Dear John and the list,  I don't have a definite number to give you on
the torque for plate bolts.  But I use approximately the same force to
tighten the plate bolts that it takes to move a tuning pin in the
pinblock of a brand new Baldwin piano, or maybe a bit more.

Before you tighten the bolts all the way, may I suggest the following:

1.  Clean out the opening between the pinblock and back frame of the
piano to remove any debris.

2.  Take 4 or 5 large pipe clamps and clamp the pinblock/back frame of
the piano together.  At this point don't tighten to the point where the
gap is closing yet.

3.  Get a good quality marine epoxy from your local hardware store,
mix the epoxy and the hardener according to the instructions, and use
a thin flexible putty knife (or artists pallet knife) and work as much
epoxy into the opening as you can.

4.  Tighten the pipe clamps and the plate bolts to close the gap
between the pinblock and the back frame of the piano.

5.  As the opening narrows, from the increased pressure of the clamps
and plate bolts, you should see the epoxy begin to squeeze out of the
crack.  Wipe up the excess with an old rag and epoxy thinner.

6.  If the plate screws are stripped out, then remove them, drill the
screw holes all the way through the back of the piano, and then install
3/8" carriage bolts, lock washers and lock nuts (or decorative acorn
nuts).

7.  Leave the pipe clamps on for at least overnight, or until the epoxy
has completely hardened.

8.  Recheck the tightness of the plate bolts, remove the pipe clamps,
clean up any sawdust, and reinstall the pinblock cap.

This procedure is outlined on page 100-101 in the book, "Piano
Servicing, Tuning and Rebuilding", by Arthur A. Reblitz, Second
Edition.

In this case, step 6 wouldn't apply because the piano already has
through bolts installed.

Good Luck!

David A. Vanderhoofven, Registered Piano Technician
Vanderhoofven Piano Service,  Joplin, MO
http://www.clandjop.com/~dkvander/