20-09-13, 11:19 AM
Its more to do with the tolerances of the nut /shaft interface. I didn't measure the thread pitch but I guess its M18x1.25 (fine thread) which means the shaft size should be between 17.76mm and 17.97mm this variation allows for worn tools or machinery when cutting the thread. Outside of this the thread SHOULD fail quality inspection.
The nut thread Should be 16.65mm to 16.91mm internal diameter. Again I didn't measure mine but I'm guessing that the mod nuts are cut to nearer the 16.65mm to give better overlap on the shaft thread. (The troughs of the nut thread should be 18-18.3mm)
In this case it looks like the shafts have been cut to the wrong diameter. However it appears that Yamaha feel that with the thicker nut there is enough surface area in contact to keep the nut tight using bearing retainer even if the shaft is down to 17.5mm (bearing retainer is similar to threadlock but has better gap filling properties).
I think that explains it clearly, if not I'll draw a picture to make it easier to understand
The nut thread Should be 16.65mm to 16.91mm internal diameter. Again I didn't measure mine but I'm guessing that the mod nuts are cut to nearer the 16.65mm to give better overlap on the shaft thread. (The troughs of the nut thread should be 18-18.3mm)
In this case it looks like the shafts have been cut to the wrong diameter. However it appears that Yamaha feel that with the thicker nut there is enough surface area in contact to keep the nut tight using bearing retainer even if the shaft is down to 17.5mm (bearing retainer is similar to threadlock but has better gap filling properties).
I think that explains it clearly, if not I'll draw a picture to make it easier to understand