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914World.com _ 914World Garage _ HeliCoil Seat Belt Mounts

Posted by: McMark Oct 21 2005, 11:43 AM

I have seat belt mounts that have stripped threads. I'm wonding if HeliCoil's are strong enough in case of an accident. Logic tells me, yes they would be fine if installed correctly, but my gut is a little more nervous than my head. Can anyone tip the scales one way or the other for me?

Posted by: ArtechnikA Oct 21 2005, 11:47 AM

i have been down this path too, since there are *no* seat belt mounts in the tub of a '71 911 Targa...

i think i'd prefer a TimeSert over a HeliCoil, possibly with a bit of tack weld on the head...

Posted by: Aaron Cox Oct 21 2005, 11:49 AM

id be scared.

weld up and retap?

Posted by: jhadler Oct 21 2005, 12:46 PM

Reminds me of an old Bell Hemlet add campaign...

"If you've got a $10 head, buy a $10 helmet".

In my opinion, seat belt anchors are one of those things that you can ill afford to do half-assed. Get it welded up and do it right. Timeserts and helicoils are great for some things. And while I might trust my engine to one, where precision is the most important thing, I don't think I'd be as comfortable trusting my life to one, where -strength- is the most important thing.

Just my $0.02...

-Josh2

Posted by: ArtechnikA Oct 21 2005, 12:58 PM

QUOTE (jhadler @ Oct 21 2005, 02:46 PM)
-strength- is the most important thing.

strength is important, but it is not absolute.

i haven't cut up a tunnel or a B pillar to verify, but my understanding is that the OEM part is not merely tapped into the sheet metal, but is itself a threaded insert welded into the local structure.

so at best, you could weld the outermost part closed and retap that, but i don't think you're going to be able to fill the whole insert with good weld material.

the seat belt anchor mostly takes up a load in shear, so that's the strength you're concerned about. as long as the anchor can take a load perpendicular to the shank axis, it'll do its job.

Posted by: jhadler Oct 21 2005, 01:03 PM

Yes, it's a welded in insert. But look at the insert, it's a far cry heavier than a timesert. And shear is certainly the key. I dunno, I guess I'm just a stickler for things like that. I like the knowledge that my belts will do their best to keep from hitting hard things (dash board, windshied, etc.) with soft things (me).

-Josh2

Posted by: goose2 Oct 21 2005, 02:37 PM

drill them larger and re tap for a bigger bolt?

Posted by: dlo914 Oct 21 2005, 03:03 PM

speaking of seat belts...has anyone tried installing a 3rd seat belt for the center "seat"?

Posted by: McMark Oct 21 2005, 06:33 PM

Early cars have the supports and threads in place already. Stopped in 73 I think. For those cars it's a bolt in and you're done.

Posted by: banksyinoz Oct 21 2005, 07:43 PM

QUOTE (goose2 @ Oct 21 2005, 12:37 PM)
drill them larger and re tap for a bigger bolt?

this is probably the best option if u cant get to the other side of the panel in question (i havent had to look yet) if you can you could replace the nut (weld nut to 3inx1in 10mm thick plate if doubtful this seems to be a manufacturing standard) then away you go or just use a bigger bolt
i know i certainly wouldnt use a helicoil

Posted by: trekkor Oct 21 2005, 08:41 PM

QUOTE
I have seat belt mounts that have stripped threads

Mark, is this your car or a customer's car?

KT

Posted by: nebreitling Oct 21 2005, 09:17 PM

boy, that's going to be a tough fix. even over-tapping might not be great unless you can ensure some good threads.

but i would not personally strap into a heli-coiled belt ancor, and i'd sure as hell not put a loved one in that seat.

Posted by: McMark Oct 21 2005, 09:19 PM

I'll have to agree with Rich's point that it's a shear load issue. The threads aren't what are doing the life saving. The threads are just to keep the bolt from falling out. I think that for the amount of material that is there makes TimeSerts less appealing to me (you'd end up thinning the wall of the support structure). Welding is probably the best option as long as you are a certified and trained welder with years of experience. Welding sheet metal and body pieces is one thing, welding shut what will basically become 3/4 thick material is completely another. Having heard some opinions and had a chance to think about things, I think HeliCoil is probably what I'll go with.

Posted by: JOHNMAN Oct 22 2005, 12:13 AM

As I recall, the seat belt bolts have an unusual (at least to me) thread. I once had to get a tap to chase the threads out on one of mine and I had some difficulty finding the right tap. If you can get helicoils in that size I would be surprised. I would think about replacing the seat belt anchor. I know it might be a big pain, but at least you would know that the new anchor was strong.

just my $0.02

Posted by: McMark Oct 22 2005, 12:25 AM

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.

[EDIT]M11x1.25 NOT M12. Whoops..

Posted by: neo914-6 Oct 22 2005, 12:28 AM

The band-aids aren't worth the liability. Did the bolt mount get cross-threaded or was it worked free by belt movement over time?

Weld a nut or thread insert to a small plate with a hole, drill out the hole in the chassis to fit the nut and weld the assembly...

Posted by: McMark Oct 22 2005, 12:37 AM

It looks like the threads were torn out during disassembly. A thread inset in a factory boss seems like a much safer solution to me than a complicated construction. I prefer a single point of potential failure, rather than a whole mess of them.

Posted by: banksyinoz Oct 22 2005, 11:27 PM

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.

nissan exhaust manifold stud size from memory?
doesnt sound real big though

i will go check

Posted by: JOHNMAN Oct 22 2005, 11:46 PM

If you feel comfortable doing it, go with the helicoils.

The load is in shear, so we are not talking pullout. The only problem I see with the helicoils is getting the tang to break off correctly to allow the bolt to fully seat without screwing the helicoil out the back side of the anchor point.

In other words, go for it.

Posted by: ArtechnikA Oct 23 2005, 06:19 AM

QUOTE (McMark @ Oct 22 2005, 02:25 AM)
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.

you have started the research; it will be complete when you discover that seat belt bolts are 7/16" 20 tpi.

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

Posted by: JOHNMAN Oct 23 2005, 09:41 AM

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.

Posted by: Cap'n Krusty Oct 23 2005, 10:10 AM

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.

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

Posted by: ArtechnikA Oct 23 2005, 10:15 AM

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.

http://www.wescoperformance.com/non-retractable-install.html 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...)

Posted by: ArtechnikA Oct 23 2005, 10:28 AM

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))

Attached image(s)

Posted by: JOHNMAN Oct 23 2005, 11:35 AM

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.

Attached thumbnail(s)

Posted by: ArtechnikA Oct 23 2005, 12:53 PM

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.

Posted by: McMark Oct 23 2005, 01:00 PM

Whoops, I meant 11mm x 1.25.