Context: my ebike has 19mm nuts on the rear axle (and 15mm on the front). I have an old Proto wrench that’s a 15mm/18mm, so i’m wondering if i could replace the rear nuts with 18mm nuts instead, so i can carry around one less wrench.
The bike is an Eahora Cupid with a 1000w motor if that makes any difference.
Some locknuts have a different size hex flat than a standard size and can be sourced locally. There are not “custom” sizes for the most part, and if you find something like that, you should be extremely skeptical of what you find.
Generally, hardware should always fall inside an engineering standard size. This will be like an ISO, DIN, or JIS number. This standards number will specify all of the material properties and tolerances. While you may not be interested in these details, their existence helps to verify you’re buying something relatively safe. Failure at the rear is not as bad as the front, but it can still make a really bad day.
Here is the wiki. The table should help you find combos that exist: https://en.m.wikipedia.org/wiki/Nut_(hardware)