I am currently in the process of installing a piggyback oil drain plug (a bolt that has a bolt through it) and was confused to learn that the inner bolt didn't need a gasket because it was an NPT (National Pipe Thread) bolt. Every other car that I have dealt with has always required a gasket for the oil drain plug.

According to Wikipedia, NPT screw threads and pipe threads "include both tapered and straight thread series for various purposes including rigidity, pressure-tight sealing, or both." If there is a way to design threads to make a proper seal, then why don't oil pans not sure this for their drains and instead require a gasket?

I understand NPT is a U.S. standard, but the concept remains the same. If there is a way to not need a gasket, why would automakers choose not to use this?

closed as primarily opinion-based by cory, tlhIngan, MooseLucifer, Pᴀᴜʟsᴛᴇʀ2 Mar 4 '17 at 2:42

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    Things that come to mind: Temperature range, -40...+150C should be accounted for at the oil pan. Vibration. Regular removal of the plug. – JimmyB Mar 3 '17 at 7:45

Best guess - NPT threading requires a certain depth to have the sealing effect - so your oil pan would need to be thicker at that spot and probably require more torque. A normal thread won't seal - that's why you use a gasket, but may require less threads to hold so the pan hole can be thinner and you need less torque since you are requiring the gasket to hold.

  • That makes sense. Also I realized the threading being tapered means the threading isn't the same all the way through which might make drilling/tapping it more difficult. – FreakyDan Mar 2 '17 at 22:19

I'm not sure if there is some kind of purely mechanical reason (such as, it doesn't seal well enough against oil), but my money would be on cost.

It costs money to design and test a new bolt, when the old bolt and gasket combo already works well enough. Additionally, such an NPT bolt would not affect safety, performance, reliability, or emissions. I would argue these are the main concerns of automotive industry R&D.

From some light googling, 1 NPT bolt costs more than 1 drain bolt and 1 gasket.

  • Interesting point regarding costs of parts. All the savings from not needing new gaskets would only benefit the buyer. – FreakyDan Mar 2 '17 at 22:17

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