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Join Date: Jan 2006
Location: Alameda, CA
Posts: 419
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Mysterious Shrinking Screw
Today I replaced a burnt-out driver-side turning-signal bulb on my 82 SC. Unscrewed the two screws on either side of the lens, replaced the bulb, lined up the lens and tried to screw it back in... the screws wouldn't catch. Odd, this should be the kind of no-brainer activity I'm qualified for.
I took out the bulb and tried again, same result. I took out the lens and tried the screws by themselves. Yup, once in far enough they grab nicely. I pulled them out until they barely grabbed and counted threads. Then I put the screws through the lens and counted threads. Hey, not enough threads! Even without the little plastic washer (which may possibly have expanded through the years). So my next thought was that this car was assembled in Germany, where it's clearly always overcast, cold, and somewhat serious. I looked up the thermal expansion coefficients of the materials. The brass screw is about 19E-6 per decree C. I'm not sure what kind the lens was made of, but I figure pyrex (4E-6) may be a pretty close. Now I'm really confused - it would have to have been WARMER in Germany for the screw to have been relatively longer. Something's wrong. And so I come to you. What could have changed? Any history of the seal around the lens expanding? There's an actual metal bolt in the back of the lens-assembly, right? ('cause the plastic is pretty thread-less) Babak Gratuitous photo: 
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04-25-2006, 05:38 PM
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Location: rockland NY
Posts: 399
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Del cross B = J(c^2)/epislon (static case of course)
I think it looks like the screw is broken, I think that normal they are considerably longer with more of a threaded area.
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________________________________ '67 912 '89 944 S2 '87 944 Turbo '82 MB 300CD |
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04-25-2006, 06:51 PM
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=) Can you tell I'm preparing for my qualifying exam?
Would be odd if it were broken- both screws are the same length (and won't work interchanged either). Both ends look the same too: a little lip on the outside and then a smooth depression in the middle. Babak |
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04-25-2006, 06:56 PM
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It's probably not much help but on my car they are longer screws and they go into metal tabs that clip onto the inside of the housing. Are you an engineering student?
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________________________________ '67 912 '89 944 S2 '87 944 Turbo '82 MB 300CD |
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04-25-2006, 07:12 PM
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Join Date: Nov 2004
Location: Greater Metropolitan Nimrod, Oregun
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put the little bars on top so everybody knows you mean a cross-product of vectors, not a mess of scalars being multiplied
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"A man with his priorities so far out of whack doesn't deserve such a fine automobile." - Ferris Bueller's Day Off |
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04-25-2006, 07:53 PM
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Location: Alameda, CA
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How do you know I wasn't using "Bold" notation, per Jackson? =)
Yup, engineering student. My background is in optics; I came to grad school to do biophysics. Babak |
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04-25-2006, 08:10 PM
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