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Just take a ruler or caliper and measure across the "flats" on the hex head.
FYI- HF doesn't sell individual sockets, only complete sets. Is there a pawn shop near you? I pay $1 or $2 for USA sockets at my local pawn shop.
 

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It should not be tight enough to require a socket. An adjustable wrench is more than sufficient.
 

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The biggest socket I have is 1 1/2 and its too small. Does anyone know what size this is? I'm going to stop by harbor freight tomorrow and grab whatever I need. Thanks!

This is out of my 67 S code car.

The reason why no one can tell you what size the drain plug is is because that is not a stock drain plug. Most 9" rear housings did not have any drain plug. Someone added that drain plug and its not in the best location which would be on the bottom of the housing. When you pull that out much of the old fluid - and any 'junk at the bottom - is going to remain.

Paul
 

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Discussion Starter #6 (Edited)
The reason why no one can tell you what size the drain plug is is because that is not a stock drain plug. Most 9" rear housings did not have any drain plug. Someone added that drain plug and its not in the best location which would be on the bottom of the housing. When you pull that out much of the old fluid - and any 'junk at the bottom - is going to remain.

Paul
Edit -

It appears this was an early 67 thing with the factory plug. I've read it in a few places and seen some pictures of others that look exactly like mine. Here is an older thread on here...

http://forums.vintage-mustang.com/concours-forum/637417-67-9-inch-rear-axle-question.html
 

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Edit -

It appears this was an early 67 thing with the factory plug. I've read it in a few places and seen some pictures of others that look exactly like mine. Here is an older thread on here...

http://forums.vintage-mustang.com/concours-forum/637417-67-9-inch-rear-axle-question.html

It may be factory but at the height that plug is it would seem like it is a fill plug than a drain plug.

Can you post a pic of it from directly behind (not at an angle like that).

Thanks
Paul
 

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Looks exactly like mine. My car was built 12/66. You can drain most of it out through the plug like I did and when you pull the gears take some super absorbent rags and leave them in there soaking up the residual for a bit and it'll be all gone before you know it. Paul 1958 you are correct it is more of a fill plug than a drain plug
 

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I'll grab a picture next time I'm out there... My car was scheduled for 12/22/66 but was actually built on Jan 10th, 1967. :grin2:
Good to see you're still working on your car. Been awhile since we.ve spoken.
For the record, my May 10th '67 built with orig 9" has no drain plug.
 

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I have done an intensive investigation of this. I have posted the results here before, but i can't seem to find the post.

The plug is a FORD part.
I have the part # on a different computer, I can't access at this time.
It is a fill plug. It was put on 9 inch rear diff housings as far back as 1962, and I have seen it on several High Performance Ford cars, and mustangs up to about 1970.
It is not just a 66/67 part.
I suspect Ford originally intended to use this on cars with gear ratio's 3.73, 3.89, 4.11, etc. So only High Performance cars.
These ring gears are much bigger/thicker, and it makes filling the housing with diff fluid, at the side of the diff case more difficult.
There is not a lot of room between the thicker ring gears, and the the wall of the diff case with large gears. Maybe a 1/4 inch IIRC. After I rebuilt my 1963, 9 inch, NOD, 3.89, rear end, with the fill plug. The rear came out of a 63, 427 Ford Fairlane car.
With what was called the highway performance package.
I wish I could have been around to pull more out of that car.
This was the start of Ford's Total Performance program.
So, this type of rear end housing is not out of the ordinary.
I will agree that it is very common to find this type of rear end housing fill plug on 67 cars.
BUT.
It is not a 67 only part.

IIRC. The size was 1 7/8. But my memory is not great. I just used a large crescent wrench, like others.
Also.
Last year I put a rebuilt 9 inch rear end with 3.25 gears, and large fill plug into my 66 Ranchero.
The rear end was pulled from a 1967 Mustang GTA.
 
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