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I am getting ready to install the nutplates (MS21075L3) on my lower fuselage to secure the access panel. The screws are AN525-10R7 and they start to thread easily, but them stop. It takes considerable force for them to thread all the way in, and appears they are tapping themselves. Is this correct? If so, what is the proper technique for these? Should I thread a screw into each nutplate before installing it to create the threads, or do you just allow this to happen the first time the screw is installed?
the nutplates are crimped to create the locking action.You can screw the screws in and it will work but is sometimes extremely hard on the screws. Some have ran a tap in but you want to check the fit and make sure they are not to loose after tapping.(i took a used tap and lightly ground the tops of the threads so it was slightly undersized. This reformed the threads rather than cutting them) You can also run a "sacrificial" screw in the first time and it will usually give the proper fit and the screw is good for a few nutplates before it is disposed of and replaced.
Excellent, thanks. I think I will do the sacrificial screw idea. I mostly wanted to make sure that this is correct and I am not trying to force a round plug in a square hole!
I also like the "sacrificial screw" method and usually lubricate the screw with a little wax to help ease the effort required.
I have on occasion run a tap through a nutplate if it is a non-structural fastener and especially if the screws are going to have to be removed frequently.
Try putting ExTurn on the screws. That worked for me.
I also used a tap to chase the threads after my third annual condition inspection. I got tired of fighting them! It works well and still holds the screws securely.