What makes servo fast/torquey?
 

is it little motor or gearings ?

Whats stopping me from changing the motor for faster ones if i can find them ? apart from my dodgy soldering...

The casings might be a different size. I know the last servo I stripped down was well packed in. The motor was in a sleeved section of the casing.

Also the circuitry might not be able to provide the current a faster/stronger motor requires.

And with servos, same as cars, the speed and torque is related to motor winding, gear ratio and current draw/voltage.

It's probably worth trying for a bit of an experiment, but I'd not try it with a good, functioning servo. I'd buy a cheap/knackered one and see how you go.

I can tell you how they are different on older ko pds servo's....the motors are the same size, speed and power but the motor pinion is smaller ( this is press fitted and can't be removed ) and the gears inside are also different sizes to a high speed version and alter the overall gear ratio to give lower speed and more torque, the circuit board is exactly the same but on some of the earlier ones the board is the same as the 7.2v high speed servo and where the 7.2v fet wire is connected is just bridged with a 0 ohm link to the 6v supply to make it a 6v servo without the blue 7.2v fet wire, so if you have this type then you can make it faster and even more torque by turning it into a 7.2v supply;)

There are a few servos where the centre section is glued to the motor and electronics. Trying to change that is the best way to needing a new one!

If you can find some older servos, like Futaba's 131 and 132 series then you could play around with motors and gears as they will come apart completely.

With the price of a servo, and the high chance (in my case!) that fiddling with them will end in tears, it's best to buy the servo that does what you need. Check this out...

http://www.servodatabase.com/servos/all

...for all the data you need on the vast majority of servos out there. HTH :)