Rear Bumper Holes' Thread Size?

[K8PJ]

Hello,

Does anyone know what the diameter, pitch, and type or style of thread are the holes behind the little plastic round black covers on the rear bumper? I am an Amateur (Ham) Radio Operator and would like to use these holes as a mounting point for an antenna mount.

Thanks!

John, K8PJ
k8pj@yahoo.com

[BigWarpGuy]

At this post, it says the tow hooks are under the carpet on the passenger size. If they are there, perhaps one could measure it? Here is another post on it. Here is a site with stainless steel bolts. I hope this helps.

[Old smart]

The threading is ISO metric "Acme," M16x4. I would guess that the female thread is the full M16, however, the threading on the towing eye is slightly undersized at 15.5mm, with the shoulders of the teeth rounded off, probably to make sure it can be threaded in and removed easily by hand.

Acme thread is not a fastener thread like UFN, UTS, etc. that binds due to friction between the threads. Acme is most commonly used to translate rotational motion to linear motion (think bench vise, screw jack, "C" clamp, lead screws on lathes). By nature an Acme thread is free moving and will not bind, and why, I'm sure, it was chosen for the towing eyes. Acme thread is also very strong when stressed linearly (again, towing).

The Acme's characteristics may not make for a solid fastening that will not loosen. Even my very light GoMINIGo front license plate bracket vibrated loose once.

[jdlynch99]

Originally Posted by Old smart

The Acme's characteristics may not make for a solid fastening that will not loosen. Even my very light GoMINIGo front license plate bracket vibrated loose once.

Thats why I asked the other day about the pitch (Thank You Again) thinking about making some kind of expanding componet in the screw so that when you tighten what ever it is that is attached to the Acme screw (in your example License Plate bracket), it expands and actually tightens to where it won't come loose until you want it to loosen.

Got a rough design in my head, all I gotta do is find time to make a couple of prototypes. Long as I don't loose my head I'm OK. LOL.

Used to cut Acme threads for a lot of years they are not that hard to cut if you know how.

[sdixon747]

Use Loctite on the threads and you shouldn't have problems with it vibrating loose.

[Old smart]

Depending upon clearances and what the face of the frame socket looks like, it may be possible to cut the M16x4 Acme on the first 18mm or so of oversize round stock, ending at a 90-degree shoulder. Then beginning at the shoulder, cut a larger diameter UFN thread for some distance that can carry a washer and lock nut. Once the Acme is seated in the socket as far as the shoulder, the lock nut can be run-in to solidly bind the whole affair.

An alternative might be to cut the UFN somewhat smaller than the Acme and with no shoulder, in which case you would have a UFN stud bound tightly with the lock nut - maybe such that the lock nut could be run-in and tightened with a deep socket wrench. Or an inside threaded collet tube acting as the lock nut.

[The Rigger]

A 14mm x 2.00 thread pitch bolt will thread into the Acme inserts and bind tight. It's not the best fit in the world, but it works and won't come loose. The bumper kits from Breathless Performance include that size hex-head cap screw for mounting the bumpers on the car.

(Or maybe it's 16mm... I forget. Oh, Ernie?)

[jetfuel]

...one word or two? ....jam nut

jetfuel

[JimDantin]

You might find this helpful 5/8 - 8 ACME Threaded Rod & 4 nuts - smart USA insider

[K8PJ]

Thank you all for your replies. Some of you mention Metric thread size and others mention SAE. I will look at the tow hooks (didn't realize they were included) and go to my local fastener store. I plan on using jam nuts, and/or collet tubes. I'll have to send pictures when I'm done with the installation.

John, K8PJ