1977 Jeep CJ7 AMC 20 Differential Fill Plug Stuck

[marcus59]

Hi All:  

I have completely butchered my rear diff drain plug on my 77 CJ7 and I guess I will have to drill it out.  I cannot find a drain plug here in Canada so I was wondering what I could replace it with once I have it out as far as size and thread.  The hex is a pain anyway and a nice square head bolt would have been nicer to work with.  Thanks for your help.

Mark

[Moses Ludel]

Mark, that happens...Do you mean the forward facing plug on the yoke side of the rear axle?  The original OEM part number is:  J4004751  PLUG.  This is a standard NPT tapered thread.  You can replace the original plug with a common NPT plug of the correct size.  You have the option of female square drive or male square drive.  Zinc plated NPT plugs like this are common for engine block oil galleries.  Hardware stores also carry common cast NPT plugs.  Make sure the plug depth does not interfere with anything inside the differential.

Here's an example of the part with the current superseded part number.  The Mopar plug is now male square head.  Any Jeep or Mopar dealer can access the part: 

https://www.moparpartsgiant.com/parts/mopar-plug-differential-cover~j4004751.html

Crown sells a plug but refers to it as a differential cover plug.  This may be the correct plug and just misnamed.  It has a female square drive:

https://www.crownautomotive.net/CJ-Rear-Axle-20-Parts.html [scroll to bottom of parts listing]

Trust this helps.  See whether this information is correct for your application.

Moses

[marcus59]

Hey Moses... thanks so much for your quick reply.  I think that's exactly what I need.  I've attached a couple pics to show you.  As you see, it's been rusted in there and I've tried heat, lubricants and brute force to the point that it's pretty mangled.  Do you recommend drilling it out?  I have not done that before but I'm guessing I will have to be very careful not to mess up the hole threads.  Thanks again, Mark

[Moses Ludel]

Mark...I would drill carefully through the center of the hex with grease on the drill bit to catch filings from the drilling.  Use a shop vacuum as you go.  You want to prevent filings from getting into the differential.  That's the biggest concern.  A 3/8" hole would be plenty for a 1/2-inch NPT plug.  This may require two drill sizes (1/4" and 3/8") to do properly.  Stay on center to avoid damaging the axle housing threads.

Do not break a drill bit in the plug.  Take your time here.  The plug material should not be exceptionally hard.  You know that already from how easily the hex rounded out.  If you drill through the plug, it will be stress relieved.  A partial hole will put more stress on the extractor. 

Use a screw extractor of the spiral type with a left-hand twist.  (See generic pic below.)  Be very careful not to break the extractor, don't use a cheap one...The extractor is hard steel and not easy to drill.  It will not take extraordinary force to get the plug to turn, this is a tapered NPT.  As soon as it "breaks loose", you should be on your way.

Send us some pics of your success story...The rounding of the hex in these plugs is not uncommon.  The design was poor, likely the reason for the change to a male square head NPT plug.  Traditional fill plugs, which incidentally this is the inspection/fill plug and not the drain, are square male.  The drain method for this axle is to loosen and remove the differential cover.  You then install the cleaned cover with a bead of fresh RTV sealant and fill the fresh oil through the plug hole where you are now making the repair.  With the axle level, the gear lube fill is to the bottom of the fill plug hole/threads.

Moses