Bolt sizes for sealt belts

[Jaybird12]

Greetings everyone. Could someone tell me the bolt size and thread for the two that attach the front seat belts to the B pillar? I need longer ones to accommodate the thickness of Targa body braces and a harness bar I want to install. BTW its a 1975 911 Targa.
Can anyone foresee any issues with a longer bolt?

[Esa911sc]

Normally 7/16 UNF.

[Jaybird12]

What does UNF stand for? I would have expected it to be a metric size. Everything else is.

[spuggy]

Originally Posted by Jaybird12

What does UNF stand for? I would have expected it to be a metric size. Everything else is.

UNF = Unified Fine.

Seat belt bolts are a standard size/rating; mandated by safety regulations/testing, IIRC. Seem to recall first encountering this on an old Datsun in the 80's when a "captive" nut dropped into the B pillar - and metric nuts about the right size didn't fit the original bolt (yeh, least I checked before we welded it to the pillar to pass inspection!)

[javadog]

These bolts typically have a shoulder on them, to allow the seat belt guide to pivot. A standard bolt won't work in this application, unless you make a spacer. You need to measure the depth of the hole that you have and the thickness of the guide you wish to use, to come up with the length of the bolt and the shoulder or spacer length.

7/16ths UNF = 7/16-20.

JR

[BK911]

You may want to remove yours to verify it is 7/16" UNF.
Most people say they are all standard size, but in my '73 they are metric.

[Jaybird12]

Mine's a '75. I wonder if the next generation became UNF. I won't have access to mine until next week. I'll post the results. Thanks everyone.

[JP911]

My '74 has fine thread 7/16" bolts for the seat belts (so did my '76).

[moneymanager]

To my knowledge they have always been UNF, the world standard since the earliest days.

[BK911]

It would be interesting to hear what other early car owners have.
I have a box of a dozen or so rear seat belts I removed from early cars over the years and most are metric. A couple of 7/16" mixed in, but most are metric.

[Arne2]

I've never encountered anything other than 7/16 UNF in any of my cars over the years. Old Datsuns, MGs, BMWs, VWs, etc.

[Grady Clay]

A little history: USA had the first government-mandated seat belts so the specification became SAE (Society of Automotive Engineers) size 7/16” UNF (Unified National Fine). It is specifically an uncommon size so common ‘hardware store’ bolts won’t fit.
SAE 7/16” UNF = 7/16” (nominal OD 0.4375”) with 20 TPI (Threads Per Inch), standardized in 1918.

A Search on Pelican yields:
Seatbelt bolt diameter and pitch

There are many automotive dimensions that are still specified in inches: wheel rim diameter and width (although the French tried to convert to metric), brake caliper mounting and more. There are even combinations: our sparkplugs are metric 14 mm OD X 1.25 mm/thread X ¾” length and use a 13/16” socket wrench.

Best,
Grady

[donporfi]

Is this possible ?
The head can be metric but the thread 7/16 UNF ?
I have here a set of Schroth seat belts and it comes with several bolts.
The head measures 17mm and the threads are 7/16 UNF.
Or these are 11/16 heads ?
The heads are a bit loose if I use a 11/16 wrench and are tight enough if I use a 17mm wrench.
The difference between 11/16 and 17mm is 0.46mm, half a milimeter.
The head have these markings:
3F 8.8
LOSO 8.8
YES 8.8 UNF
KWS 8.8
DERA 8.8

[HarryD]

You may want to remove yours to verify it is 7/16" UNF.

Most people say they are all standard size, but in my '73 they are metric.

The head is metric but the threaded part is 7/16 UNF

[HarryD]

Is this possible ?

The head can be metric but the thread 7/16 UNF ?

I have here a set of Schroth seat belts and it comes with several bolts.

The head measures 17mm and the threads are 7/16 UNF.

Or these are 11/16 heads ?

The heads are a bit loose if I use a 11/16 wrench and are tight enough if I use a 17mm wrench.

The difference between 11/16 and 17mm is 0.46mm, half a milimeter.

The head have these markings:

3F 8.8

LOSO 8.8

YES 8.8 UNF

KWS 8.8

DERA 8.8

Strange but true.

[donporfi]

I checked one of the bolts with a 7/16 UNF die and it is difficult to screw it.
Unfortunatelly I do not have a 11 x 1.25 die to check it.
The difference between 11 mm and 7/16 is 0.11mm.
The thread pitch difference is very small a 20 thread per inch would be a 1.27 metric thread very close to 1.25