Many M17s have experienced bent turnbuckle T-bolts on the upper shrouds when raising the mast. This can be prevented once you understand how it happens. The photos here show the problem.

The shape of the chainplate allows the outer turnbuckle's shackle to rotate to the outside when the shrouds are loose. If it rotates far enough, tension on the cable will translate into bending motion on the T-bolt, at an angle that does not tend to turn the shackle upright. Eventually it snaps into place, but the bolt may already be severely bent by that point.

Preventing bending requires only that the shackle be turned upright before raising the mast. You can even do this by hand, although if you're unlucky it could slip back before you get the mast up. At Thomas Buzzi's suggestion, an easy and permanent fix is to slip a screw through the shackles of both inner and outer turnbuckles and add a nut on the end to hold the outer one upright. I used a #10 screw cut to about 1-3/4" with a nylon locknut. This is small enough to be a loose fit, ensuring that no additional strain is added when the mast is up. The head of the screw is to the inside, requiring the inner shackle to be removed to install it, because there is not quite enough room for the locknut on that end.