Sport mirror plates with weld nuts?

[cheyenne shortstep]

Maybe SSTim or another with more knowledge than me can explain these vs. the plates for sheetmetal screws. I am not looking for opinions as I have a few of my own, I am looking for facts as I have seen no rhyme or reason to these.

[Ol Blue K20]

Are they for the 67-70 style mirror arms?
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[cheyenne shortstep]

No, these are for the '71-'72 sport mirrors.

[SS Tim]

Not exactly sure what you are looking for as far as facts.

A backing plate of any real size will spread the mirror point mounting loads and reduce local flex from vibration or airflow. This makes it a much better choice than just surface mounting something strictly one sided.

The use of machine screws vs sheetmetal screws allows a secure fastener that can be torqued much tighter that is much less likely to strip, however it must be started correctly. The finer threads for a given diameter of a machine screw can also make them less likely to loosen in use.

[Hawssie]

I just mounted some aftermarket sport mirrors which of course came with the sheet metal BPlates. The first screw I put in wouldnt create threads and stripped out the screw along with next two others I tried. I ended up welding a square nut on the back of that one and threaded out the other two for 10 32 threads and used machine screws with the same type of head as the sheet metal screws. Works much better and a lot easier to mount as your not trying to hold up the bracket and make screws as you go. It would be nice but not necessary to find a way to attach the bracket to the inside door so you could take off the mirrors with out opening up the door to put them back on. Thought about using some Ureathane window weld but havent got around to it.

[Scott>72-GMC]

Exactly. Spreads the load. A lot less likely to just pop out too if someone bumps into the mirror in the Walmart parking lot
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[cheyenne shortstep]

I should have been more specific, my question really is why some trucks have the threaded nut and some take sheetmetal screws. Maybe different plants sourced them from different manufacturers. Thanks for the input and info. anyhow.
I have always had the idea to drill a couple small holes through the door and rosette weld the plates on, they are pain in the ass with the windows in the doors.

[SS Tim]

Sorry I was not aware there was a factory variation on the screw type. The only kind I remember seeing were the machine screw style.

As far as the backing plate, a big blob of weatherstrip adhesive works well to hold it in place. For a long term solution just line it up and put a single pop rivet throught the door and plate. It is easily reversable later for bigger mirrors. Or if you wanted a little wiggle room use an oversize hole in the backup plate and a longer rivet with a backup washer through the oversize hole.

[Lee H]

My NOS backing plates do not have the nuts. They have threads for the machine screws.
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[cheyenne shortstep]

The sheetmetal screw type are the most common I've seen. The threaded nut type I have only found in a handful of trucks over the years.
Here is some non thread nut type.