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I've been working recently from a Honda service manual. There are, of course, torque specifications documented for various fasteners used throughout the vehicle. I cannot find, however, any mention of the thread lubrication necessary to achieve the specified torque values. I'm curious what Honda, and other manufacturers, intend as standard best practice for thread lubrication when installing fasteners to their specified torque.

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In general, the assumption is that you are not using a lubricant. When a lubricant is specified, obviously then you use it.

Using a lubricant will result in the same torque readings, but higher tension in the fasteners. This is, more or less, the same as over torquing and may lead to failures of the fastener (or the part, or the gasket).

Lubricants can give you more consistent and accurate torques, so they are specified when this matters – head gaskets for example. But torque is a "secondary measurement" it doesn't tell you directly how tight the fastener is, only how hard you had to turn it to get it where it is. So when you add lubricant you make it easier to turn the fastener, so the same amount of twisting actually results in a tighter fastener. But since we have no easy way to directly measure tension in a faster, we use torque and take care to get the procedure right.

But in general lubricants are a hassle and the assumption is that they aren't necessary and won't be used.

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Best practice for all fasteners is clean and dry threads.

There are three exceptions.

  1. If your using thread locker use the same torque. It is generally assumed that thread locker does not change the torque value.
  2. Fasteners that require thread sealant, such as ones that go into water jackets, have torque that is already compensated for the sealant.
  3. For head bolts best practice is a light coat of oil (engine or transmission). If a head bolt requires sealant, don't use oil, use the the sealant instead and see 2 from above. If the head bolts have special instructions then use those.

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