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If an upright regulation is to manufacturer's specification what is the most likely cause of the jack starting to barely clearing the wippen? Applying more pressure to the key after let off does provide more clearance. No changes have been made to key felts.
Ian

The specs are wrong for the current configuration...

My quick guess would involve setting the blow distance to specs with shorter hammers due to being reshaped - slightly changing the starting angle of the hammer shank?

That's why I usually focus on functional regulation as opposed to any numbers. Try reducing the blow distance just a bit and see if that makes it all work better.

Ron Koval

Oh, and I find setting let-off too close often will cause blocking hammers as the seasons change, so especially on older or instruments serviced less often, I will set that a little wide of the specs.

Ron Koval

In pianos specs are just a starting point. If they don't work you must make an adjustment or two. There are so many variables and some are only in .000" but can have big impacts elsewhere. Good factory workers know how to make these adjustments on the fly, as do techs who understand the action inside and out.

All that being said...there is always a REASON any spec(s) does not work. If the reason is correctable, great. If not, adjust.

Peter Grey Piano Doctor

My thanks to you both for your answers. I'm referring to my own piano which I have been regulating since it was new. I do have to change the let off as the humidity changes (damp chasers are not normally fitted in UK homes). I have never needed to change the blow distance but I will give this a try.
I'm still curious as to which component has, during the period between regulations, the greatest effect in reducing the jack clearance? e.g. felt or wood?
Ian

Both 😁

BTW (and I think you already know this), let-off is in fact a tolerance to account for seasonal humidity change. Yours may just need a dight more for your situation.

Peter Grey Piano Doctor