What is a BMG extruder? Gear ratio, clones and setups

A BMG extruder is the Bondtech Mini Geared, a filament feeder that grips 1.75 mm filament between two hardened steel drive gears and turns them from a NEMA 17 motor through 3:1 gearing. It works as a Bowden or a direct drive extruder. “BMG” also names its clones and the BMG gear sets inside Voron’s Clockwork extruders.

What is a BMG extruder?

A BMG extruder is the Bondtech Mini Geared, a compact dual-drive filament extruder that Bondtech introduced in 2017. Two counter-rotating hardened steel drive gears grip 1.75 mm filament from both sides, and a 3:1 gear reduction lets a standard NEMA 17 motor push it. Bondtech lists it at 75 g without the motor, which is sold separately.

“Dual drive” is Bondtech’s name for the two-gear grip: the motor turns the primary drive gear through the gearing, and that gear turns the secondary gear on the hinged idler, so filament is pushed from both sides instead of being pressed against a plain bearing. What a dual gear extruder is covers the idea across brands.

Bondtech now makes the primary drive gear and the main shaft as one steel part, the Integrated Drive Gear Assembly (IDGA), to cut run-out (wobble). It introduced the IDGA on its DDX for Creality printers in 2020 and now fits it to every BMG-type extruder, which is why current listings end in “IDGA”. Bondtech’s BMG IDGA retrofit set converts older BMGs.

What is the BMG extruder gear ratio?

The BMG extruder gear ratio is 3:1 as Bondtech advertises it. The gears have 17 and 50 teeth, so the exact ratio is 50:17, about 2.94:1. Bondtech gives 50:17 for Klipper’s gear_ratio setting, and Klipper’s own documentation uses the BMG as its example of gearing that differs from the advertised ratio.

Klipper explains the odd count: gears whose tooth numbers share no common divisor “don’t always mesh the same way with each revolution”, which may spread wear. In practice:

• Marlin and RepRapFirmware need only the steps/mm value. Bondtech’s 415 already includes the gearing.
• Klipper takes either rotation_distance: 7.71 alone, as Bondtech recommends, or rotation_distance: 22.679 with gear_ratio: 50:17. Voron’s configs use the second form: 50:17 for the original Clockwork (“BMG Gear Ratio”) and 50:10 for the Clockwork 2. Use both lines or neither: 7.71 with gear_ratio: 50:17 feeds 2.94 times too much filament.
• Calibrate either way. The published value is a starting point; measure 50 mm of extrusion and correct it as shown in BMG extruder settings.

What is the difference between a Bondtech BMG and a BMG clone?

A genuine Bondtech BMG has CNC-machined steel drive gears that are heat-treated and nickel-coated, in a nylon housing made by selective laser sintering (SLS). Clones copy the design, and their quality varies by maker. In Bondtech’s 2020 teardown of one clone, the unhardened drive gears scratched under a file and the tooth profile differed.

Bondtech published that comparison as the seller of the original, on a single clone a friend sent it. Its findings on that unit:

• Drive gears. The clone’s drive gear scratched in a file test and Bondtech’s did not. Bondtech’s slightly yellow color comes from heat treatment followed by nickel coating.
• Tooth profile. The teeth on both clone gears were cut to a different, less defined profile. Bondtech links this to reports that clones tear filament and leave plastic debris.
• Housing. The injection-molded clone body had warped edges, and its hinge would not close without force because the secondary gear’s teeth hit the front housing.

Clones are not all alike. TriangleLab sells “a high end clone of BMG internal components” with “hardened steel and nickel plated gears” or nano-coated gears. West3D listed the two at $24.99 and $32.99 on November 4, 2026. The kit has the same 50-tooth main gear, and Klipper’s documentation gives 50:17 for “the common BMG extruder” gearing. Start a clone at Bondtech’s 415 steps/mm, then calibrate.

Voron’s Stealthburner manual, which builds its extruder from BMG-type parts, says “Poorly made gears often cause print quality issues. For best performance consider sourcing the original Bondtech parts.” For a tension spring from another vendor, it says to check for about 12 mm length, 6 mm outer diameter and 1 mm wire.

To check a BMG you already have:

1. Look for Bondtech’s markings. In 2020 Bondtech listed a resin badge with its logo, the SLS surface texture, a black main gear, a threaded metal push-fit and a black thumbscrew. Its current IDGA listing mentions a new white plastic main gear, so a white gear no longer marks a clone.
2. Close the idler with no filament loaded. The hinge should close freely, with no gear tooth touching the housing.
3. Check the filament path. Insert a piece of filament: it should run through the middle of the toothed section of both gears. If not, loosen the set screw and move the drive gear, as Voron’s manual directs.
4. Watch for debris and set the tension. Bondtech ties torn filament and plastic debris to badly cut teeth. Voron’s manual adds that too much tension also causes print issues and deforms soft filament, so tighten the thumbscrew only until the filament feeds.

Can a BMG extruder be used as a Bowden or direct drive?

Yes. Bondtech sells the BMG for both. For direct drive, a groove-mount hotend such as the E3D V6 clamps into the extruder body with two bolts, and the BMG-M version takes a Slice Engineering Mosquito instead. For Bowden use, add Bondtech’s Bowden adapter (sold separately) to hold the PTFE tube.

• Left or right. The Standard BMG is the right-hand version. The Mirrored (left-hand) version is its mirror image, for mounts or second extruders laid out the other way.
• Retraction. As a direct drive the BMG retracts direct-drive lengths: Bondtech’s PLA profile for its CR-10 kit uses 0.8 mm at 40 mm/s. As a Bowden it keeps Bowden lengths, because the filament still travels the whole tube. Details are in BMG extruder settings, and the Bowden-to-direct trade-off is in what a direct drive extruder is.
• Mass on the carriage. As a direct drive, the BMG puts the extruder, a NEMA 17 motor and the hotend on the carriage. Lighter direct drive extruders built for that job include the Orbiter, about 135 g with its motor, and the Sherpa Mini.

How much does a BMG extruder weigh?

A Bondtech BMG weighs 75 g without the motor, by its product page and datasheet. Bondtech’s Gen2 comparison chart lists 99 g for the BMG, BMG-M and DDX,. The motor is extra: Bondtech offers 22 mm and 25 mm long NEMA 17 motors for the BMG.

BMG extruder vs E3D Titan

Both are 3:1 geared extruders driven by a NEMA 17 motor, with starting values 3.5 steps/mm apart. The difference is the grip: the BMG turns two gears that pinch the filament from both sides, while the Titan presses it against an idler bearing with one toothed drive gear. E3D’s store no longer lists the Titan; Bondtech still sells the BMG.

E3D explained its choice of 3:1 for the Titan: around 5:1 gives more torque than the drive gear can grip with and lowers top speed, while 3:1 keeps the torque needed and triples the resolution. Swapping one extruder for the other moves the starting steps/mm by under 1%, so calibrate after the swap.

Which BMG extruder kits are there?

Bondtech’s BMG family covers single and dual extruders for groove-mount and Mosquito hotends, plus BMG-type upgrade kits for Creality and Prusa printers. Voron’s Clockwork extruders are built around BMG gear sets. By name:

For a Creality Ender 3, extruder options and prices are compared in Ender 3 extruder upgrade. For Voron builds, see Voron. Every other extruder type, from metal Bowden replacements to lightweight direct drives, is in extruder upgrades.