Making zerk fittings stay in place?

This is for a disk, an implement that is towed behind a tractor to till the ground. It has 16 disks, and 16 zerk fittings. The holes for the zerks are in the axle bearing housings. The housings are cast. The holes are not threaded, and they are more than 3/16" in diameter, but I can not put a 1/4" drill bit in them. The closest zerks that I can find are 3/16" In the past, I had to cut a little piece of copper wire to put in the hole with the zerk and then pound it in. But they still come out. What's another way of keeping them in place? Could I tack weld them in place? I can pick the fittings up with a magnet.