Seat post bolt sheared off - twice I use a BMC TM02 on CycleOps Magnus. The seat bolt has sheared off twice while riding on the trainer.
The seat is attached to the post by a single stainless steel bolt. KSFC 12.9 appears on the head of the bolt.
The first time I had adjusted the bolt and may have overtightened it. The second time, the shop that sold me the bike installed the seat and tightened it to manufacturer's spec. The seat is positioned with the clamp in the middle of the rails. The seat is a Selle SMP Glider.
The second bolt broke after 23 rides averaging 75 minutes each. The first bolt lasted far longer.
I am not pushing high watts ever. I am not overweight (6 ft and 175 lbs).
Any experience with this? Any idea why the bolt fails?





UPDATE AS OF December 10, 2020
LBS talked to BMC. BMC said bolt can be torqued to 2x recommendation. LBS set it to 20Nm (2x the recommended 10Nm). I bought a torque wrench capable of 20Nm and check the bolt regularly. So far, no issues.
The photos look like you had the saddle clamped at the very rear end of the rails. This, especially if you sit at the tip of the saddle like time trialists often do, puts unusually high torque on the seat rails and clamp. In your case, the rails were stronger than seat clamp bolt.
Since there are four options for mounting the seat clamp, try to choose the seat clamp position so that it is close to center of the saddle rails to avoid unnecessary stress on the seat clamp.