Thrust Bearing The idea behind using a thrust bearing is to take the longitudinal load off of the motors bearings. Motors bearings are not designed to support a load parallel to the shaft (thrust), they are designed to support the loading perpendicular (radially) to the motor shaft.  When running a flex cable leave a gap between the drive dog and the drive strut to allow for cable shorting due to the torsional wind up. This in turns means that more of the prop thrust is transferred up the flex cable to the motor shaft. Although at moderate levels this is OK it will wear out the motors bearings faster. So this is where the thrust bearing comes into play.
 There are several types of thrust bearings, the ones I use include 3 separate parts. These parts include two hardened washers and a nylon bearing ring with pressed in ball bearings (see picture). To use these you sandwich the nylon ring between the two washers which allows for free spinning while taking up the load.
 You install a thrust bearing between the motor and the flex coupling. With it in place the prop thrust will be transferred to the motors case and not to its bearings. You can see the installed thrust bearing in the picture.
Why not just have the drive dog push/rest on the strut? Remember when running a flex cable it shortens under the torsional (twist) load we put on them. So in this setup the thrust would be on the strut via the drive dog which a first sounds like a good thing. However remember that the flex shaft is getting shorter which significantly increases the driveline friction and increases the load on the flex which makes it want to shorten even more!. This may even snap the flex or may it may just pull apart between the flex and the coupler.
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