Introduction: One Tire Fire? Weld Your Differential

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Hello instructables! Long time listener, first time caller.

Do you have an "off road use only" vehicle that needs more traction? Would you rather have more predictable power transition and control while driving to the extreme? Do you not really mind if your tires chirp around slow, tight corners? 

Well, I bet you'd be interested in an aggressive limited slip differential, or some kind of locker (rock crawlers). But if you're like me, anywhere near $1000 for an aftermarket performance differential is just out of the question. Especially when you own or have access to one of the kings of sticking-metal-to-other-pieces-of-similar-alloy-metals.  The versatile, the amazing, the humble. The Home Welder!

I happen to be building a drift car, read more about it here, and welded my first differential not too long ago.   Disclaimer: welding your diff wrong can lead to a diffsplosion. Actually, depending on how you drive, welding a diff right can lead to diffsplosion. Hey, driving hard breaks things, deal with it.

Do yourself a favor, trust me, go out and buy  small wire feed mig welder. They aren't that expensive, and as you can see later, they can do things you didn't  think they can do. It's pretty cray.  I bought my Hobart Handler 125 at Northern Tools for about $350. You can do a lot with it out of the box, but to get the most out of it, go gas. I won't go into too much detail about welding here, I'm sure you've read more than one instructable on how that mule kicks. So, let's get onto the metal melllttiinnngg.  I mean, welding can be dangerous. Please, kids, safety first, wear protection.

Step 1: Yank Out Your Open Diff

In order to weld a diff, you need a diff. Most people will probably remove the one that's in their car.  It might be a good idea to buy a used one to work on so you can still drive your car if you need to.  Mine is a long-term project, so I wasn't worried about down time.

Make sure you follow your Factory Service Manual (FSM) to remove, disassemble, and reinstall your differential.  There's no way to walk you through your removal here.  Sorry, you'll have to figure this one out on your own. Make sure you get an adult to help!

Ignore the the clean, rear uprights with new 5 lug hubs and polyurethane bushings in the foreground. That heavy, dirty lump in the background is what we're interested in.

Step 2: Break Open the Housing

Mmmmm, I love the smell of old gear oil. Drain the diff housing, and remove the rear cover.  Some people leave the the actual differential case assembly installed and just rotate it 180 degrees to weld the spider gears from both sides through the "windows".

I wanted to make sure I could reach as much of the gears as I could, so I removed the diff case assembly from the diff housing.  I also wanted to inspect everything and such as.

You can see the ring gear separated from the diff case in the first picture. Do as I say and not as I do by replacing the seals, maybe even the bearings if your car has a lot of miles.  Be prepared to order any parts that are out of spec according to the FSM. 

The second picture is of the differential itself inside the diff case.  These spider gears are what let the tires rotate at different speeds when going around a corner, or when doing a lame burnout.  On the lower gear, you can see the splines where one of the stub axles slide into.  The stub axles are connected via a flange and bolts to the half-shafts. The half-shaft's connected to the hub bone, the hub bone's connected to the wheel bone.

The diff is still soaked in gear oil, yucky.  Make sure you brake clean the shiz out of everything you can see. Well, maybe not everything, keep it off your sandwich. 

Step 3: Buzz Buzz Hot

Once the parts are clean and dry of brake clean, you can start the fun part.  I had the power of the Hobart turned all the way up, with the wire speed set a bit lower than full.  I don't remember the exact settings, but the idea is to heat the teeth of the gears up really well.  I had mine glowing.  I wasn't expecting the Hobart 125 to be able to heat up the metal enough, but I was sure wrong.  It had no problem melting the teeth and laying a nice deep puddle.  

Here you can see where I started on the "corners" where the teeth of the spider gears actually meet.  Some people weld where the gears meet and leave it at that.  I wanted mine to have more steel connected. So I welded all four corners and then added some steel plate to completely connect all the teeth of the spider gears together.

Step 4: Ball of Steel

It's welded! Yay!  Make sure to clean it up real well.  Even though I was using gas here and not just flux core, there is still a bit of slag.  Make sure you get as much out as you can, you don't want anything getting in the way of any moving parts.  

This thing is solid.  The housing will break before any of the welds fail.  I am a little concerned about whether or not gear oil can get inside that rock of steel.  I'm hoping it will still be able to through the stub axle opening.  I don't think it will, if it does grenade, stock diffs aren't too expensive.  I'll just weld shut another one!

Now that its all cleaned up, put it back together man!  Don't forget a new cover gasket, and of course, don't forget fresh gear oil!  

I removed my subframe to gain access to the fuel tank and to replace the torn, rubber subframe bushings with solid aluminum bushings.  It was pretty easy to get the diff back into the subframe, then raise the whole assembly up to the car with my floor jack.

I also added a short video of it in action.  Don't hate on my iPhone video skills yet, I was just a iVideo noob at the time.

Done!  Go drive and enjoy your poor man's limited slip!