The bobbin winder atop the Kenmore 158 sewing machine has a rubber tire that contacts a ribbed ring on the inside surface of the handwheel; the clutch knob disengages the main shaft and you run the motor at top speed. As you’d expect, both age and wear take their toll on the rubber, to the extent that the winder on Mary’s machine stopped turning. I swapped it for the slightly less decrepit winder on the Crash Test Dummy, but that was obviously a stop-gap measure.
I mistakenly thought the metal wheel consisted of two plates that clamped a rubber disk in place, with no possibility of removal:
The fact that the spare parts list didn’t include the rubber disk helped convince me.
Eventually, I stumbled over replacement “tires” on, of course, eBay that suggested how to dismount them:
Yup, that sucker slides right off.
Anyhow, the replacements seem to be standard industrial O-rings, rather than the original tire with a flattened rim:
The new tires measure 28.94 mm OD on the bench (I don’t trust that last digit, either) and 29.56 mm OD installed. The (hardened and cracked) old tires measure 29.94, 30.06, and 30.28 mm OD on the bench; that’s a radius anywhere from 0.2 mm to 0.4 mm larger. The winder’s mounting screws provide a very small adjustment range that helps a bit.
Knowing that I needed an O-ring, I checked the assortment of “standard size” O-rings I bought many, many years ago, which once again failed to offer up anything suitable. To the best of my knowledge, that kit has never had the right size; apparently, every application uses a different standard.
The O-ring definitely puts less rubber on the handwheel than the tire, but seems to drive the bobbin winder well enough to fill a handful of bobbins without any of the previous drama.