separated back

[BSimon1234]

Clyde Hollinger wrote:

<<5.  Remove the plate screws across the top edge, to be replaced with
carriage bolts.  4.  Drill through the entire back to take carriage bolts big
enough to do the job (what diameter?). >>

Let me relate a mistake I made in the first year of rebuilding pianos, - a
long while back. I also had a separated back, epoxied it ( or titebond?)
together and installed 4 or 5 bolts with a diameter of  FIVE EIGHTS OF AN
INCH!  Why so huge???? - because the heads on these bolts matched the heads of
the plate bolts I was replacing!  I wanted that authentic look. Also, I really
wanted to be sure that they were strong enough.  It was my own piano, later
sold, but an excellent rebuild was partially  spoiled by huge 1 1/8" nuts  on
the bolts sticking out the back.

Later, as my understanding of materials and the strength of steel developed, I
found that a 5/8" bolt will hold at least 20,000 pounds of tension. The proper
bolts to use should have been 1/4", although even 1/8" will hold many times
the needed tension. A 1/8" bolt will hold perhaps 400 pounds of tension,
safely,  when all you need in 20  to 40 pounds to resist the plate from
pulling forward. Look at the Everette console backs, I think they use 1/4".

There are bolt tables in most hardware stores showing the strength of bolts
versus the size. The lowest grade of bolts, SAE grade 1, low carbon, has a
minimum strength of 65,000 pounds per square inch. Do the math on the cross
section area of a 1/8" bolt, and if I did it correctly you get 797 pounds
minimum tensile strength. And that for a 1/8" bolt! a grade 8 bolt would be
about three times as much. (a  1/4" bolt  is therefore about 3000 pounds) 

incidentally, a carriage bolt isn't the "be all and end all" - a regular cap
screw (machine screw) with a slotted flat or oval head, in conjunction with a
wood nut ( has prongs that sink into the wood, winds up  flush with surface)
makes an excellent repair. A 1/4" diameter will probably give you the exact
size for the plate screw hole depression.

Two points:

There was no pianotech list at that time to save me from my error, you are
blessed.

and 

Not much tension is needed. I don't think that very much stress is put on
plate bolts to resist the plate lifting moment, so not much force needs to be
used to hold it down, especially since plates have many bolts, all joining
forces. Think of the loose plate screws you have tightened and how easily they
pulled the plate down with a light turn of the screwdriver. ANY OPINIONS?

Bill Simon
Phoenix