I have read that one should not leave greasy fingerprints on their spark plugs or on their light bulbs. Why is this a thing? On what else one should not leave fingerprints? How does one avoid leaving fingerprints (latex gloves or, say, cleaning the light bulb after putting it in)? How do you clean your fingerprints if you leave any?

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    What else should one not leave finger prints on? Wow, it's a pretty extensive list, but I'd say bank vaults and murder victims unless you had a pretty good justification for why they were left there. A lawyer might have a more comprehensive list! And one that amuses the geek in me: high vacuum equipment. Finger grease outgasses, and with scientific grade vacuums, that can actually slow the process of pumping it down because you create a gassious atmosphere of fingerprint that takes forever to finally pump out. – Cort Ammon Mar 15 '19 at 16:05
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    @CortAmmon Please clarify. Do you mean that one should sand down the fingertips of the person one has just murdered, or that one should avoid leaving one's own prints on them? I, er, ... my friend -- definitely my friend -- needs to know this. Quite urgently, actually. – David Richerby Mar 15 '19 at 16:39

Why is this a thing?

For bulbs it is - spark plugs don't care. The problem with getting fingerprints on bulbs (specifically halogen-type bulbs) is actually the grease that gets left behind - the bulb will get quite hot when lit and grease on a portion of the bulb will cause differences in the rate of heating on the bulb surface and thanks to the way materials expand when heated this means you get different parts of the bulb expanding at different rates and... crack.

Where the bulbs are made from quartz glass (such as in certain more powerful bulbs from some manufacturers) there is also the issue of devitrification - where the oils/salts left behind in the finger print can potentially cause the material to weaken although typically car bulbs aren't going to get hot enough for this to occur although some fogging of the glass may occur.

On what else one should not leave finger prints?

Probably a bit broad to answer here I'm afraid.

How does one avoid leaving finger prints (latex gloves or cleaning say the light bulb after putting it in)?

Yep clean latex gloves (cotton gloves are even better if you have them - but latex or nitrile works just fine) or a holding the bulb in a clean cloth works.

How do you clean your finger prints if you leave any?

You can clean them afterwards as well, a good wipe down with a clean microfibre cloth will do the job.

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    If you don't have glove or a clean cloth, a paper towel works good as well. The ultimate here would be to use cotton gloves, but they aren't nearly as prevalent as latex or nitrile gloves are. – Pᴀᴜʟsᴛᴇʀ2 Mar 15 '19 at 14:51
  • Clean halogen lights using a glass cleaner, make sure it's dry before you turn it on, that should only take a minute. – GdD Mar 15 '19 at 14:51
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    Rubbing alcohol works well if you have touched the bulb. – David Watson Mar 15 '19 at 14:52
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    @GdD once the glass part of the bulb is inside the headlight unit then removing the foam cover from the glass will be challenging... I have always managed to get halogen bulbs fitted by just holding the metal or plastic parts... – Solar Mike Mar 15 '19 at 15:15
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    It depends on the car I guess @SolarMike. – GdD Mar 15 '19 at 15:19

Fingerprints on spark plugs aren't a thing, the natural oils on your skin won't affect their operation. If, however, your fingers are covered in car grease or dirt when you handle the plugs then that can cause issues, either interfering with the electrode or the cable connection to the plug terminal.

If you foul the plugs with dirt or grease then clean them before use. Any dirt on the electrode is going to end up in your cylinder!

For halogen lights @Motosubatsu's answer is pretty comprehensive, I would only add to be careful if handling with facial tissues (i.e. Kleenex), which some people do. Some types of tissues have been treated with moisturizers which will contaminate the bulb and cause the same issues as fingerprints. Toilet tissue generally doesn't, neither do paper towels.

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    I would say even tissues without moisturizers would be a bad idea. The oils from your fingers would probably go right through a normal tissue – Aethenosity Mar 15 '19 at 21:48
  • I tend to use the cardboard box the lamp came in. I figure its clean enough for the lamp's glass for transit - its clean enough for the install. Yet another reason why plastic blister packaging sucks. – Criggie Mar 16 '19 at 20:53

Despite the two previous answers stating that fingerprints on spark plugs are not a problem, I'll mention that they are.

I have personally replaced plug wires that showed traces of carbon where the spark was "leaking" around the plug due to grease on the plug ceramic. This is especially apparent on aftermarket ignition systems with higher voltage and lightly-coloured (orange) plug wires. Granted these were vehicles modified for competition twenty years ago, however today's ignition systems are on par with those for voltage.

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