Is the M4x0.8 set screw mythological?

I need to replace and have on hand a few spare key set screws for my OTK rear hubs. The set screws have a very odd thread of M4x0.8. I find tons of M4x0.7, but the only thing I find in the size I need are taps. None of the usual outlets list my hub or set screw online and as I’ve found with other parts when I call, if it’s not on their website, they don’t have it and can’t get it.

So… anyone have a source of some set screws sized M4x0.8 that are 19mm long? I’ll take what I can get, but if I get my choice, they’d be cup point. A last resort would be to drill and tap for an M5x0.8, of course.

I might be out of my depth here, but are you sure they are a metric thread. I don’t think OTK hubs come with grub screws as standard, so they may have been added later.

I asked myself the same thing because I’ve never heard of a 0.8 thread pitch on an M4 before today. I had to triple check with my pitch gauge and even then, I was in doubt and went a size on either side to make sure… It’s definitely not 0.7, and it’s definitely not 1.0, either. Holding it against an M5x0.8 fastener and then measuring tooth to tooth of the screw to verify pitch did indeed confirm what my pitch gauge told me. The closest 'murican equivalent would be a 5/32" diameter with 23 TPI, which absolutely does not exist anywhere. Even still, the 23 TPI thread gauge does not mesh as completely as the 0.8 metric gauge.

I can’t see how this was added later, as it’s such an oddball size. Even if someone were to be irrational, the lack of availability would prevent them from committing such nonsense. Then again, were someone mad enough, they could cut this on a lathe…

#8-32 UNC. 4.166mm. OD. 0.794mm pitch.???

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Unfortunately that is not it. It is definitely metrique.

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drill another hole and tap it to something you can find?


That was the plan to punch it out to M5, but I wanted to see if anyone had a secret stash of these things before I did. I like to keep things as designed unless there is no other option.