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oil drain plug size issue

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I have a K301 (for an AC712), which (based on the parts list) has a 3/4" drain plug. I repowering it with a different model K301 that uses a different oil pan (one without the flanges). I have an oil pan for it, but it didn't come with a drain plug, but the plug from my old K301 is too large. I was assuming (based on the parts manual) that it must be a 3/8" plug, but when I try to screw a 3/8" bolt into the drain plug hole, it won't fit (tried both a course and find thread). My question is, are these drain plugs "special" sizes, or are the threads messed up, or what am I doing wrong?


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There are a few differences between pipe and bolt threads. One of the main ones, is that pipe threads are tapered. That's why they get tighter as they are cranked down more.

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Here's the skinny on threads.

External pipe threads are sized by the inner diameter of the pipe. 3/8" pipe threads will have a 3/8" diameter hole passage.

NPT stands for "National Pipe Tapered". Tapered pipe threads are the most common pipe thread.

NPS stands for "National Pipe Straight". Straight pipe threads are mostly found on electrical conduit fittings.

On bolts, 3/8" is the diameter on the bolt.

NC stands for "National Coarse"

NF stands for "National Fine"

A 3/8x16 NC bolt is 3/8" in diameter with 16 threads per inch.

My guess here is that there are 1/2" pipe threads on the new oil pan.

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Actually went to a local hardware store and determined that it's 1/4" NPT. Very easy to find a plug when you know what to look for! =)

Thanks for all the help. Sorry for such a noob question ... but, now I know!

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I should specify SMALLER compressors and fittings. Pretty much anything in a home garage sort of thing.

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Pipe thread, not fine or course bolt thread. You can use a pipe thread bushing to reduce size if needed. I reduced my Kohler 3/4" drain tube down to 1/2" for ease of engine installation

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