T Thomas is right when he says the Orange Spur is a 90 tooth.
I would ask what track you race at?
Is it indoor,Outdoor?
What rear tires do you guys use.(This will help me with what the surface is like)
How big is the track dimension wise?
Is it a fast free flowing track or is it kinda tight and twisty with a long straight away?
On gearing MOD motor's I will try to help describe what we do.
We usually gear 5-8 over the wind.5,6 over would be for tighter tracks and 7,8 would be for faster Free Flowing tracks.
OVER THE WIND means Teeth of the Pinion OVER the wind of the motor.
10 turn 6 over the wing would be a 16 tooth pinion 10+6=16
12 turn 5 over the wind would be a 17 tooth pinion 12+5=17
When it comes to stocks alot of it will be determined by Spring and Brush Combinations.Heavier Springs can usually be geared a lil higher where as softer springs usually have to be geared Lowered to help with torque.
The only thing I disagree with T Thomas on is gearing of stock.I know Tol and he's a mod guy for the most part so he may have forgot stock gearing.lol.
WHATS UP TOLLLLLLLLL.
Get a Red Spur.It is the 86 tooth spur.
Gearing is like this on a range of stock motor's.
Reedy MVP...18-21(Depending on brush and Spring Setup and Size of track)
Orion Core/Peak Hellfire(Same Motor).18-20
Green Machine 3..15-17
Revenge of the Monster..16-18(usually 16,17).18 for FAST TRACK.
Binary 4 Magnet...15-17(Dont buy this motor EVER for truck)
Binary Roar Legal 2 Mag...15-17(This motor is VERY picky on gearing and ONLY good for BIG FAST FLOWING TRACKS).
If you notice above some motor's need to be geared LOWER than others.These motor's are RPMISH motor's and need to be geared lower to help with Umph or Torque out of corners or from a dead stop.
The motor's that have HIGHER gearing are usually Torquey motor's that will take some gear to help with RPM.
Most Popular Motor's
Tight Track and small to medium in size.Trinity P2k
Tight track Medium to big in size...Trinity P2k2
Fast Flowing Track BIG IN SIZE...Revenge of the Monster.
Instead of Buying TRINITY motor's I would go more for a Aftermarket Tuner that will go through the motor for you and Get it Fully Race Ready out of the package.
****NOTE****---I DO NOT practice with my race motor's.If I happen to,I practice for 6 minutes and pull it off.HEAT KILLS magnets,arms and brushes.So if you wanna Bash or Practice with a motor use a different motor than what you actually race with.
If your practicing for LONG periods of time or bashing for long periods of time even in a stock motor check out Trinity's new XXX laydown brushes.They wont be as fast as other brushes out there but they will LAST alot longer and WONT burn up like other brushes when your running for more than 5 minutes of racing.
Last but not least.Tol was right about gearing on the 14 turn.A 20 sounds great which would be 6 over.
*****TIP***** for MOST offroad racer's there is NOOOO reason to be buying Hand wound $60.00-75.00 mod motor's.Machine wound motor's are just fine for most offroad applications and less than 15-20% of racers can actually feel or tell the difference on dirt.If the only thing holding you back is motor THEN MOVE up to hand wounds.Till then Machine wounds it is.Saves you some money and they work great.
Goodluck to your son racing and I hope you enjoy racing.
*****Look me up on Ebay. I will have alot of stuff going up as 7/7/2016. User name is Dalracer.**** This is going to be a BIG ONE!!!!!!
Just wanted to say Thanks to RcTech for all these years of Great Information.