Originally Posted by Luddite

As Dave G said, with the horizontal cylinder at TDC power, the cam on the vertical cylinder will try to rotate under the influence of the helper springs, which would, (in theory), affect the tension when you're checking the belt.

If you use the cam locking pins, then you can check both horizontal and vertical belt tensions with the motor at horizontal TDC. If you're not using the locking pins, then rotate the motor until the vertical cylinder is at TDC power.