I have two shields and an M&P 9c on order. At some point I may wish to install a different rear sight. My concern is replacement rear sight may not be tight fit and require loctite. Specifically concerned that loctite could get into plunger under rear sight that acts as firing pin safety and jam it up. Does anyone have experience with use of loctite on rear sight? Info and suggestions appreciated. Thanks.

Not sure this will help you but a couple of the Shields I purchased had evidence of red Loctite around the rear sight set screw new out of the box.

You can put some thin shim stock under the sight to tighten it up. Loctite is just for the set screw.

The sights come out easy. The new sight should fit good and the set screw will hold
It the rest of the way

My favorite securing method with sights is confirm they are shooting to the desired POA, then apply Green (wicking) Loctite to the front and/or rear dovetails if they did not offer much resistance to the pusher. Apply just enough to wick into the dovetail, then in the case of the rear sight, invert the gun while the Loctite cures, to use gravity to discourage wicking into the drop safety. After an hour check that the drop safety plunger moves freely, and you are good to go. For set screws Blue or Purple (weaker than Blue) Loctite should be applied to the removed screw, then tightened in place. Red Loctite is a really bad idea on sights, and heat is often needed to loosen it, if necessary.

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Clyde Washburn
NRA Cert Inst

I think your getting ahead of yourself. Why assume the replacement rear sight will fit sloppy?
It's my experience they come over sized to just right. Often requiring fitting IF they are loose, a few prick punch marks in the cut can make up some space. Then blue locktight applied sparingly(!)to the top surfaces of the sights bevel will keep the juice away from the plunger assy.

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Dave

You should only need locktite on the set screw that holds the sight in place. I have replaced the rear sights on four Shields and they were very tight to get into place. I only added locktite on the screw, not the sight or dovetail. After over 1000 rounds through my own I have not had any problem of the sight moving out of zero.