Like many I bought the Hot Racing motor mount and center differential holder. They are well made parts and the motor mount offers unheard of convenience when servicing your truck. The major problem is that when used together, they create front and rearward play in the center differential that equals .4mm. This is unacceptable. Having the spur gear move backwards and forwards on the pinion .4mm while under power is a recipe for bad things to happen. What to do? I contacted Hot Racing about this issue. They said they were unaware of this issue but would look into it. That's great but how does that help those of us who bought the parts but can't use them? I experimented with many fixes and finally found one that works. It allows you to use both the HR motor mount and the HR center differential holder without any play. The fix is rock solid. You have to buy these shims: https://www.amainhobbies.com/protek-rc-8x12x0.1mm-pinion-gear-washer-10-ptk-h-5909/p188797 Take 4 shims (which equal .4mm) and place them between the center differential spur gear and the ball bearing. This is on the external, rearward side of the center differential. It is essentially externally shimming the center diff. It takes all the play out. Their you go. Go ahead and enjoy the convenience of the motor mount and the strength of the center diff holder......without the dealbreaking front rear play.