Mystery Nut--Anyone Identify?

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Can anyone please identify the nut pictured at the following site?
http://home.earthlink.net/~honda.lioness/id16.html
It's head is 12 mm across. It has some sort of locking
mechanism. It sheared off its stud, presumably due to age and the vibrations that resulted while I was grinding off the lower inboard control arm bolt on the driver's side. Part of the stud appears in the photo. At the end of the day, I found it beneath the cardboard on which I lie to do this work.
I will be inspecting the engine compartment to see if I can see a similar one.
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Mystery photo. How are we to tell?
Elle wrote:

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Can you identify the nut who took this terrible picture? As blurry as this photograph is, it's most likely to remain a mystery, though I'd venture a wild guess at a nylon-self-locking nut.

Did you shear this nut off yourself or just find it? Are you sure it's not really a old metal tire valve stem cap?
nb
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You didn't say what model Honda you have. But it looks like a manifold nut to me. On my '00 Accord both the intake and exhaust are 12mm.

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Beg pardon. It's a 91 Honda Civic. Nasty, thanks for the suggestion. My Chilton's does indeed show a bunch of exhaust and intake manifold nuts that seem to fit the bill. Majestic's online parts site confirms that the exhaust manifold has nine self-locking nuts. It's also certainly rusted enough that I would expect it to be either in a weather path or on the exhaust system somewhere. I'll take the shroud off tomorrow and confirm. Thank you!
Curly, good tip. I know it's a lousy photo.
RWM, it's some kind of self-locking nut and so is a little unique looking. I thought someone might recognize it before, ya know, I discover the hard way and my engine falls apart or something on the highway later today.

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Elle wrote:

-------------------------------
I think it's an out-of-focus picture of one of them alien space craft that they used to publish back in the 70's. Looks a bit taller though :-(
Tip: On digital cameras that don't have a MACRO or close-up setting, you can hold a magnifying glass or Grandpa's reading glasses in front of the lens, and it works just fine.
'Curly'
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"'Curly Q. Links'" wrote:

snip
That quite a tip Curly... Who would have known???
I'll pass that onn to my cheap friends who have cheap cameras!
JT
(Yep, you can learn something new every day)
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It looks like a standard flanged nut with some sort of a PAL nut as locking insurance.
JT
Elle wrote:

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Elle wrote:

That would be TeGGeR... oh wait, wrong picture ;)

Do you have a pic that's not quite so fuzzy?
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Ha. We're all gearheads in this together. :-)

I'm afraid not. I am looking for a magnifying glass. Little busy today.
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Whaaaat? I resemble that remark. >:^
It's an exhaust manifold nut. Looks like part of the stud is still in it too, so that's broken.
Remove the front heat shield for the exhaust manifold and have a look at all the studs.
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"TeGGeR" wrote:

I also thought it looked like an exhaust manifold nut. The question remains though, how did it get under the dash? Was Elle under the dash recently and unknowingly transferred the nut there?
Eric
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No; miscommunication. My Civic was on ramps, and I was under the car, grinding out one of the control arm bolts, for a long time (see other thread). About mid-day I moved the cardboard and wood planks on which I lie for this sort of work and found this lock-nut (with, as Tegger said, part of a stud attached). I figured the grinding vibrations knocked it free.
I'll get a closer look tomorrow. Car's driving okay, all things considered (especially all my monkeying with everything).
Thanks, folks, this will save me a lot of time.
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Elle wrote:

i would put money on it being a stud/nut for the exhaust system. either the a/b/c pipes, the catalyst, or possibly the exhaust manifold. its got the same patina of a nut/stud thats been in high heat all its life.
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I took the exhaust manifold shroud off today. It is indeed an exhaust manifold nut (with broken-off stud stuck in it). It's the one nearest the distributor.
I am amazed at how many people (Nasty, Tegger, Eric, SoCalMike) nailed this, and with a fuzzy photo, too. What a phenomenal newsgroup!
I see the stud on sale at Majestic online for a couple bucks.
I reckon I'll see how my ball joint castle nut/stud work goes today and then make a trip to the junkyard. I think the front desk man and I could become an item... :-)
Much obliged for everyone's input. This all is valued work to me, and I couldn't do it nearly as efficiently (and at some points, not at all) without the extensive experience of so many of you.
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Broken studs are very common. I had one too. Mine broke below the surface of the head. My mechanic removed it, but wouldn't tell me how (trade secret, or so he says...). It was done at the same time as my head gasket replacement, so the machine shop that trued the head probably did it.
The problem is that exhaust pulses are quite violent. This is the reason you need heat-resistant "prevailing torque" nuts. The nuts don't vibrate off, but the pulses often break a stud.
Funny, the one that broke on mine was the one nearest the timing belt. It must be a random thing.
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That combined with the exhaust system flexing as the engine torques on its mounts, of course...
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Mine is broken off at the outer surface of the exhaust manifold. Hopefully "all" I have to do is remove the manifold, and I'll plenty of leftover stud to drill/double-nut/etc.
I'm not going to pursue it until I get my new front lower control arm bushings in (knock on wood). Tomorrow's a big day.
Doggone Napa said they had the stud. I go there and they insist it's a 10 mm stud. I didn't have the broken nut-stud with me and bought it. Back I go.

Interesting. Maybe I'll see about replacing all nine or so, before they break within the cylinder head.
Thanks for the info. As always, it's very helpful.
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What torque should be applied to the exhaust manifold stud (91 Civic LX, 176k miles) when installing it in the cylinder head? This torque specification does not seem to appear in the service manual or my Chilton's.
I see the locking nuts get 23 ft-lbs, so I suppose that's reasonable for the studs, too. Still, perhaps someone here has some more insight or thoughts on what is reasonable here.
Also, ISTM I should replace all nine studs. Any good reason not to? "If it ain't broke, don't fix it," is a good rule, but on a car this old, and since broken exhaust manifold studs are fairly common, I think an ounce of prevention may be appropriate here. Your experience on this matter is welcome.
I plan to double nut the studs to remove them, being mindful that they are fatigued and could very well break off in an undesirable way.
Again, they're 8 mm nominal diameter studs.
My Civic yesterday recorded the lowest gas mileage since I started keeping careful records two years ago--36.3 mpg vs. my average of about 40 mpg (higher in late spring and summer). It's never been this low for this time of year. I googled and it certainly seems that the missing nut may be causing an exhaust leak and causing the engine control system to run rich.
I am pricing a new exhaust manifold gasket, studs, and nuts locally today.
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Elle wrote:

Studs should have slightly more torque than nuts as it is desired that the nut turn freely (when loosening) while the stud remains in place. this can be done by locking two nuts together when installing the stud. Using "lock tight" is also a possibility but may not be as effective due to the high heat this application must endure.

Yes, I would replace all of 'em if it is not a big hassle.

That's ok. Just be patient and work the studs in both directions to gradually loosen 'em up. Also, a direct blow or two to the top of the stud itself can be beneficial. Just make sure a nut is present to protect the threads.

Ethanol mix maybe??

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