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Is a 30mm m6 likely to have a shoulder?
The part you're talking about is not a shoulder, it's the shank. If you don't specify full threads when ordering, you might get a short length of unthreaded shank on M6×30 because the thread length is typically around 24 thread pitches for screws/bolts* without full length threads.
*see argument above, and previously. The defining difference is not whether it has full threads, but whether it screws into a nut or into the workpiece. A short bolt will always have full length threads if it is shorter than the minimum thread length for it's pitch, a long screw might not since the assumption would be that anybody using a screw that long is passing through a deep clearance hole before getting to the female thread in the underlying piece, not that they are wasting a lot of time engaging threads for more than two or three times the diameter.
"Full length thread" is always just an approximation, since you can't cut threads all the way to the underside of a bolt head, there will always be either a thread relief or a runout of about one thread pitch.
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gbj_tester
Interesting.
I thought it was to do with whether is had a point (to be screwed into something) or flat (requiring a bolt)
Nonetheless, a 20mm m6 thing with an allen head has a thread the whole way up. Is a 30mm m6 likely to have a shoulder?