KS 8: Bedding the Backrail

KS 8: Bedding the Backrail

How to proceed when a backrail tests positive for a gap? So far, with no excessive friction or need for repairs, the action has stayed together. Maybe it received a screw tightening or superficial cleaning. But it's possible, only minutes into this job, that we could soon have samples taken and be on our way.

There could have been the delay of an extensive keybed cleanup (as with the Welte). Or vertigris in the centers might have added more hours or required replacement parts. Such variants, as with this one, a misfitting backrail, change job scope and need to be accommodated in initial estimates, acknowledging time and dollar implications.

Whatever is needed, our hoped-for course should provide valuable improvements at an agreed-upon price structure with least anxiety for our customer and a reasonable profit for ourselves. This week's photo shows a balance-punching shim, glued to a keyframe cleat with its hole centered over the topstack screw hole. When assembled, the topstack forces shim and backrail down to complete the fit with substantial savings of time and money.

If a sanding solution is needed, however, remove topstack and keys from keyframe. The dags we have shimmed to correctly self-position the keyframe. The return spring we have removed, so a light nudge left will seat it in its at-rest position. And we have backed up the glidebolts so they won't influence our perception of the backrail fit. With no keys or topstack in the way, reach a hand in and tap along the backrail cloth for quick discovery of areas that tap back. Mark gaps with chalk on the backrail cloth.

If the rail mostly fits, except for an area under or near a cleat, try touching the cleat lightly. If now there is no tap, a balance-punching shim will solve the problem. With no keys or topstack it may tap, but that is not how the action sits on the keybed to be played. If it fits when assembled, it fits. And the speed of adding a shim, compared to reducing the areas of contact until the tap goes away, is compelling. Inverse alternatives to this approach: planing a cleat, grinding a bracket foot, or in the case of a Steinway brass-railed topstack, twisting the topstack - all of these can be part of a solution depending on what is needed and why.

The shim solution has the merit of simplicity. All's fair in this mating game, but the superior solution is simple. The ideal would have keyframe backrail fully fit keybed on its own. Then, topstack sits on keyframe, bracket feet fitting cleats or shimmed to fit them. And when tightly secured, the topstack reinforces the fit. The factories employ this passive approach to good keyframe structure as the most stable and longlasting solution.

Two spots in a keyframe's backrail interrupt its integrity: the tenor break for sostenuto pitman clearance and the treble channel for softpedal lever access and clearance. They can cause stability issues, particularly the softpedal slot, producing a warp that may be exacerbated by removal of material. In this case, a shim may easily solve what sanding cannot, or at least, cannot easily.

If part of the backrail has been removed for a player system, its lip has likely been lost. This, coupled with the shortened arc of the keyframe front-to-back, will have significantly changed topstack elevations. Glue on a strip of maple to rough this back in, identify areas lacking contact, and proceed with the process below.

Sand the areas of contact surrounding a gap until the gap disappears. There can be the unhappy challenge of misinterpretation, so validate areas of contact in case there is a gap that is too big to tap. Sounds obvious, but sanding a gap is a really big setback. 

I recommend 220 open grit sandpaper to remove modest amounts at a time. If faster removal is needed, use 120 (80 is too coarse). With the keybed pre-cleaned, the work should all be sanding the keyframe. It's too awkward, too slow, and too athletic to pull strips of sandpaper between keybed and backrail as in fitting a frontrail. 

Pull action out of action cavity and place frontrail on floor (protecting the floor as needed), keypins toward you. Using a sanding block to keep work general so that neither softpedal shifting nor position tweaking for tone will undo the resulting fit, reach over the backrail to sand the areas of contact. Feather as you go, sanding most in the middle of these areas, least at the edges. Go slowly, wear a mask, and keep your work dusted. Patience will help you complete this in the least time with the least effort. And finish with a balance punching shim if the opportunity presents.

Next week: Bedding the Frontrail

(Index of all articles in this series)

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