The Garage Cars 101

How To Open a Square Drain Plug Without a Square Socket

Square drain plugs are common on old trucks. An open-ended wrench is your go-to tool here, but I'll share some alternative tactics if that doesn't work for you.
Andrew P. Collins

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Changing the fluid in my 1975 International Scout‘s manual transmission took me a few hours. A lot of that time was spent pumping it’s-barely-a-liquid 90W GL-1 gear oil three feet uphill through a stupid straw (more on that later), but first I had to figure out how to open a square drain plug, which is easy but maybe not obvious to everyone.

Most drain plugs or bolts that you’ll encounter on any car or motorcycle are hexagonal. Those that aren’t tend to have a star-shaped indentation for a Torx bit. But if you find yourself working on an old American vehicle like my Scout or a piece of agricultural equipment, you might find a four-point (square) head on a drain plug.

International Scout transmission oil drain plug.
Yeah, I know it’s a little mucky. Andrew P. Collins

I saw that and, in one of my dumber moments, thought, “well shucks, I ain’t got square sockets.” I took that picture down to my friends at the local NAPA, and they were kind enough not to laugh at me when they explained, that no, they didn’t stock square sockets but an open-ended wrench should fit over that just fine.

Of course! A five-eighths open wrench did indeed fit on the five-eighths square drain plug. But I still had a problem—the damn thing wouldn’t budge. In that situation, the easiest thing to do would be to get a longer five-eighths wrench to break the seal. If you don’t have that, slide a pipe or long jack handle over your open-ended wrench to effectively give it a really long handle. That should provide enough leverage to break the plug open. If you can’t do that, or a long wrench just doesn’t fit where your drain plug is, you could very carefully try what I did and stick a metric 12-point socket onto the square.

I found that a 19-mm socket fit perfectly onto my five-eighths-inch drain plug, which seemed kind of odd because 19 mm is more like three-fourths of an inch. This is just because of where and how the tools are measured. Square sockets actually do exist, by the way. But you’re going to have to order them online, and based on my cursory research they ain’t cheap. If you’re really concerned about stripping the plug you’re working with, one of those specialty sockets might be worth the investment.

So let’s recap and run it down for anyone who finds this post in search of solutions.

How To Open a Square Drain Plug

  1. First, try an open-ended wrench. A five-eighths wrench is a good size to try right off the bat.
  2. It might be worth trying one size down just in case. If you’re working with a plug that’s very old, it might not be its factory size anymore.
  3. If the plug won’t move with a standard open-ender, see if you’ve got a longer open-ended wrench in the right size.
  4. If you don’t have a longer wrench, try sliding a pipe or a long jack handle over your wrench. This will effectively extend your wrench handle and give you a leverage boost.
  5. If you can’t fit a long wrench or handle, or it’s not working, dig around in your 12-point socket set and see what fits.
    • Be very careful while test-fitting 12-point sockets onto a four-point drain plug. You don’t want to force it on and damage the plug’s shape.
    • If a wrench couldn’t fit where you need to work, a ratchet might.
  6. If you find a 12-point socket that fits nicely over the plug (and don’t forget to try metric sizes even on an American vehicle), try loosening it very gently.
    • You have a higher chance of stripping the four-point plug with a 12-point socket than you do with an open-ended wrench, so watch the plug very closely as you turn your tool.
  7. If you’re still not getting traction (somebody must have really been yoked when they torqued your drain plug down) your last hope is to find one of those square sockets and run with that.

Pro Tips for Fluid Changes on Old Trucks With Square Drain Plugs

  1. Make sure you can loosen the fill plug as well as the drain before you empty fluid from anything. You don’t want to be stuck with a dry system!
  2. Triple-check what type of oil you should be using in your specific vehicle’s transfer case, transmission, differential, or anywhere else. My Scout, for example, calls for an ancient formula known as GL-1 which is not all that easy to find today. Modern transmission oils, classified as GL-5, can be incompatible with some of the metals in the Scout’s tranny and cause major damage. Other old vehicles might have similar requirements.
  3. Many people recommend warming your transmission up by running or driving the vehicle before a fluid change. Warmer fluid will drain more effectively, but I never do this. I’d rather have a long drain time or risk a little bit of the old stuff getting left behind than get burned. However, I would save this project for a hot day or do it in a well-heated garage if you can.
  4. Watch your fluid as it drains—mostly to spot metal chunks. If you see some little ones, try being more gentle during your shifts. If you see a lot or you see big ones, share some images with mechanics who specialize in old trucks and see what they think.
  5. When it’s time to refill—have you wondered how you’re going to get a gallon of oil into a tiny hole on the side of your transmission?—you’ll either need to get new fluid in a Capri-Sun style squeeze bottle or a hand pump that’s compatible with oil and the container it comes in.

How To Find Transmission/Transfer Case Drain and Fill Plugs on an International Scout

Just in case anybody finds this post specifically looking for help on drivetrain fluid changes on an IH Scout, here’s a little visual guide on where exactly to find the drain and fill plugs referenced in this post. This is a ’75 Scout Traveller with a four-speed manual, but I bet you’ll find these plugs in a similar spot on any manual-shift Scout.

Andrew P. Collins

Get Ready for a Workout Because Transmission Filling Takes Forever

Once my Scout’s tranny was empty of old oil and the drain plug re-installed, I had to fill it up again with new blood. I mentioned the 90W GL-1 gear oil I had to buy for this thing—that stuff is so thick you might be able to make a sculpture with it. Now, the fill hole was tucked up high and perpendicular to the ground. So what I had to do was run a long tube into it, then connect that tube to what is basically a giant soap dispenser pump which I then screwed onto the new oil container.

My Scout’s transmission sweats oil, but not enough to consistently drip. I park it over a pee pad all the same. It only looks gross because it’s never been fully cleaned. Andrew P. Collins

After a few furious hand pumps I had to laugh—I’d barely moved the oil up the tube! I sat and pumped a while longer, sweating and swearing over my stupid car hobby. Why do we do this for fun, again? Finally, I opened up Netflix on my phone and ran Seinfeld to keep me company since my wife wasn’t interested in watching me wriggle around our driveway all afternoon. I got through almost three episodes of the show before oil started spilling out of the fill hole, indicating I’d gotten all the juice I needed to into the transmission.

So my last piece of insight is this: If you’re gearing up for this job yourself, be advised that you might be at it for a minute.

Any other adventures in trans fluid changing you’d like to share? That’s what the comment section’s for, my friends. Otherwise, good luck and happy wrenching!

1975 International Scout Traveller
I gave my Scout a “Pan-American Safari” theme to make the camo look more “adventure” and less “paramilitary.” I’ll be giving it a whole new look over the next couple of years. I did buy a new hood though, so I could save my parrot and hang it up in my garage. Andrew P. Collins

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Andrew P. Collins

Executive Editor

Andrew’s been an off-road tour guide, car owner-operator-modifier, writer, and editor in the automotive media business for a long time. Previously Reviews Editor at Jalopnik and EIC of Car Bibles, now Executive Editor at The Drive. He's really into old trucks and tuner cars and tends to spend more time tinkering with his vehicles than driving them.