HeliCoil Seat Belt Mounts, bad idea? Options

[John]

QUOTE

there are a few places where Amrican sizing is used; this is one of them.

You mean like Spark plug sockets, and wheel diameter?

You may be close with 7/16-20, but it isn't quite the same. These are M11x1.25. Even the shoulder bolts that thread into them are metric.

Please step away from the crack pipe.

[Cap'n Krusty]

QUOTE (McMark @ Oct 21 2005, 10:25 PM)

Yeah, it's 12 mm x 1.25. Most 12 mm bolts are 1.5 or 1.75. I can get the HeliCoils. I've already done the research. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

IT'S NOT 12MM! It's 11mm, and SAE standard cars are 7/16" fine. Same thread, BTW, and it's been that way from the gitgo. Seems the "governing bodies", whoever THAT is, had their act together, just this once. IF you can find an 11mm or 7/16" helicoil of the correct thread pitch, it would be fine. Helicoils, installed properly, are stronger than the parent metal ever was. Generally, you DO have to consider wall thickness, but in this case, the wall thickness is there. As for Rich's claim the bosses aren't there, I don't recall seeing any Porsche since the 356C that didn't have them under the carpet somewhere. My 65 911 even has the shoulder belt anchors, years before they were standard on Porsches. BTW, in cases like this, I suspect the thread damage was caused by someone trying to install, with force, the wrong hardware. The Cap'n

[ArtechnikA]

my spark plug threads are 14mm - i donno about yours.

master cylinder bores are American sizes - what we call 19mm (for example) is actually 3/4".
brake caliper mounting ears are 3" and 3-1/2".

7/16" UNF has always been the standard seat belt anchor hardware specification.

go buy a set of racing harness eyebolts from Simpson and observe that they thread right into the factory receptacles. use a 7/16"-20 thread gauge on the factory hardware and observe the match. 20 threads per inch == 1.27mm pitch so it's easy to see how they'd look close.

This is a typical example.

but you don't have to believe me; do the research.

(i have no doubt that *somewhere* in the world someone has buggered a set of seat belt mounts for some other thread size...)

[ArtechnikA]

QUOTE (Cap'n Krusty @ Oct 23 2005, 12:10 PM)

...As for Rich's claim the bosses aren't there, I don't recall seeing any Porsche since the 356C that didn't have them under the carpet somewhere. My 65 911 even has the shoulder belt anchors...

donno about coupes, but in (at least) '70 and '71 Targas, the seat belt mounts were attached tp the seat sliders (outboard) and the center tunnel top (inboard) both using 2 6mm bolts in tension. i do not consider it a very good design. it does have the usual 7/16-UNF receptacle at the top of the B-pillar for the shoulder belt mount.

looks just like this, from drawing 812-00 in the ETKA ('71, Main Group 8, SubGroup 12 (belts))

[John]

All I know about these is what I have actual experience with.

These bolts are metric. As stated before M11x1.25.

The bolts supplied are Grade 8.8. If they had been SAE, I would expect them to have been grade 5 fasteners (roughly equivalent to metric 8.8).

Refer to the pic below. While 7/16"-20 bolts will screw in, I don't believe the fit is quite the same.

(And yes I do have racing harnesses and use the supplied fasteners)



just my opinion.

[ArtechnikA]

7/16" == 11.11mm
i donno if a 7/16" fastener would fit into an 11mm hole, but i bet the smaller 11mm would fit a 7/16".

20 tpi == 1,27mm pitch.
i bet there's enough tolerance in the standard thread profiles that a 0,02mm difference is insignificant; especially when there's a small diametric difference.

i've used my 20 tpi thread gage on the factory fastener and i can't see a difference - 0,02mm is probably too small to eyeball with a cheap gauge.

i see no reason a bolt can't have both a metric 8,8 strength rating and nonstandard (i.e. American) threads. unlike most "generic" hardware shown in the ETKA (which carry 'N' or 999 numbers, typically) all the restraint fasteners i've seen (i checked several years and models) use 901 numbers. IOW - they do not seem to be 'standard' fasteners.

[McMark]

Whoops, I meant 11mm x 1.25. (IMG:http://www.914world.com/bbs2/html/emoticons/pinch.gif)