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.

andrewpcollins

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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.

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.

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.

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.

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!