Supporting grand damper underlevers on a rigid, straight fixture during damper installation is the best way to ensure a perfectly even lever height. That in turn allows uniform damper lift with the pedal and the keys, even sostenuto operation, and accurate damper stop rail adjustment. This jig does the job beautifully, with simple knurled brass height adjusters and a clean, functional design.
Correct jig height can be set directly from the key end felts, as explained in the included instruction sheet. The jig slips apart into two pieces for easy carrying.

You will get a more even lever height, and less tendency for damper heads to rotate as screws are tightened, by weighting the damper heads as you adjust them. This seats the felt to the strings, simulating the height that the damper heads will assume once the weight of the levers is pulling down on them.
This Damper Bean Bag does the trick. Just lay it on the damper heads, and move it along as you proceed to tighten the top flange screws. This tool is a perfect companion to the Damper Underlever Jig above.