Oil Pan Plug Threads - Mercruiser Cast Aluminum
03-16-2019 | 03:55 PM
#1
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From: Kemah, Tx
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
Anybody know? Pretty hard to get down that low in the boat with a thread pitch gauge.
- Alex
03-16-2019 | 04:07 PM
#4
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From: Kemah, Tx
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?
So do I just have bastard pans or were the early pans (these are 1984 I think) tapered thread plugs?
03-16-2019 | 06:02 PM
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From: West Michigan
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
https://www.homedepot.com/p/General-...600A/205748240
03-16-2019 | 09:25 PM
#10
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From: Kemah, Tx
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.
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.
