wrx rear transmission drain plug tool

13 mm x (1 inch / 25.4 mm) = 0.51 inch

So a 1/2" drive socket is only 0.01 inch larger; however, the "1/2-inch" really applies to the socket rather than the drive head which has to be a bit smaller to insure it fits (it can't be the same size or it would be too hard to insert) plus it needs to allow space for the ball bearing (which doesn't recede completely into the head) that holds onto the socket. Considering it is square, there is a lot more metal between points versus hex or star drives. The more "round" the head the more likely it'll strip or mar the points, so 4 points is better than 6 points which is better than 8; however, more points provides better flexibility in positioning the socket and wrench. The same is true if there were a 13mm socket wrench in that the square head would have to be less than 13mm square recess for it to fit and allow the ball bearing to enter the recess. They use metric but 13mm is close enough to 1/2".

Instead of a shorty 1/2 socket drive (i.e., has maybe a 8-10" long bar to the ratchet head) and because it may require more torque than the ratchet mechanism can withstand without damage (it just has a gear with little points on it), get an extended manual 1/2" wrench. This doesn't have a ratchet end. It just has the 1/2" square head (with spring ball bearing for socket retention) on a pivot attached to a much longer bar. It's often called a "breaker bar" because of the greater torque you can apply to break the grip on the threads for a bolt (see pic at

This gives you better leverage so you can more easily apply ever increasing torque to only what is sufficient to break the bolt's grip. Since there is no ratchet mechanism at the end and because of the longer bar length, you need enough room to rotate the bar which means the car should be elevated. Besides having enough room to rotate the bolt to break its grip, you need enough room for the flex on the bar and perhaps to get both hands on it. I've seen these with 17-20 inch length bars but there are jumbo ones with a 40-inch length bar. Obviously the longer the bar then the more elevated must be the car to rotate the bolt. I think my 1/2" breaker bar is about 18 inches long.

With a shorty socket wrench, you're more likely to apply more torque than needed and then there's the sudden release which can bang your knuckles against the car or pavement or whatever. You can also damage the ratchet mechanism which means you have to buy another 1/2" ratcheting wrench to not lose the use of all those 1/2" sockets. With a longer breaker bar, you can also angle the bar a bit to get past obstructions and still have plenty of leverage.

I haven't bothered to even look for a 13mm breaker bar. Never seen one. I've used a 1/2" breaker bar in the diff plug with no problem. The extra bar length is handy for more control of how much torque I apply and permits greater torque than a shorty wrench.

Subaru put that 13mm plug there so that you can use a 1/2" drive breaker bar. It's also used on serpentine belt tensioners on motors too. I like seeing plugs and tensioners with these provisions because you don't have to use any sockets at all. Very convenient. Of course, having to find a

13mm square socket would be troublesome, but that's not Subaru intended.

I've always ridden Japanese designed motorcycles. I assume all the design specs are metric.

There actually are tools for that plug. From Amazon:

Harbor Freight has a whole set of them: