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My car has a small oil leak which caused one of the spark plug holes to have some oil in it. When I replaced the spark plugs today, oil got onto the threads of the old spark plug, so I'm fairly confident oil got onto the spark plug hole threads (and thus onto the threads of the new spark plug). Could this cause any issues? I'm planning on re-torquing the plug again in a day or so in case the oil prevented it from being torqued down properly.

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When you get oil or any other kind of lubricant on the threads of a fastener that has a dry torque spec, you need to reduce the amount of torque applied while tightening. This is because the reduced friction leads to less force required to get the fastener to its proper tension.

I found this handy chart here that shows how much to reduce the torque (in %) depending on the type of lubricant.

Lubricant                       Torque Reduction(%)
----------------------------------------------------
Graphite                            50 - 55
White Grease                        35 - 45
SAE 30 oil                          35 - 45
SAE 40 oil                          30 - 40
No Lube                                0
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