Beware the 13mm Square Drive Drain Bolt: A Mechanic’s Horror Story (Read On…IF YOU DARE!)

It was a dark and stormy afternoon. Lightning crackled across the sky, briefly overpowering the cacophony of howling winds and driving rain slamming against the garage door.

…But the bad weather didn’t bug us, because we were staying in the shop to do some easy car maintenance anyway. First up: changing the rear gear oil in our Subaru Outback.

Little did we know what was in store for us. (Dramatic Gopher)

A Warning?

We rolled underneath our Outback to start the differential fluid change.

It all looked pretty ordinary: an upper fill plug, a lower drain plug, and…a Pentagram?!? Surely, our eyes were deceiving us.

As we smeared away some grease with our thumb however, it was clear the mark was cast into the cover intentionally in an attempt to alert us to something.

But what?

Pushing Ahead Anyway

While we were a bit surprised, the suspicious mark was quickly dismissed. After all, it was just a simple gear oil change—something we’d done dozens of times on other cars. Just insert a half-inch drive socket extension into the square receptacle, remove the bolts, drain the oil, refill, and replace. Nothing to worry about.

TIP! Always make sure you can reach and remove the fill bolt FIRST, before draining the oil. That way you know you’ll be able to fill it back up when it’s empty.

We jammed our half-inch drive extension bar into the upper fill bolt and started turning.

Something wasn’t right.

We kept tuning. The bolt didn’t budge. Yet our wrench continued to creep around. We pushed a little harder and with a sharp “SKRIIIIIINK,” the wrench handle spun down with alarming ease. The sound of that terrible grind pierced through clamor of the thunderstorm raging outside.

Oh no. What had we done?

Our Terrible Mistake

Turns out, that star was a warning. What looked like standard half-inch square drive drain bolts were actually 13mm square drive drain bolts—and that’s a big, big deal.

Why? Well, let’s do the math…

1/2 of an inch is 12.7mm.

Which means when we put a 12.7mm square drive head into a 13mm box and started cranking, we rounded the corners inside the bolt’s receptacle.

Upon that realization, we stood there sweating. Our stomach began to ache. Was the plug receptacle rounded? Would we need to drill it out? Is the differential gear oil now trapped inside forever?

A violent CRASH! of thunder brought us to our senses, and we began to ponder our next steps.

A Hero Appears

As if by some divine intervention, a rarely-used drawer on our tool chest grooooaned open—it was where we kept a mishmash of loose sockets. And, with the slightest glimmer of sunlight through the window, one socket set in particular caught our eye.

It was a collection of specialized drain plug sockets we had purchased years earlier. The set contained an assortment of unique socket shapes and sizes for specific drain/fill bolt applications. There were triangle drives, star bits, square drives, and hexes. (No, not those hexes…)

Lo-and-behold, the set also had a 13mm square drive bit.

We raced over to the drawer, grabbed the socket, popped it onto our 3/8-inch drive ratchet, and rolled back underneath the Subaru.

…Our only hope was that we hadn’t completely wallowed out the fill plug’s receptacle with our earlier impetuousness.

Car-tharsis

You know that part in Mussorgsky’s “Night On Bald Mountain” where the bells begin to ring, symbolizing dawn and the vanquishing of demons?

That’s basically what happened as we inserted the correct 13mm square drive bit into the differential’s fill bolt. It snugged in tightly with zero wiggle, and we began cautiously turning our wrench.

With an agonizing slowness, the fill bolt creaked around and a few moments later, both the fill and drain plugs were resting in the palm of our hand.

Suddenly, the skies parted, the rain slowed to a drizzle, and a family of doves flew in to perch on the Outback’s roof.

“This Car is Clean.“

But let our tale of woe be a lesson to you: If you’re ever wrenching somewhere and see a mark that’s been purposely stamped there, (be it a star, exclamation point, or skull-n-crossbones) take heed!

The stamp mark is likely a warning left by a wise mystic (or factory engineer) telling you that you should grab some lucky talisman (or shop manual) and proceed with caution.