Newbi Q

Hi new to the forum i have recently purchased a hip firestorm a hpi mt2 and a ofna crt5. My question is what is up with the back plates on these i have replaced the ofna a nd the firestorm with the shaft start back plates. They are on correctly but when you put them on they seem to tight to spin it?? my roto start will not turn it over untill i loosen the screws. Is there some sort of trick.

Did it come with any kind of shims , you may need thicker ones , the gasket type material between the engine and the back plate to get the proper gear mess without binding .