I have HS-645MG's in my GP Ultimate Bipe 160. I have issues with looseness. I can move the control surfaces with little to no effort a noticable amount, not a lot but enough for me to be concerned. I have tightned all my linkages and replaced half of them and I am convinced they are tight. Do I need new gears? Am I mis-applying the servos? I notice a lot of people use these on trucks where maybe a little slop is no big deal.

I had these on my YAK and I could see the ailerons shuddering in fast flight.

If I buy the digital version will they be tighter?

Metal gears will wear with use and will eventually have to be replaced. What you can do to tighten up the gears a bit is to pull the top case off and rotate the center gear 180* so it rides in a fresh part of the gear/teeth. Also double check the splines on the horn to make sure the teeth have not been rounded off causing some of the "slop".