Bought the xtm mammoth and while breaking in the engine I grinded down the spur gear . could it had been not matched to the motor right from the factory ?

Could of been. Before you buy any RC, you have to check everything to make sure its all put together right. Make sure the gear mesh is right. To test the mesh, hold the cluth bell and move the spur gear up and down. There should be very very little play, and I mean little.

sorry for the ignorance but what is the mesh ? The distance between the 2 gears ? I use to use a dolor bill

Yeah mesh is distance between the gears. The dollar bill method is a good way to set mesh as well.