If you really, honestly and truly, have a non-standard thread size or one so obscure that you cannot find an off the shelf die or tap, you can always go custom:
https://www.tapcotaps.com. I'm sure there are plenty more around the country or world.
Back in the mid 1970s when working on an Austin America (Austin 1200 to those across the world) we ran into such a threaded hole; I cannot be sure but it may have been on a flywheel or something. My father, then working in a research lab like Bell Labs, brought the part into the machine shop at work. Run by a German and so neat and clean, you could eat off the floor. Guenther measured the undamaged threads, decided they don't have a tap for that size...
and made one for my father. We used it to chase the threads with a little bit of cutting oil. During the same era, he also made a clutch alignment tool for my Datsun 510. The local Datsun dealer was nice enough to let me look at it but wouldn't let it out of the shop. Armed with a caliper and graph paper, I drew the tool and Guenther made it for us. Sometimes that's what you have to do.
