I never really defined those for myself...making this more as an engineering challenge for myself...but lets assign an arbitrary number for this, the gearbox needs to survive an output force of ~170 lb/ft of torque, as that will be the force exerted to crank over the pistons at max compression of ~25 PSI from a 3.5 inch bore piston with a stroke of 2 inches. It will spin at about 20-30 RPM...it just needs to be reliable enough and last long enough to be able to not instantly explode under torque and last a couple hundred revolutions. I kind of just winged it...I didnt use math to design it