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"Short Ears" edition of BNB (Bunny and Bear) Retrograde extruder better balances the extruder by moving motor ultimately back and down. Motor is now 16.5 mm from back of extruder body and closer to level of x-belts. Think of this as a "mid-engined" extruder.

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This extruder is only for the MK3 laser filament sensor. For MK3S or MMU2S printers, see BNBSX extruder https://www.thingiverse.com/thing:3626993

For comparison, the following are how far various extruders place motor from rear of extruder body.

MK3 R3 30.5 mm
MK3S R4 28 mm
Bear 27 mm
BNB Retrograde 23 mm
BNB Short Ears Edition 16.5 mm (with BNB universal x-carriage, 18 mm otherwise)

Bunny scientists moved the motor so far back that it will hit the normal Bear x-end. On the Bear X-axis you will also need to install their special "Short Ears" version of the Bear Motor X-End. On a Prusa x-axis, the motor x-end should already clear this extruder.

Build differences from the regular Bunny and Bear Geared Extruder...

• Short Ears X-End must be installed

• Compact shaft locks required

• Bunny and Bear Short Ears Shaft is different
  Length 55 mm for 2 bearings
  Pulley Flat 0 to 9 mm
  Bondtech Flat 42 mm to end

• Has Vulcan and Mosquito compatibility features.

Upgrade replaces motor plate, extruder body, and idler door, and motor-x-end, x-carriage, x-carriage covers of Bear extruder.

I recommend also changing to the BNB Universal X-carriage while doing upgrade. https://www.thingiverse.com/thing:3610593. That carriage lets you remove the motor plate WITHOUT taking off the back of the extruder and reduces risk of hot end cable damage during XYZ calibration

Motor plate / gearbox MUST be printed with supports. 3mf file supplied with suggested plating and support enforcement block. 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. There will be a small amount of support material near filament path. Clear those out as well.

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 55 mm
Pulley Flat 0 to 9 mm
Bondtech Flat 42 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
4 x M3 square nuts
3 x M3 set screws
M3 hex nuts & bolts (various lengths from 12 to 30 mm long. Some must be cut 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 microstepping and e-steps. Depending on which type of motor you have installed (usually 1.8 degree) the e-steps differ.

geared extruder, 1.8 degree motor (usual)
M350 E16
M92 E490
M500

geared extruder, 0.9 degree motor
M350 E8
M92 E490
M500

Notice that M92 must be AFTER M350 because M350 can alter existing M92 value
Follow with power off/on and M503 to verify settings are correct.

Pulley gearing originally inspired by Skelestruder

Reversible motor x-end inspired by Cristian2281 https://www.thingiverse.com/make:667387

Video of BNB Short Ears Extruder running https://youtu.be/5elxXwZDiMQ

Issues:
Check motor x-end top clamp bolt. Depending on your print tolerances, the motor may come into slight contact with the head of that bolt. If that is happening, either file off a bit of the bolt head or change to button cap bolt (with smaller head profile)

Changes

Added NEMA 17 Direct Motor Plate - allows reversion to plain motor without gearing. This is only to support testing of motors without gearing. Not needed normal use.

Added axis rod push out feature to x-end.

Switched to BNB Universal x-carriage to move 1.5 mm closer to x-rods and allow use on either Prusa or Short Ears Bear axis.

Added Short Ears X-end Tensioner and Idler.

Updated BNB Universal x-carriage to be ready for Short Ears MK3S edition. Intended to avoid future reprinting and re-installation x-carriage.

Changed extruder body and hot end cover to versions with hot fins that can be separately printed in higher termperture material

Corrected error in join alignment of BNB Short Ears X-End Tensioner

Changed to Mosquito compatible extruder body, hot end cover with E3DV6 plenum panes.

Fixed hot end cover chamfer to fit Mosquito adapter. Actual E3DV6 hot ends were fitting correctly, but taper of Mosquito adapter was impinging hot end cover.

B02M3 - adjusted E3DV6 panes for better fit

B02M4 added Mosquito anti-rotation flats to the hotend cover.

B02M5 Dual position fan mount hotend cover is Vulcan kit compatible.

B02M8 Hot end cover now uses standard length M3 x 30 mm screw instead of 27 mm

B03 Changed PINDA clamp to M3 Nylock to better resist loosening over time.

B03m1 Implemented fan shroud interlock to prevent right side tilt. New hotend hot fin and fan shroud are needed to gain new feature. Engage hook of fan shroud before inserting screw.

Added remixed RHD fan shroud with interlock hook

B03M4 Increased PINDA nylock nut pocket size. This nut must slide into place without resistance. Drawing in with tension will split print layers in this location. Enlarge pocket as needed until nut slide into place.

B03M5 Improved fit of fan shroud. Strengthened fan shroud Evan hook

B04 Extruder body and its E3D pane adjusted to allow thicker Sunon 2010 fan with Mosquito kit.

X-end idler modified to better clear PINDA mounting clamp.