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Talked with a colleague yesterday about this and wondering if anyone has some theories. I was working on an 80 y/o Canadian upright and there were about 25 sluggish hammers between the mid bichords and the central part of the keyboard. Upon closer inspection, it turned out that the sluggishness was due to the wippens center pins all drifed out to the right and dragging on the wippen beside it. The piano is not going to be kept much longer so i just quick fixed it with a flat blade pushing them back in place.

Anyways, this got me thinking. Why would they all shift out to the right and not haphazardly this way and that?

I'm thinking it might be that the CP's were snipped off with dull tool and mushroomed a bit on one side with factory production. Any other possibilities?

The angle of the strings?

large humidity swings create center pins going on holidays in my experience (as the absence of tightening of the little plate on the vertical butts , of course)

The earth turns to the left……

Ok seriously I had the same thing once with those Japanese butts with the little metal plate and the flange attaches to the metal rail.

One drifted out and pushed directly onto the next one, then consecutively for about a half dozen flanges.

The mushrooming is a plausible explanation. Could be the whippens were loose and vibrating.

There must be a lateral force moving those pins to the right. If the hammer does not hit the strings entirely squarely then the forces at the flange will not be entirely square. There will be a component of force along the pin.

The hammers for those notes in many pianos are angled as are the strings so nothing is entirely square. The asymmetry is reasonably consistent.

Just a possibility.

Hi Dan, all Renner butts have that little plate , here.

But for the whippens also, I noticed direct relation with humidity swings , never find that problem in normal moderate conditions.

Just a guess...

Maybe the pins were all inserted from the right side during the manufacturing process?

Then, maybe they weren't all pushed in the whole way when the piano was manufactured?

Then, expansion and contraction of the hole in the jack over the years allowed the pins to become loose enough?

one side of the pin is "pre cut" and does not have the same termination than the other ( a little rounded because of the groove allowing the cut)
May be that is enough to generate a move with moisture change, the cloth swell and may be able to push a little more on that side than the other.

AT the same time, when humidity is high, the wood swell and the fit is less tight for the pin (I suppose)

The grooved side allows me to know what direction the pin was inserted originally

On the German Steinway parts, (Renner made) the center pins have a groove in the middle.

I wonder if this is not done to allow a better fixation of the center, the wood may swell in the groove and any lateral motion is avoided then.

Never find that groove on other parts