I did a little more research on the motors used in the extruder of the BST 798. The manufacturer is micromo (in the US) and other names abroad. It appears that they have small stepper motors http://micromo.com/Data/Sites/1/Datasheets/Stepper%20Motors/AM_2224-R3_PCS.pdf that will fit the same gearbox the DC motors are currently attached to. (I was hoping to not have to change the mounting/gear layout, so that’s a good thing.) Could someone with more knowledge of electronics let me know if any of those motors and their drivers on the spec sheet are compatible with RepRap electronics?
I’m aware that the steppers are 15 degrees/step but the gear reduction is large. The gear head is the 43:1 version of this http://www.micromo.com/Data/Sites/1/Datasheets/GEARHEAD/23-1_MIN.pdf After that there is another pair of gears. I didn’t count the teeth but it looks like at least a 4:1 reduction. Some rough maths show that the steppers should easily achieve the speeds required for extrusion. The original DC motor appears to be the the 2233012S. The actual motor reads 2233U012S. http://micromo.com/Data/Sites/1/Datasheets/DC%20Motors/2233_S_DFF.pdf