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| p.1 #4 · Canon EF-S 17-55 Parts Question |
The track moves the element forward and backward a bit - you can move it and see the difference it makes in sharpness, but mark where it is supposed to be - it takes a lot of trial and error to get it back once you've lost it.
Most commonly, the plastic screw holes are stripped, not the screws. Unless it's rotated so far that only one of the three screw holes under each slot are accessible, you might be better off simply inserting the screws in a different hole.