Testing the Backrail Fit:

With action fully assembled, keybed clean, spring out, cheekblocks bedded, and glidebolts off keybed, reach through dampers, strings, and action parts with a long screwdriver (or other device), rest tip on backrail cloth, and tap. If there is a gap in the fit, a high-pitched tap should answer back. Try 2-3 places in each section.

Hidden Glidebolts:

Not all glidebolts show. Unless you are familiar with the make and model of a grand piano, it might be worth the extra effort to pull the action enough to have a look at its underside.

Keys must clear a protruding bolt or provide space for screwdriver access to a slotted bolt. In some well-known keyframes, one of the glidebolts will be offset toward the back to facilitate the placement of that clearance. But there are keyframes, like the one shown above, that simply offer no access except by key removal.