Hi!
Me an my friend have the D4 SS from 2 diffrent "tuners"!
My question is in what way does the internal resistance in the stator affect the performance... One motor has number on the A-B-C poles in the range 20.8 ->21ohm and the other has number from 19.0 -> 19.1ohm..
Both motors are running 12.5HT rotors.
So my question is what are the benefits and diffrences of these 2 motors, performancewise?
Or are these numbers just mumbojumbo that doesnt affect performance at all?

All else being equal, lower resistance means more power.

I'd be concerned about the legality of the 19.0-19.1. Only one I saw that low was an Ultra-low resistance motor which is not a ROAR approved motor for stock racing.

How do you test for resistance? Is it as simple as using a multimeter set to ohms checking A-B, then B-C then A-C?

Multimeter must be able to measure milli ohms. That's where the MMS meter comes in. Yes, A-B, A-C, B-C.

I doubt if the 19~21 ohm is right. It is just about 1 meter of copper wire.

As Andy said, you need a special meter to measure low resistances accurately. I use a GW Instek GOM-802.

Terminal resistances (AB, BC, AC) will be about twice the numbers given in the OP, which are coil resistances. Measuring coil resistance is done from each terminal, A, B, or C, to the collector ring on the side of the stator away from the terminals. Measuring terminal resistance is obviously more convenient, and is fully acceptable, since that's what the ESC "sees". (You can also use the terminal resistances and some algebra to solve for the coil resistances.)

Expect a good 17.5 to be in the low 40 milliohm range for terminal resistances. As Andy and Roeloff stated, 19 milliohms for each coil (38 for terminals) is suspect.

You'll also need to know what temperature the stator is at, as copper resistance has a large (+0.39% per degree C) temperature coefficient. It is customary to report the result corrected back to what it would be at some reference temperature, usually 20 or 25 degrees C (I use 25C).

I havent measured it myself, just what it says on the stator sticker...