Need bolt strength advice, please

[Quickstep192]

I took my seats out to have them re-upholstered. Many of the socket head bolts that held them in were rusted or stripped, so I decided to replace them all.

I got a box of socket head cap screws only to discover that they were too tall for the bolts that hold the rail to the underside of the seat. The bolts that hold the rail to the seat have to be low profile to clear the other half of the rail.

I haven’t been able to find the low profile, 8.8 grade plated low profile socket head screws in anything but quantities that are 7 times what I need.

I realized today that although there’s a height issue, there’s no side to side clearance issue for the bolts that hold the rail to the seat. There’s plenty of room to turn a regular hex head bolt.

I’m sure these bolts are spec’d for safety reasons to ensure the seat doesn’t come out in a wreck.

I was wondering if there’s any safety reason that I shouldn’t use hex head bolt (still rated 8.8) for bolting the rail to the seat instead of the low profile socket head bolt.

I appreciate any wisdom.

[faapgar]

A lot of the door hardware has the same 5mm allen head bolts.Try DC Automotive if you do not want to buy new from Porsche OEM.Fred

[Bill Verburg]

Originally Posted by Quickstep192

I took my seats out to have them re-upholstered. Many of the socket head bolts that held them in were rusted or stripped, so I decided to replace them all.

I got a box of socket head cap screws only to discover that they were too tall for the bolts that hold the rail to the underside of the seat. The bolts that hold the rail to the seat have to be low profile to clear the other half of the rail.

I haven’t been able to find the low profile, 8.8 grade plated low profile socket head screws in anything but quantities that are 7 times what I need.

I realized today that although there’s a height issue, there’s no side to side clearance issue for the bolts that hold the rail to the seat. There’s plenty of room to turn a regular hex head bolt.

I’m sure these bolts are spec’d for safety reasons to ensure the seat doesn’t come out in a wreck.

I was wondering if there’s any safety reason that I shouldn’t use hex head bolt (still rated 8.8) for bolting the rail to the seat instead of the low profile socket head bolt.

I appreciate any wisdom.

McMaster-Carr can provide most any bolt you could want.

[Evans, Marv]

If the bolts are the grade you need and fit, use them.

[HaroldMHedge]

I ordered a set of seat bolts from Classic 9 Leather Shop but they only had the short ones. I needed longer ones for the passenger seat in my 87 Carrera since it had a 20mm spacer to bring up the manual seat to the power drivers seat height. These longer ones I found on Belmetric's site.

[Locker537]

https://www.fastenal.com/ also tends to have them.

I was able to find them at a local Ace Hardware store that has a great selection. I was surprised to see the proper grade 12.9 hardware there.

[chrismorse]

It is academic , but, wouldn't a bolt, (of equal or better strength), provide superior retention due to the larger clamping area under the head?

Sunday morning coffee hasn't kicked in yet,
Chris

[Quickstep192]

[QUOTE=chrismorse;10724480]It is academic , but, wouldn't a bolt, (of equal or better strength), provide superior retention due to the larger clamping area under the head? /QUOTE]

That was kind of my thinking. Seems to me that a hex head rated 8.8 should be equally strong as a socket head rated 8.8.

I just wanted some validation from people more knowledgeable than me.

[Locker537]

Permalink to a post from Grady on this topic:

I Searched (Seat bolt replacement)

Wil is right on.


I will repeat:

Porsche specifies the 12.9 bolt

6MM x15 Seat & rail, socket head cap screws

Those are very unique and important screws.
They are not the common, garden variety M6, 8.8 socket-head cap screws.

These screws are grade 12.9 and have a shorter head than the common variety.
The part number is 900.119.042.02, M6x15, DIN 6912 – 12.9 vis.

The strength is critical to keep the seat and seat belts in the car in a crash.
The short heads are necessary for clearance so the seat will slide without interference from the screw heads.

Best,
Grady

[Harpo]

The bolt is only as strong as it's weakest link. The problem with shallow head (button head) internal drive fasteners is the risk of cam out. Socket headed cap screw have a much deeper drive and there fore the risk of cam out is much lower.

[Quickstep192]

Originally Posted by Locker537

Permalink to a post from Grady on this topic:

I Searched (Seat bolt replacement)

The info from Grady is interesting. The bolts that came out of the car are 8.8 and the searches I’ve done for p/n 900.119.042.02 show pictures of a bolt stamped 8.8.

In my early quest to find these bolts, I found 10.9 bolts more easily than 8.8, but got advice that 8.8 bolts might have been chosen because they’d be better able to deform and therefore be possibly less likely to shear.

Now what?

[Hotshot]

Interesting enough I went to Porsche to get some seat bolts. The ones they gave me were too long. As it turns out there are 2 bolts, one for auto seats and one without. I need the without as I have power up/down but the fad/aft is manual. The Parts Dept didn’t specify, but our hosts site does. Add those to the order..

[David]

I wouldn’t worry about seat bolt grade. I would worry about the age of your seatbelts since they do the heavy lifting in a crash.

[76FJ55]

Originally Posted by David

I wouldn’t worry about seat bolt grade. I would worry about the age of your seatbelts since they do the heavy lifting in a crash.

Unless you are rear ended, then it is seat frame and seat bolts. Shear strength and tensile strength, both should increase with bolt grade.

[KTL]

Folks need to recognize that the size of the seat mounting bolts changed. Through 1984 the 911 mounting bolt is M6. In 1985 they changed it to M8.

As Bill V mentioned, you can get the same low head pan head, not button head) M8x20mm length bolts, in common 8.8 property class/strength, from McMaster Carr. They have black oxide coated 8.8 (catalog # 93070A173) for $9.19 box of 25 or zinc plated 8.8 (cat # 90327A157) for $12.70 pack of 25. Each seat takes six bolts so you'll have a spare set for the future? At least it’s not a box of 50 or 100. These McMaster bolts have a 5mm key size instead of the larger key size of the original bolts

The bolt size for the seats with a spacer underneath is M8x35

[Quickstep192]

On my seats, there are 8 bolts (for per side of each seat) that hold the rail to the floor.

There are 14 bolts that hold the rail to the underside of each seat, 2 of those are also holding the seat belt buckle.

To do both seats requires 44 bolts. All are M6.

[KTL]

If what came out is 8.8, I would tend to think that is original. However, Grady (RIP) was a wealth of knowledge and it’s hard to argue with his info. I looked on McMaster and see that bolts meeting DIN 6912 are either 8.8 or 10.9. I would choose the 10.9 since M6 seems small for seat safety and maybe that’s why Porsche/Recaro made the bolts bigger starting in 1985?

McMaster has three bolts to choose from in M6x16 (1mm longer should not matter) and meeting n DIN 6912. Two are 8.8 and one is 10.9. Note that DIN 6912 is for the key tooling. It has a deeper recess below the hex to pilot the hex key (which requires the DIN 6911 key......... ) and hold it better when tightening/loosening the bolt. I honestly don’t think the DIN rating here is ultra critical. All of the DIN 6912 bolts we are looking at are not in stock and 1-2 weeks to source

Or you could get a low head M6x16 bolt (8.8 class) that’s not DIN 6912, fully threaded (are yours partially threaded?) that’s in stock ready to ship for $10.88 for a box of 50

Are we over-thinking this? Probably. But safety is important and we should get it right

[Cad.Guy]

Mine only has 4 M6 bolts per seat.
The self proclaimed " Excellent " mechanical skill knuckle head I bought it from put standard M6 bolts back in.
I have been contemplating drilling out the floor mounts and tapping them to M8 and re fitting the seats.

[KTL]

I agree with you that it's not a bad idea to remove the M6 blind nuts and install M8 pan head bolts with nut plates. The later cars have thick metal rectangles with two or four M8x1.25 holes in them. Makes for an easier installation with the nut plates because you don't need a backup wrench underneath the seat rail to counterhold the nut (plate) while you tighten the bolt from above.

But the reason for the nut plates is the later seat rails are U-shaped and you can't get a nut and wrench (or socket) underneath the rail. Whereas your '79 SC seat mounting in the chassis is a shelf-like mounting area. So you should be able to get a nut underneath the shelf with a wrench to hold it pretty easy. I would recommend a flange nut or an extra wide and thick washer to spread the load out on the mounting area.

[Cad.Guy]

@KTL
Yep kinda what I was thinking.
I have the nut plates now as well