Overview
This replacement gear head assembly transfers rotational power from the trimmer shaft to the cutting head at the proper angle and gear ratio. A worn gearbox develops play, makes grinding noises, and eventually stops driving the trimmer head altogether. Swapping in a new aftermarket gear housing restores smooth, vibration-free operation.
Key specifications
- Replacement gear head/gearbox assembly
- Includes housing, bevel gears, and bearings
- Pre-lubricated for immediate installation
- Direct-fit OEM-equivalent design
- Hardened steel gears for long service life
Compatibility
This gear assembly is designed for Stihl trimmers and brush cutters as specified above. Verify the shaft diameter, thread direction, and mounting style before ordering.
Installation notes
Remove the trimmer head or cutting attachment from the old gearbox. Loosen the clamp bolt securing the gearbox to the drive shaft and slide it off. Install the new assembly onto the shaft, align the drive coupling, and tighten the clamp bolt. Re-attach the cutting head and verify that the head spins freely without wobble.
Common questions
Q: Will this 4180 640 0114 gear housing assembly fit my Stihl FS 91 R, or is it only for the FS 56 RC-E?
Yes, this gear housing assembly is compatible with the Stihl FS 91 R as well as the FS 56 RC-E, FS 90 R, FS 94 R, FS 111 R, FS 131 R, FS 70 R, FS 85 R, FS 80 R, and FS 240 R. The safest check is to match your original part number and confirm your machine is one of the listed straight-shaft trimmer models.
Q: Does the 4180 640 0114 gear head box come ready to install, and do I need to add grease before using it?
It’s a good idea to inspect the housing and add the proper gearbox grease before first use unless it is clearly pre-greased. After installation, make sure the driveshaft engages fully and check that the head spins smoothly without binding before running the trimmer at full speed.
Q: My FS 94 R has noise and wobble at the cutting head—would replacing the gear housing assembly fix that?
If the wobble or grinding is coming from worn gears, bearings, or damage inside the lower gear head, replacing the gear housing assembly is often the correct fix. It’s also smart to inspect the driveshaft, cutting head, and mounting hardware at the same time, because wear in those parts can cause similar symptoms.
