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Oil Pan Plug Threads - Mercruiser Cast Aluminum

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Oil Pan Plug Threads - Mercruiser Cast Aluminum

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Old 03-16-2019, 03:55 PM
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Default Oil Pan Plug Threads - Mercruiser Cast Aluminum

I bought some engines with Mercruiser Cast Aluminum oil pans (stock on a 454, 7 quart I think?), and one of the drains plugs was leaking. I pulled it and It appears to be a 3/8" NPT pipe plug. Everything online says it should be a 1/2-20 thread with a sealing washer...

Anybody know? Pretty hard to get down that low in the boat with a thread pitch gauge.

- Alex
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Old 03-16-2019, 03:59 PM
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depending on the year, some are metric thread
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Old 03-16-2019, 04:03 PM
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METRIC tapered threads? because the plug that was in it, which threads in very nicely, is 3/8 NPT. I dont want to put sealant on this thing, tighten it down, and strip the pan.
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Old 03-16-2019, 04:07 PM
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According to old OSO posts, its either 12mmx1.75 (too small) or 1/2"x20 (again too small)

So do I just have bastard pans or were the early pans (these are 1984 I think) tapered thread plugs?
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Old 03-16-2019, 04:13 PM
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take the bastard plug to a local hardware store and see what the thread pitch is.......put some flex seal on it and go have a beer.....
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Old 03-16-2019, 04:39 PM
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the plug is 3/8 NPT I measured it. flex seal lol it would work but it ain’t righttt

ill go stand on my head and see if I can get a better look at the pan
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Old 03-16-2019, 05:29 PM
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I doubt it is a npt. Should be thread with a sealing washer. Are the threads stripped? The plug you have sounds like a plug they use on stripped threads found in the help section.
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Old 03-16-2019, 06:02 PM
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Since age is not kind to us I keep an inspection camera similar to this in my roller cabinet.

https://www.homedepot.com/p/General-...600A/205748240
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Old 03-16-2019, 07:22 PM
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I have never seen an oil pan with pipe or tapered threads. Are there grooves cut length ways on the drain plug? If so its one of those repaire plugs for a jacked up pan
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Old 03-16-2019, 09:25 PM
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The plug is the same on both engines/pans, it is a hollow core plug if that makes sense, with an 11/16” hex head.

I am 100% certain it is a 3/8” NPT tapered thread plug, threads in nicely but goes all the way until the hex head hits the pan, so probably was forced into a straight thread hole as a “fix” in the past.

I have both pans draining overnight so I can really clean the pan threads up and see if someone tapped the pans to NPT or just jammed these plugs in. There is a flat area that looks like it was for a plug with sealing washer originally. Really gonna suck if I have to pull these engines to fix these stupid plugs.
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