I arrived home last night (first time on the bike to work in 6 months, and boy did it hurt) to a small box on the doorstep. Inside were a collection of interesting items which I'll go through in the next week or so. The one that related to yesterdays blog post was an Atlas Sd mech from 2000, with a motor in it.
OK, so out with the measuring stick, and guess what, its a completely different size to the older (c.1990) motors. The dimensions for this motor (which I'm guessing is the same size as the 540100 slow speed motor) are 26.5mm long by 9mm wide by 13.5 mm high. The flywheels are smaller at 9mm wide by 7mm long, and the overall length is 42mm (with flywheels).
UPDATE. Talking with Steve this afternoon, and checking again tonight the motor body length is indeed 23.6mm. No idea where I got 26mm and I'll blame it on the Pinot.
And Steve has kindly sent in some pictures showing a few different aspects of the motors.
On the left is a slow speed Atlas motor for non DCC ready split frame drives (motor contacts the split frame chassis directly, the one on the right is for DCC ready drives (contacts the Light/DCC board).
On the left is the motor with the step back inside the flywheel to fit in the plastic hex nut drive (most 4 axle locos, old 6 axle locos), on the right is the same motor with a different flywheel to fit in the plastic joint for the drive train (most new 6 axle locos). If you look closely you can see that the plastic part is cracked and will just leave the loco sitting on the rack making noises without doing much else. This is apparently a problem with the new Atlas 6 axle locos.
On the left is the Atlas slow speed motor , on the right the Kato WARP I motor. Note that the Kato flywheels are noticeably bigger which leads to an overall better drive performance (so Steve says), but at the expense of a scale width hood on their N scale models.
More info as it comes to hand.