Good day Florian!
I'm not sure whether you've glanced at my ongoing seat belt thread: May I ask for advice on Beam's 140" WSCH300 seat beats?
Your question remains my unsolved mystery as well, yet I've gathered these facts:
- Some trucks came with those shoulder anchor inserts pre-threaded, while some require threading.
- VernF110's 1969 F100 4x4 came with the inserts pre-threaded with 3/8-24 threads.
- The 3/8-24 threads on Vern's truck fit a 3/8-24 bolt in a nice and tight fashion; they weren't loose at all.
- I ran a 3/8-24 tap through one of my inserts, yet the fit of a bolt in there feels loose, as though its beginning diameter was too large for proper 3/8-24 threads.
- 70_F100 mentioned that he'd check the status of the shoulder anchor inserts on his '70, to add another data point.
- The 1972 Ford Truck Shop Manual seems to claim that the bolt's supposed to be a 7/16-20 bolt, not a 3/8-24 bolt.
Since I tapped one of my 1972 inserts with 3/8-24 and it seems a little loose, I'm probably going to have to drill it out slightly and re-tap with 7/16-20 threads. At the moment I'm at a stand-still though, focusing on other things, and anticipating the results of 70_F100's truck inspection!
Florian, is there a local store where you can buy one 7/16-20 bolt, and one 3/8-24 bolt? These are fine-thread bolts, and just to be certain they're strong enough, you'll probably want to buy grade eight (six lines on the bolt heads). If you bought these two bolts, you could test them in your inserts to see which one's the correct thread.
Yet perhaps before that, are you sure your inserts are currently threaded? Some came threaded, while others did not.
Florian, I hope you're having a great weekend!