Also make sure to put some grease on the gears and in the case before you screw the alum diff cases together. If you noticed when you took the stock diff cases apart there was light grease on the gears and in the diff cases. I use kyosho 15k grease, not diff oil. This grease is very fine and I use it on all my diffs. Take the alum diff case apart and remove the diff and input drive shaft with bearings. With a q-tip (of which you will use many), apply (very lightly) some grease just inside of where the bearings will sit. What I mean by very lightly is, you should be able to see a shiny glaze to the area where you apply the grease. You don't want any clumps of grease in these areas. Do this on both the top and bottom half of the alum diff case. Next, apply some in the groove (where the diff gear will be) inside the diff case, top and bottom. Here you can apply just a little more, still no clumps of grease. If you look inside the case with the groove for the diff gear to the left, you will see the area that the diff cup itself will be, to the right. Starting from the front bearing retainers apply a little grease down toward the center of the diff area, do this top and bottom. Apply grease to the diff gear and drive shaft input gear. Here you can be more liberal, but still you don't want to put gobs of the grease on. You want to apply enough grease so that when they mesh, the grease will get down in the teeth. Finally, put a light coating of the grease on the outside of the diff cup (not the alum diff case). Put your input drive shaft with bearings in the bottom half of the alum diff, and then the diff itself. Before you screw on the top half, turn the diff either by the diff outdrive or the input shaft. Rotate it one way for a few turns, then the opposite direction. Do this gently as to not pop the diff or input shaft out of place. This will smooth out the grease in the areas you applied it too and spread it uniformly on moving parts. Once you are satisfied with the smoothness of the rotating parts, then put the top half of the alum diff case on. Again, rotate the internals either by the diff outdrive or input drive shaft for the same reason as before. Run your screws in snug, not tight, and repeat the rotating process. Then tighten the screws all the way and check for free rotating internals. When you rotate the internals it should not bind, and spin smoothly with very little resistance. If after you tighten the diffs, the internals do not rotate, you've applied too much grease and need to lighten the amount of grease between the gears. This takes a little patience and practice but it will improve the performance and durability of you alum diff cases. Hope this helps.