Bumper Plunger
This is on a bolt attached to the bumper, slides into the bumper mount with a hole.
Slides onto the round headed 1/4-20, 3.5" long through bolts (https://www.mcmaster.com/90185A414/) with the plunger side facing into the hole. After the plunger is on the bolt, slide a 1.125", 0.36" OD, 0.308" ID Spring (https://www.mcmaster.com/9434K71/) onto the bolt, and finally a a #10-32 nyloc nut (https://www.mcmaster.com/90631A411/) is screwed on the bolt to keep the full assembly together (Onshape has an updated model of this).
STEP:
STL:
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