Then I had a closer look at the old belts. As you can see they sit low down in the pulley, a tell-tale sign of wear. Luckily, the belt was not bottoming out:
Fitted the same make and size of belt, but unworn (bottom one in LH picture). Worn belt on top of and unworn one - note the obvious difference in length!
Approximate dimensions (mm) of BTR 51050 V-belt: worn | unworn
Internal length (Li): 1373 | 1355
Upper belt width: 16 | 17
Lower belt width: 8.5| 9.5
So, the width of the belt decreased by 1 mm, and the length increased by 18 mm - they do strech!
Generator bracket is now fitted on the adjustment bolt properly (compare with above), but still at the end of its adjusting range:
New B52 belts are on order, should be available over the counter tomorrow. While the B52 belts are 35 mm shorter, it will help getting the generator lower down on the adjustment bolt. Stay tuned!
Hanno