ORIGINAL: XJet
I'm sorry but your "x-spurtise" is showing again :-) Nylon geared servos are *not* suitable for gas-powered planes (especially an aerobatic ship like a Yak). The stresses put upon them by a vibrating gas engine and those large control surfaces is just asking for trouble.
What do you think guys were using 15yrs ago when there were no digitals? Guys flying giant scale 35% gasser planes were using nylon geared servos because thats all they had. They were using 3 and 4 nylon geared servos ganged together and adjusting them mechanically. You might want to confirm that by talking to some guys that have been flying these giant airplanes for more than just a couple years.
Don't forget, the original poster is flying a rather small airframe. It's 74" and it might have a 50cc engine at best. Nylong gears will work just fine.
I installed JR8231 NYLON geared 88oz digitals on the elevators of my 80" World Models Patty Wagstaff with G62 engine. Flew it 450+ flights without a single gear related problem. Actually not ANY problems. It would still be flying today if I hadn't slowed it down too much and knocked the gear off on landing.
This MYTH that gas airplanes need metal gears is just that a MYTH.