I'm installing a new Stwy back action in a 1920 M, and following David Hughes' excellent protocols, measured the old and new distances (to the keybed front) of the underlever noses and sostenuto tabs. It turns out that the new tabs have moved 1/4" forward from the old tabs. This was confirmed by trying to regulate the sostenuto (with the action unchanged from its original location). To get the lip to engage the tabs by 1/16" during a proper regulation, the rod and its brackets do have to be moved back by 1/4".
So how to recover the 1/4" lost to the backcheck/sostenuto clearance in this situation. If I move the entire back action back so that the new tabs match the location of the old, the pick-up point of the underlevers on the key end felt moves from 50% of the key end length to 20% ( - at 0% you fall off the end of the felt). I've never see anything like that, and can only guess at whether the wear over the years would be less, applied to the middle than towards the end. As for whether the damper posts, moving 1/4" backwards, would be far enough off of directly under their guide rail bushings to lose that advantage of damper wires traveling purely vertical…I can only guess there, too. There is room behind the damper assembly for this 1/4" move.
The other idea is to do a Forstner bit cove in the backside of the backcheck head. (This action is getting new backchecks and wires.) In the accompanying photo, there are two 5/16" arcs, one going 1/4' into the head and the other, 1/8". Clearly, the 100% solution (asking the backcheck head to cough up the missing 1/4") doesn't leave enough thickness to convince me that the head won't crack under heavy use. But the 50% solution (1/8" into the head looks do-able). (Assuming that the backcheck wire will be clear of the cove.)
So I'm thinking the best way out of this situation is to get an 1/8" from both of these locations.
Anyone else been in this situation? And come up with a better solution? (I thought for a while that converting to the Hamburg belly-rail-mounted sostenuto might work, but that's out of the question.)
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William Ballard RPT
WBPS
Saxtons River VT
802-869-9107
"Our lives contain a thousand springs
and dies if one be gone
Strange that a harp of a thousand strings
should keep in tune so long."
...........Dr. Watts, "The continental Harmony,1774
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