I‘ve got a problem with a 7/16-20 UNF...

[Durango2k]

....it‘s not in my E-Type, but in my Benz. But it should apply equally to almost any car with seat belts, because 7/16-20 is the worldwide standard for seatbelt bolts it seams?

I had a dead seatbelt lock in my Merc. Tried to remove the T45 torx, killed the bolt when the rust in the thread blocked and the tory bit rounded the hole. Damn.

Had to cut the bolt. Then drilled up in 5 steps to 9.5 mm, bought a 7/16-20 unf cutter from ebay, and tried to cut a new thread in the hole. Of course out came the fragments of the drilled ultra-hard bolt, and now I have an incomplete thread in my car‘s chassis.

Tip - what I should have done - drill to say 8.5 mm, the use a thin blade, and cut three grooves into the bolt remains. Use a screwdriver as a chisel and remove the 3 segments. Saving the old thread.... but I was not that clever.

So, I then tried a new 7/16 bolt, it becomes loose at about 12 Nm. No chance, not enough thread left.

Now I thought I might use M12 instead, in 8.8 grade ?

But then I found that M12 standard has an 1.75 mm pitch. I have a hole depth in the chassis of about 15 mm. So an M12 would give me about 9 threads in the flesh (say 15 mm/ 1,75 = 9).

The 7/16 - 20 would give me about 15 mm /25.4 mm = 3/5 x 20 = 12 threads, which is 1/3 more.

Even if the M12 is larger, has more circumference, I have a feeling the 7/16 would withstand more force.

Now - I have no idea. I cannot reach the rear side of the chassis nut, I tried to find it, but it smells like. „Take the rear axle and tank out“ before you can see the nut or similar. I could of course cut M12 and leave it well alone.

But its a rear seat belt, so I am a bit reluctant to just „fettle something“. If an accident happens and this bolt comes out I am maybe ruined.

Helicoil has a set, but would that be better then an M12 ? How many threads would it have on the outer side ?

Carsten

P.S. I could use one of those:

https://www.kaeferland-shop.de/innenaus ... 6-unf-x-11

Cut a slot like a L into the metal, bend it out. Cut out the old nut, put in the new one. Fix it using an 7/16 bolt and some washers. Drill some 6 mm holes around it, weld through them to lock the new nut-plate into place. Hammer the L shaped part back and close the L cutout with welding. Remove the 7/16 bolt, use a 90 deg paint spray head, flood cavity behind with paint, paint on front after flattening, job done.

[abowie]

To Helicoil it back to 7/16 UNF would be my personal preference. It will have the same number of external threads as internal.

If not I think your idea of a 12mm bolt would do fine. If you were worried about your 1.75 thread, you can also get M12 1.25. 12mm is about 0.9mm bigger in diameter than 7/16 so the threads should have enough bite.

[Durango2k]

I just did some maths. The M12 at 1.75 will give 9 threads in the 15 mm meat I have. The 7/16 would give around 12.

Now, the M12 has a larger diameter, so 3.14x12x9 is a total circumference of 339 something. The 7/16 gives 3.14x11.1x12 wich is 418 something, so it gives 23% more „clamping area“ in the meat.

The M12 at 1.25 is a nice idea. In fact in metric the UNF 20 is 25.4/20 = 1.27 which is very close. No need to calculate further, because 1.25 is smaller then 1.27 so there will be a tad more clamping area with the M 12 at 1.25.

I‘ll do that, and use an M12 at grade 8.8 in 1.25. Anything else is a lot of work.

Carsten

P. S. 15 mm meat / 1.25 = 12, and 3.14x12x12 = 452 clamping area which is indeed a bit more then the UNF -20 offers.

[abowie]

To be honest, leaving it standard with a Helicoil would be best for resale and the next owner, but if it is very hard then the M12 1.25 would be OK.

[JerryL770]

I think your concern would be governed by whether, when the seat belt is stressed in an accident, "is the load on the bolt in tension or shear"?

If in shear, it will not matter much which thread you use.

[Durango2k]

Its the lower bolt, so I fear its not shear but pull.....

One idea came up today which is nice - drill nearby hole, large enough to put a nut through. Weld 7/16 nut on thick wire, and fiddle it behind the original nut. Pre-drill the previous nut in M12 so the new bolt would slide through, and fix in new nut behind.

Carsten