Wood Screws
I typically sort the hex-head wood screws from the bottom of the playfield into 2 containers. One gets the short screws, which are used with thin sheet metal. The other gets the longer screws, which should only be used with thicker metal parts (e.g. flipper assemblies).
Some screws are even longer than the normal "long" screws. For example, this playfield bracket:
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When in doubt, keep the screws with the part in a large ziplock bag. I had a screw pop on my CSI playfield because I used a PF bracket screw on a standup target assembly. That was painful! Learn from my fail.
One place to be extra-careful on WW are the plates that attach to the bottom of the playfield. These plates are thick sheet metal, but they're going into a recessed area! So you MUST use the shorter screws.
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If the area is not recessed, it's really easy to tell whether a particular screw/part combination is safe to use. Just hold them both against the playfield and make sure that the screw doesn't reach the last layer of plywood.
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After reading all of this, you might be tempted to use the shorter screws everywhere. That would also be a mistake - the mechanisms could rip out of the bottom of the playfield, perhaps causing damage when they do.