The following guide will show you how to lock your gear differential. These differentials are common in Tamiya TT-01, HPI Sprint 2, and many other kits and RTR chassis. This method is fully reversible if you do not like the behaviour of the locked diff. Many RC Drifters recommend this especially for the rear diff, but it can easily be performed on the front diff as well. This gives users an inexpensive way to lock a diff so that they can make a better decision whether a spool axle is worth purchasing in the future.
For this mod I will be using a Tamiya TA-03 gear diff. Gear differentials are designed the same way so this method should work for any differential utilizing the same spider gear system. Other common methods includes jamming the differential with rubber tubing, glue, solder, and many other foreign objects to jam the gears and prevent them from turning. Those methods will also work, but this is one of the best we have seen out there.
Carefully remove the differential from your chassis. Take out the drive cups and set them aside. Now carefully remove the screws that fasten the differential together. Carefully remove the the top bevel gear.
Now replace the Y-shaped part with the two gears into the differential, with the 3rd gear removed earlier between the two. This will ensure that the gears will not turn and are locked in place. Refer to the picture below.
Now re-install into your chassis and the differential is now locked, the rear wheels will now spin in unison. Remember to shim (thin spacers) the differential correctly on each side so that it has very minimal play. Careful not to over-shim, this will cause unnecessary heat and friction. All done, happy drifting! XD