The What and Why of Welding the Differential.
Most 240sx’s come equipped with what is called an open differential. An open differential is designed with four gears set inside a casing that is rotated by the driveshaft. One gear connects to each output shaft and the remaining two gears connect to a center pinion and rotate in opposite directions perpendicular to the rotation of the casing. In this manner, power can only be sent to one wheel at a time (the path of least resistance) because the pinion gears counter the rotation of the casing for one output shaft at a time. In short, one wheel gets power while the other one coasts.
Open differentials work great for street and a lot of grip vehicles but when it comes to drifting, an open diff is a serious handicap. During a drift it is very important to be able to modulate the traction of both rear wheels in unison. To achieve this, many drifters use a mechanical 2-way limited slip differential (LSD) which distributes torque equally (or as equally as possible) to both drive wheels during acceleration and deceleration. Although great at what they do, a good 2-way LSD is going to cost around $2000 uninstalled. For a wear item that endures a lot of abuse during drifts, $2000 can be a little much for those new to the drift world or on a tight budget. And this is where welding the differential comes into play. There are several methods used to weld a differential with the end result being a permanent “locking” of the two output shafts. In this manner the rear wheels will achieve a 50/50 torque split without the addition of an expensive aftermarket LSD.
More often then not, the differential is locked by welding the side gears to the pinion gears. This can work well in some milder application but this method has a history of breaking under extended use. This guide details an alternate method that involves removing the pinion gears and welding the side gears directly to the casing. This not only gives a much more direct connection of the gears to the casing but it also yields a much stronger weld as the surface area for the attachment is greatly increased. Also, by removing the pinion assembly, the extra drivetrain noise commonly associated with this setup is all but eliminated. Finally, it is very important that you thoroughly clean and sand the surfaces before welding. I recommend a brass finishing wheel to quickly remove any debris or oil that will inhibit the weld arc from penetrating into the metal surface of the casing or the side gears.
The pictures below with associated captions detail the steps necessary to weld the differential on your 240sx. If you are not sure about one of the steps, I recommend you check out the PD section of the FSM (factory service manual) or picture one for the expanded diagram.