length, and then trimmed it with a file to get a tight joint between the wire ends. I cut the groove shallow enough to leave the wire .008 - .010" above the deck. It's been a long time since I did the last one of these. Back then, I jigged the cylinder up on a lathe faceplate to cut the groove. If I ever do another, I'll probably try doing it on the mill with a boring bar and custom ground tool. I thnk that would be a lot more efficient. Stainless safety wire is also popular for o-ringing cylinders in the car world, but I thought the copper might be a bit more forgiving in the machining tolerances, and it always worked fine for me. I did replace the wire periodically during engine tear downs.
Ken