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Walter Moore

Seat Belt Bolts

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I couldn't find a thread on this subject, so I started one.

For the future reference of others who, like me, live in a area where obscure metric bolts and taps are difficult to find, the bolts for the seat belts on a 1971 240Z are a rather unusual size.

They are M11 x 1.25.

Some of the bolt holes had some dammaged threads. Bolts would go in just a little hard, but when they came back out the threads were rounded off. I tried the old trick of trying to clean the threads with a bolt by cutting a grove in the threads to make it look like a tap, but it didn't work.

So I searched for most of the day for a local (Within a 50 mile radius) supplier of a tap in the size mentioned, without success. Finally my Mechanical Engineer son asked if there was an S.A.E. equivalent... silly question I thought until I did the math.

7/16" = 11.1mm

20 threads/inch = 1.27mm/thread

I found some SAE 7/16 x 20 bolts and nuts in my assortment of random bolts, and to my suprise the M11 X 1.25 bolt threads into a 7/16 x 20 nut perfectly. You would never know they are different.

I bought a tap for the SAE size at the local auto parts (Napa) store. The metric bolts threaded in and tightened up perfectly.

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The early Z car seat belt mounting bolts are SAE thread and not metric thread. This is due to the US DOT standards in effect at that time:

Quoted from 571.209 Standard No. 209; Seat belt assemblies:

...Seat belt assemblies designed for installation in motor vehicles equipped with seat belt assembly anchorages that do not require anchorage nuts, plates, or washers, need not have such hardware, but shall have 7/16- 20 UNF-2A or 1/2-13UNC-2A attachment bolts....

You could either use the approved SAE bolt which has been already tested and certified by DOT or you could use your own bolts but you must summit your test data. Generally, it is cheaper and safer to do the former.

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That is amazing...

I guess that what mislead me is that the new bolts that came with the belts I bought are marked "8.8", which is a metric strength class designation.

The OEM bolts are marked RYB, or something like that, and they are soft as butter.

I would have expected SAE fine thread bolts to be either grade 5, or grade 8, and to have matching markings... Oh wait, perhaps the letters on the old bolts are some obscure 60's era SAE grade markings... I am an electrical engineer, not a mechanical one. The SAE grade designations are somewhat outside my essential knowledge base.

I just assumed that this was another place where a metric bolt and an SAE bolt were interchangeable. I know from first hand experience that an M5 x 0.8 and a SAE #10-32 interchange. I have done that both by accident and on purpose.

This isn't the first time that I have found SAE bolts on the interior of the car. For some reason the bolts that hold my radio/Heater control/Map light panel are SAE #6-32, and the bolts that hold the fuse box in place are SAE #10-24. I did not re-tap them either, I just discovered this because I had lost the original bolts, and those are the sizes that fit correctly. A 4mm bolt would not thread into the face plate holes, and a 5mm would not thread into the other holes... truly strange and anoying.

Not saying that it wasn't done by the PO, but I don't think he would have had any reason to remove the fusebox.

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Still trying to tighten the right rear nut of the right seat. Seems complicated though. I guess i will have to loosen the plastic nut underneath first and then try to tighten the small nut. 

20190420_131408.jpg

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Those two black ones on top are irrelevant. Remove the black one from underneath you can barely see and then tighten the small metal one underneath

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The plastic nuts are spacers, used to 'adjust' the height of the seat.  The one on the top in your picture is in use for this purpose.  The one on the bottom is a spare that could -- if you wanted -- be moved to the top and doubled up with the one already there so as to raise the seat higher.

The actual retaining nuts are pretty fiddly to remove (there's not much space available for wrench swing).  If you've got one, a short ratcheting box-end wrench is the way to go.

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Still trying to tighten the right rear nut of the right seat. Seems complicated though. I guess i will have to loosen the plastic nut underneath first and then try to tighten the small nut. 

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