The part number for the HPI pullstartis HPI1618 It only fits on the old style back plates if you have the smaller back plate it won't fit. Depending on the model engine you have you can start it with a bump box and not have the shaft fall off. If it's a VX engine your good to go, if it's a SH engine it's a no go the shaft will fall out. You could also get the sh18 roto start back plate and use it on the VX engine or on the SH engine. If you have the new small back plate then you will have 1 inch between screw holes if you have the old style it will be 1 1/4 inch between screw holes.