I have the set for a 1" post. The threads look to be 1/2-20. I used a set of threaded gauges to measure. The cross bars thread onto an M12-1.25 and seemed OK. But the male post does not fit it M12. Post is definitely bigger than M12. I don't have M13's to try, but it doesn't "look" like it's a full mm bigger than 12mm. It almost fits the 12-1.25, definitely does not fit the coarser threads. The 1/2-20 fits nicely, both post and bars, and tightens down good.