>Any suggestions for why that one key hits bottom so often with a
>thunk?

First thing I would try is playing the key with the jackrail off. If the
thunk goes away, take a look at how the cloth on the rail's underside is
attached.  (a) if with little "cloth nails" as ZHI calls them, the top of
the jack may be hitting the nail just right (wrong).  Easy to pull and
relocate the nail slightly. Not too much or you'll end up with the same
problem on a different jack.  (b) if the cloth is glued in, perhaps the
jack is hitting a bit of cloth hardened by glue.  I've encountered both
situations, and especially with a resonant case, the thunk produced can be
quite annoying! You could also raise the jackrail very slightly (i.e. no
more than necessary  -- and ONLY if this is the source of the problem.) You
already know that the more you raise the jackrail, the more "slop" you are
liable to introduce in the action. is one of the jacks standing on the
offending key perhaps a skosh too long? It would hit deeper into the
jackrail than the others and perhaps cause the noise.

If you've eliminated the jack and are sure the thunk is coming from the
key, something you might not have checked is the clearance between the
underside of the key and the front edge of the balance rail. I had a
clavichord once with a square balance rail, and I had to chamfer the front
edge so the keys wouldn't bang against it. You could also carve away the
bottom of the key (if you can't raise the river, then lower the bridge, an
old DJW-ism), but I'd be VERY careful carving so close to the balance pin,
not to mention altering the balance of a key which - apart from the thunk -
presumably works fine.

Close observation is the key (yuk, yuk) to a proper diagnosis, which sounds
like what you're doing! Hope some of the above is some help.

Cheerio,
Brian Joyce