FASTENING
As in drilling and trimming, there are a few general principals that must be understood:
– DO NOT use rivets to attach the windows if fasteners are required through holes drilled in the window or windshield. With rivets, the clamp-up forces can not be controlled and will overstress the material, causing future cracks. Use screws, washers, and self-locking nuts.
– DO carefully control the torque of the fastener. Watch the reflection of a strong light in the surface of the acrylic around the screw head. Tighten the screw and nut just until you see a distortion in the surface around the screw head and then back off just until the surface is distortion free. At this point the screw and nut assembly may be able to be turned with a screw driver but probably not by hand. Since acrylic expands and contracts at different rates than the airframe, this method will allow the window slight movement.
– DO use upholstery-type or tinnerman-type countersunk finishing washers under flat-head countersunk screws. Flat-head screws will hold the acrylic much too rigidly without washers. If you use the tinnerman-type washers, keep in mind you will have to drill the hole much larger to accommodate the countersunk portion of the washer. Again, the general rule is to drill the hole at least one and one-third the diameter of the fastener, which in this case would be one and one-third as large as the countersunk portion of the washer.
– DO watch for anything that could cause localized stress in the material. Mounting over a rivet head, for instance, will cause a lot of stress in that area, and will probably crack when adjacent screws are tightened. Watch out for raised weld beads, sharp corners, etc. If necessary, bridge the window over the obstruction using washers as spacers, or remove the obstruction.
Drilling, trimming for fit, and fastening acrylic windshields and windows is not difficult, you just have to understand the material, the tools required, and the proper methods. With a little practice and a little time to get the feel of the processes, you will have a very satisfactory installation with a long service life.
STORAGE: DO NOT ALLOW THE WINDSHIELD OR WINDOW TO BE EXPOSED TO SUNLIGHT WITH THE PROTECTIVE COVER APPLIED. EVEN SHORT EXPOSURE WILL MAKE THE COVERING VERY DIFFICULT, IF NOT IMPOSSIBLE, TO REMOVE.