Redbullet

Thanks.
Lower marks are deeper: the difference of 0.1-0.13 mm in diameter exists only at the level of the red point, generated during previous period of using 35mm of spacers, 3-4 years / 25000 km. Upper marks are scratches, there is no decrease in diameter. Lowest marks are also scratches for the moment and were generated within 1-1.5 years / approx 10000 km with 30mm spacers. Torque was always 5Nm, for plug and for stem. The plug was the first tightened and the stem was the second.

Since normal maintenance or costs and hassle of sending the bike abroad to producer for a new fork would not eliminate the reason of the issue, I want to make a change. I am tented to proceed with changing of the stem from 28.5 to 31.8 mm clamping and use aluminum shim adapters that go down 1-2 cm below the stem edge. That would distribute clamping forces to a much larger area of the steerer, see the sketch at the 22nd post in this thread.
But I am wondering whether the stem might easily slip at 5Nm torque in this setup, since I shall have 2 surfaces of friction instead of one: one between the stem and adapters and another between the adapters and the steerer. I also wonder whether some other issues (unidentified by me) might be generated by this change...