Sohmer Model 41 cupid grand (1933)
Action ratio is high - around 6.1
Action spread is 113.5
The original reps use a low angle jack that appears to match up well with the WNG low angle jack. It also looks like I will have to modify the wippen rail with a clearance cut, no big deal.
Since the dimensions lie outside of current standards, or even if they didn't I would suggest starting your action redesign by proving the stack geometry.
In setting up S&S action frames for techs of many different action design skill sets, I have found that there is a fair amount confusion about what the stack geometry is supposed to accomplish. I'm designing a class for next years Boston NEECSO called "Understanding Spread" to address this confusion.
As I see it, and I would love input from Del, Ed Foote, et al regarding the the following, the goal of the stack's geometry is to:
1-arrange for the jack to contact the knuckle at an advantageous angle with key at rest.
2-locate the jack in the rep lever window so that there is sufficient room for both escapement and aftertouch without bottoming out on the jack stop felt
3-with the jack/knuckle angle at an efficient rest angle, the jack located with sufficient room for escapement/aftertouch, and the action feet sitting elevated on the tapered or non-tapered riser blocks, arrange for the shank, at rest, to be 5mm or so off the rest felt.
As far as overall key&action leverage, within functional limits defined by the given parts, the stack has very little to do directly with defining overall key&action leverage. Leverage is primarily defined by key balance point, capstan location, knuckle distance from shank center, heel distance from whip center and shank length.
So in changing parts, you need to prove that the 3 items above can be accomplished with these new parts, on the existing or modified action frame.
To do this...
Mount a new rep flange and shank flange on the existing stack. Measure the spread. Also with the action frame mounted on the key frame and riser assembly,measure the height differential between the whip center and the shank center.
What you have just measured is, in effect, a radius of a circle(spread is a radius with the shank center the center of the radius), and where on that radius in, relation to horizontal, the whip center lies.
-Take a 1'x1'piece of mdf of other flat smooth sheet stock.
-Set a drafting compass to draw a radius that matches the spread you just measured.
-with a .050 drill bit, drill a hole 1/4" or so deep in the center of the mdf. Set one foot of the compass in that hole(that hole is your shank center location) and draw a spread arc.
-draw a line, call it line#1, through the shank center hole and roughly perpendicular to 2 sides of your 1'x1' square mdf.
-draw a line, call it line #2 parallel to the line you just drew, offset from that line by the differential you measured between shank center and whip center heights. Current standards use 2.5" as this differential, but your Sohmer might well be different.
-you'll notice that line #2 intersects the spread radius at some point. That point is your whip center location. Drill .050" hole at this whip center location.
-Take a proposed shank and whip, with a pin punch push the center out enough so that the center pin can engage the holes you drilled in the mdf. Locate the shank and whip as defined by the drilled holes.
Set the parts so jack touches the knuckle, while the shank is resting 5mm off jack rest felt. (with rest rail, you will have to transfer this felt location from the existing stack. On an integral whip with rest felt, you are all set).
-In this position, a line drawn perpendicular to the shank, through the center of the knuckle spline, should pass to the distal side of the jack center by a 1/8" or so. This line is defining the jack/knuckle contact angle referred to above.
-Also, in this position, where is the jack in the rep lever window in relation to the "score mark"...ie, is there enough room for escapement & aftertouch.
If the parts don't line up with these standards, or somewhere reasonably close to them, draw other spread arcs and differentials until things alighn properly within the stack.
Hope that's not too long and hard to understand. The point is that, rather than recite dimensions to adhere to, understanding what you are trying to achieve gives you more scope to understand the action geometry changes you are trying to make. It will allow you to understand why you should either avoid the changes or make the changes with confidence.
ps chances are the string height on this Sohmer are outside of the 7 1/4" to 7 1/2" standards. This is more reason to make sure the new parts will be able to be aligned with the various parameters defined by the existing piano.