The backing plate is set in the glass, but not that firmly. I had sheared off a gudgeon bolt on my 17 and when I tried to extract the stub, I ended up 1) stripping the threads on the backing plate and 2) pushing the plate in so it wouldn't catch the bolts anymore.
Can you say "access port?" D'OH!
Just be careful not to push in on the bolts to much and you'll be OK. While you've got the gudgeons off, you might want to try the newer 1 piece cast aluminum gudgeons. They eliminate any alignment problems between the upper and lower old style steel gudeons. They're kinder to the pins, too.
Matt Bounds
