Plate Horn

[Avery Todd]

A question for you rebuilder types:

I guess basically it's what is the procedure, when restringing, to 
ensure the plate
horn is snug? Here's what happened:

My apprentice had restrung the plain wire section of a Yamaha G3. To 
save a little
time, I'd pulled up the middle strings for him with my SAT before he 
arrived (whole
step flat) and then he was in the process of pulling in the outside strings.

I happened to be standing on the bass side and noticed a piece of 
gold-painted metal
lying on the keybed. A little quick looking and we discovered it was 
a wedge between
the horn and the belly (is that what that area's called?). Well, it 
wouldn't go in
but just a little, so I had him start lowering tension, starting at 
the bottom of the
tenor section. After several notes, the wedge fell out, so I knocked 
it up again as
far as it would go. Had to remove the sostenuto rod to give some 
"banging" clearance.
Then, same procedure done again and once more the wedge fell out. We 
continued this
until the wedge was almost all the way in. Finally, it went all the 
way up to where
it'd been originally. Plate screws had been tightened, BTW. I'm 
assuming everything is
OK now. No plate cracks, etc. :-)

I've never had this happen before and am wondering why it did. 
Obviously, the plate moved/flexed a little for this to have happened. 
I'd never seen a wedge like this
before, so didn't think to check it. I don't know exactly when it 
fell out: while
unstringing, pounding in the pins when restringing, while pulling up 
the pitch?

Any ideas on what I should have done to prevent this from happening? 
Any other comments,
other than how lucky I am to have discovered this before the piano 
was fully strung and
up to pitch? :-) Thanks.

Avery Todd