I like the masking technique and it looks like you did a very good job with the jig saw. If you have to do more than a couple/few, one of these might come in handy.I had trouble finding spacers that the OD wasn't too big so today I just built some. Bought a cutting board at the resale shop and cut some out of it. Broke the bead on the tire that needs a tube, sanded and painted the wheel as it had black paint splotchy sprayed on it. I hope to do the conversion tomorrow.
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When using this, or a more common style of hole saw, I drill a pilot hole with a normal drill bit then put the beheaded shank of a bolt in place of the pilot bit. A bolt is MUCH stronger than a bit and this prevents bit breakage and minimizes having the pilot hole cut bigger than desired, if some chatter develops while cutting.