Friday, September 29, 2023

C3 - License Plate and Lights

As I've dug in further to my car, trying to get license plates installed, I'm finding I just can't get parts.  Machining the license plate holder was one thing, but now I have to make a light bracket, plus I have to figure out the screws for the lower plate, but I don't want to purchase them in order to make the custom brass ones.  Here we go.  Let's throw in the assembly manual diagrams :

In the first diagram (page "407"), the screws are #2, or part number 3974037, for the license plate light bezel mounting.

In the second diagram (page "473"), I'm looking at #3 (part number #3907444) for the plastic mounting nut (the bracket [#7] is part # 339142).

First, lets hit the lower screws for the plate fastening. Identifying the screws shouldn't be too difficult, as the plastic sockets do not come threaded for a specific example.  It should be as easy as identifying the largest through hole diameter (call it the largest minor diameter), and also the largest entry point diameter.  From those two measurements, I should be able to deduce the screw size using screw dimensions.

The largest hole was 0.193".  The small end was 0.128" in diameter.

It was at that time I stupidly realized the package that had been left unopened HAD THE STUPID SCREWS!

The screws had an outside diameter of 0.240", and a thread pitch of 14.  The depth isn't much, because these are essentially wood screws, but I have a sneaky suspicion that any 1/4" screw would fit.  It also tells me the threads can be 25% larger than the hole, and I expect a course thread would be just perfect, so a 1/4"-20 will be fine.

This allows me to use the above criteria in calculating the next screws for the license plate light.  Let's measure things up and see what things show as.

The largest fit diameter did not go all the way through, at 0.139".  The largest that did go through was 0.133" diameter.  Looking at the Machinery's Handbook, to see what matches :

With a minor diameter that cannot exceed 0.139" or 0.133", and a scan of the table, we find the maximum's minor diameter of a #10 screw is a 0.1376".  The #10 screw in my mind is a wee bit too big, so it is smaller than that.

The major diameter of the screw has to be at least 0.133" in order to engage the hole with the threads at all, and that means #5 (major diameter between 0.1172" and 0.1240") will absolutely not engage the hole.

That leaves a #6 and a #8 for the most common sizes (I am not going to do a #7 or a #9 - they are too odd).  With the ranges of a #6 between 0.1293" and 0.1369", it is on the "barely" side.  That leaves only a #8 screw, with a max 0.1553" and 0.1629".  Course threads would mean a #8-32 screw, or a #8 wood screw.

Okay, screws identified!  I had the upper screws for the plate made already in a previous post, so it's the same basic process.  First, turn down just a little length for the smaller light screws (#8-32's), and use a die to cut the threads (yes, I could thread it on the lathe, but I don't want to have to re-tract, and line up, and this way I know I'm not bending a long slender part).

At that point, i can turn down the next section of the screw, and the die keeps the thread perfect.

The final screws, the wood-screw-like, lower bolts that had the angled thread on the tip, was going to be fun.  I did not want to try turning an additional, tapered thread on this point because the complexity was something I did not opt to choose, so I turned a pointed tip and then threaded anyway.  Note that I am well aware that the quick change gearbox, set up for 20 thread per inch, is right on the money.  I do not need to repeat this on these screws (I'd already validated on the previous ones), but I've found that marking up the part before turning gives me a good visualization of when I am deep enough to break out the threading die for 1/4"-20 to ensure a perfect thread.

Now I have all six screws ready to go.


I just need to install the plate light, then the lower bracket (maybe I will have time in the next evening or two).

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