I ride a bow shaped handlebar mounted in quill stem. I am riding this with a somewhat sporty setup, which means I do have weight on my hands and thus handlebar grips.
Originally I torqued this to 8Nm. After a first slip, I increased it to 10Nm. This helped, but after riding over a hump and having a peak load on the handlebar, it slipped again.
The stem and handlebar were purchased new, but these are inexpensive unbranded parts, both aluminium and painted rather than anodised. I measured them and the diameters match at 25.4mm. I believe the pinch bolt is M8, and it's steel. After the slip the paint has partially stripped off (see picture). For now I am riding with spare parts, but I want to re-install these if it can be done safely.
How high can I torque the pinch bolt safely?
- IIRC I originally used some compound paste on the handlebar/stem interface
- The pinch bolt was greased
- It didn't seem to loosen over time, rather only slip on peak load
EDIT UPDATE: I removed the paint from the center of the handlebar with sandpaper, which also did not help sufficiently. I again torqued up to 10Nm, which was not enough to prevent slippage. I dared not to go higher as the handlebar is aluminium. I decided against a shim, and instead tried using another stem I had on hand. It is 10mm longer, but I installed it slightly higher and that was enough for it to be comfortable. The handlebar has not slipped in the other stem in the last weeks, so I hope this was sufficient for now. Will replace the handlebar when I find a suitable replacement just to be safe.