AR-15 trailer hitch cover, Mk 2.

Updated May 6, 2024: Further simplified print.  Now uses the Floating Holes Trick to eliminate the printed “washer cover” part.

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My original “over-engineered” AR-15 “bolt head” trailer-hitch cover had some issues, up to and including falling off on the road 😨. This new version is somewhat simpler and, I hope, stronger.

Features:

• Dimensionally-accurate (but scaled up) AR-15 bolt head.
• Installs using standard hitch-receiver pin.
• Two-part print.
• Reinforced with embedded steel threaded rod.

Additional tools/materials required:

• 105mm length of #8-32 threaded rod.
• Two #8-32 “nylock” nuts.
• Two 1/8"x3/4" flat washers.
• Good epoxy (JB-Weld, etc.)

Print Instructions

I printed it in a black PETG.

• Slice the bolt head, with the following particulars:
  • It should be oriented with the bolt-face down on the print bed.
  • Supports enabled everywhere.
  • 15% grid infill.
  • Add a support-blocker, loaded from the provided “modifier.stl” object.  (This prevents supports from being generated inside the nut/washer cavity.)
  • Add a modifier, loaded from the same provided “modifier.stl” object.  In that modified region, set the infill to 30% gyroid.  This strengthens the area immediately around the nut/washer.
  • Add a “Pause Print" (M601) at a height of 30.80mm, where it's just starting to bridge over the nut/washer cavity.
• After slicing, verify:
  • Supports are being generated inside the bolt face.
  • Supports are not being generated inside the nut/washer cavity
  • Denser infill around the nut/washer cavity.
  • A pause just before the bridging over the nut/washer cavity.
• Start printing the bolt-head.
• At the pause, drop the nylock nut into the cavity, nylon-side first.

• Place the washer over the nylock nut.

• Resume print.

For the hitch bar:

• Slice the hitch bar, with the following particulars:
  • Placed on its side, with the cross-pin hole oriented up-down. (This will get the greatest strength of the print oriented in the direction we want it.)
  • No supports (didn't need any for me.)
• Print the hitch bar.

Assembly:

• Pull support material out of the bolt face.
• Cut a piece of #8-32 threaded rod, 105mm long.
• Thread the rod into the back of the bolt-head until it bottoms out.  To make it easier to turn, spin a nut onto the end, and then a second nut jammed up against the first with a wrench. That will give you a temporary hex head on the rod you can use to turn it.

• Test fit the hitch bar onto the bolt-head, with the cross-pin holes at the far end away from the bolt-head.  Make sure the end of the threaded rod is not going to interfere with the hitch-pin.  Shorten it if necessary.  Remove the hitch bar from the bolt-head.
• Mix up epoxy and slather it in the recess in the back of the bolt head.
• Reinstall the hitch bar.  It should be oriented so that when the bolt-head's extractor hook is at 10 o'clock, and the ejector pin at 4 o'clock, the cross-pin holes in the hitch-bar will be horizontal.  Bed the hitch-bar down in the epoxy.
• Install a flat washer, and nylock nut through the back of the hitch-bar, and tighten them down.
• Optionally slather some vaseline onto the nut and washer, to protect them from rust.  A couple salty Southern Ontario winters are guaranteed to rust it solid.
• C'est finis!

Design Rationale

The previous design was too easily damaged, just by bumping into it with a knee for example.  The last thing I want is to have this thing fall off on the road and cause an accident.

That's why I printed the hitch-bar portion on its side (to get the strong axis of the print oriented the right way), and used an embedded steel rod.

May 6, 2024 Update

This update slightly simplified the print, using the Floating Holes Trick to eliminate the printed “washer cover” component that was placed on top of the washer mid-print.

The original versions of the files are still here, in the “Old (Rev 1)” folder.

Simpler, and stronger construction.