Question: How do you decide the correct thickness for replacement front rail and balance rail key felt bushings, when the old ones are so compressed and/or moth eaten that you can only guess at the original thickness? thicker is not necessarily better if it's not optimal, no? What to measure? Have to rely on expert testimonials on this one.
Thanks all. Ready to order parts, and want to get it right the first time.
thin
medium thick
thick........hmmmmm...........I do have a micrometer in my tool box now, but seems kinda silly to be mic'n felt.

pardon me, I meant punchings, not bushings

It is the back rail cloth which is most important. You can probably measure that reasonably well, but I always do a visual comparison. Everything else is measured from that. Balance rail punchings should leave you a little shy of the proper key height, and front rail punchings should be a bit more shy of the proper key dip.

It is hard to determine proper felt thicknesses without samples.

Ya, I agree with BDB. There are usually spots on the back rail cloth that haven't been compressed, so you can determine the intended felt thickness there. If you screw up on the back rail then everything is screwed up.

Then you want to add enough thickness at the balance rail to give you the right key height, then do dip. (Key frame glides must be regulated first.) Do samples first and confirm that key height and dip look correct in terms of clearance under the fallboard, at the key slip, under the key upstop rail, and how much of the center rail pin pokes above the key button. Also confirm that the key capstans are in a normal range of adjustment when the hammer distance is correct, not screwed in or out too far. If any of those confirmations don't look right, then the key height spec may not be a good one and you'll have to determine another.

Usually if you get the key height right all the other stuff will look fine. Sometimes not, because factories can vary in what they put out. That's why you need to check it against that other stuff.

I always keep keep several thicknesses of felt punchings in stock and try out a few in order to determine what I need a whole set of. Most of the time it will turn out to be thin, medium thin, or medium.

I figure the thinner the better and always use the best quality felt, because it will lose dimension less in the future.

There are always a few approaches to almost any job which will (eventually) get you where you want to go.

Remember when choosing the felt balance rail punchings that above a mf blow, a thicker punching will compress a lot more than a thin one. It essentially becomes a springy, poorly defined key fulcrum, which sucks energy out of the system (key - wippen - hammer - string - soundboard). I prefer to go with a thin, FIRM balance rail punching. Stay away from those .125" marshmallows. Shim to get the proper key height using the smallest number of paper/cardstock punchings.

A good rule of thumb is: no more than 4 paper/card punchings, and no more than 2 of the same thickness.

I love this forum
Steve