Geared extruder mod for stock Prusa MK3S extruder improves extruder uniformity via gearing like Bondtech BMG or Skelestruder. Rather than a complete extruder replacement, this is a quick upgrade for the stock Prusa R4 MK3S extruder.

This is intended for those who wish to improve uniformity, but do not want to replace their entire MK3S R4 extruder. This upgrade replaces JUST the motor plate and idler with geared Bunny Science versions. Remainder of the stock R4 MK3S extruder is left intact and even remains installed on the printer during the upgrade.

NOTE: You CANNOT put this motor plate onto the Bunny and Butterworth (BNBTR) extruder bodies. This will only fit the stock Prusa R4 extruder body. The BNBTR extruders also implement a filament path correction and tighter air gap at the bottom of the extruder. To remain compatible with the stock extruder body, this upgrade does NOT alter filament path nor air gap. Indexing pins on the BNBTR extruder body help prevent accidental installation of this motor plate on the wrong extruder body.

Advantages of Bunny Sciense MK3S Extruder Gearing Upgrade

Improve extruder uniformity via gearing like Bunny and Bear, Skelestruder or BMG extruders.

Reduce extruder mass by about 70 gm compared to stock motor.

Reduces moment arm by moving motor closer to x-axis than stock motor.

Upgrade replaces motor plate and idler door.

Leave the rest of your MK3S extruder installed.

Motor plate / gearbox MUST be printed with supports. 3mf file supplied with suggested plating and support enforcement block. Specify support only for the gearbox portion and up to the 2nd bearing. This leaves filament path unmarred by support material. 3mf's are set for 0.15 mm layers for nicer finish.

Removing support material from inside gearbox cavity takes 10 to 15 minutes. Take care to completely clean out mounting holes for bearings. Push shaft rod into gearbox from outside to remove support material within bearing mounts. If you do not clear out bearing mount holes completely, bearings will not spin freely.

Some support scarring will remain, but is largely hidden by installed extruder cover.

Build with two shaft bearings. There isn't a 3rd bearing for this upgrade.

Materials needed...

5 mm diameter shaft rod
Length 61 mm
Pulley Flat 0 to 9 mm
Bondtech Flat 50 mm to end
see https://www.thingiverse.com/thing:3595205 for shaft cutting guide and instructions

2 x 1050ZZ bearings
1 x continuous loop 140-2GT-6 Timing Belt
3 x M3 square nuts
3 x M3 set screws (3 to 4 mm length)
M3 hex nuts & bolts (various lengths from 12 to 30 mm long. Some must be ground to custom length during assembly)

M3 washers
1 x 16T drive pulley (to simplify alignment, use one for a 10 mm wide GT2 belt despite the belt being 6 mm wide)

Motor - 1.8 degree, STEPPERONLINE Nema 17 Bipolar Stepper Motor 0.7A 13Ncm (18.4oz.in) 17HS10-0704S. This is the ONLY short body motor that worked well in my testing.

Wiring harness for motor.

You MUST also set e-steps by sending g-codes to printer
M92 E980
M500

Pulley gearing originally inspired by Skelestruder

Extended flats on shaft to ends of shaft for easier alignment during assembly