gasman

I echo what druid said your tech must be crap

Had that with my Specna and cured it by dumping the stock grip and fitting a new none Specna grip new hi torque motor and new complete gear set. All shimmed perfectly with best gearbox grease. I start by shimming bevel gear to give optimum engagement to pinion gear. then locktite motor adjuster, the rest is child's play. I found the brand new E&L I bought recently only had half of the pinion gear engaged. The grip needed quite a lot of plastic removing before it seated on the gearbox rather than on the receiver and even then the adjuster required screwing in.

If you are changing only pinion gears then I would look at the bevel gear very closely, then that all gears are from the same manufacturer.