10 December 2015

Poor Fastener Selection

The Primary Arms 5x Prism sight came with a mount that attached to the rail with two nuts.

I don't want to carry a 1/2" wrench with me in case the optic goes tits up, so I ordered an LT-681 mount for it.

Primary Arms states that you can use any mount that's compatible with Trijicon's ACOG line, and LaRue's mount certainly is.

The problem isn't mount compatibility.  The problem was getting the mount that came with the scope off!

The threaded holes in the scope are M5x0.8 threads.

The screws Primary Arms uses are grade 10.9 (grade 5 equivalent) counter-sunk hex-socket heads.  The key-hole is the standard 3mm hex.

The screws are generously Loctite'd into the holes.

This is where it all goes wrong.

The first screw took a LOT of force to get out.  You can see the deformation on the hole from the stresses.  The second... the hex-key wrench stripped out the hex-socket.


Because it's counter sunk and tight quarters, and because I'm pissed now...  To the Dremel tool!  Yes, there are ways to get that screw without hurting the mount, but have you ever experienced the catharsis of destroying the offending part?  It is joyous, you simply must try it sometime!

So I made it into a flat-head slotted countersunk M5x0.8 screw.  And took a lot of the Primary Arms mount with it getting the cutting head in there.  I also applied a judicious amount of heat to the area to help break the Loctite.  GRRRRRRR!

I will say it again:  Countersunk screws should be flat-head slotted.  They bind like no other screw so they need more torque to get unstuck than any other type.  Flat heads Slotted are the best at getting torque applied to them.  Every, EVERY other form of screw driver is to speed manufacture (Phillips) or to lock the customer out of repairs (Torx) none were designed to be stronger than the basic flat-head slotted.

I cannot help but wonder what someone who is not mechanically inclined would have had to do to get this mount off.  I have tools and some know-how.

Good thing the LaRue mount came with its own fasteners!  Metric threads with an inch spec hex-socket button head...  Mark LaRue must have worked at GM once.

All's well that ends well!

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