I get anti-tamper Torx license plate fasteners at Auto Zone, Pimp Boys, and I've seen them at local metric supply houses. I use them on all my vehicles including motorcycles.

I've also seen people fill the cavity of an allen drive bolt with JB Weld epoxy after they tightened it up on the license frame. If they want it off later they pop it with an automatic center punch. Thieves will move on to easier prey.

My father used to attach his plates with phillips head drive bolts and after they were tight he would drill the center of the bolt to remove the slots. To get the bolts off he simply drilled down farther until the head pooped off.

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You can get farther with a smile and a gun than you can with just a smile.

1984 300D
1985 300CD
1980 300SD
1993 SL500