makita grinder makes a loud grinding noise from the gear head, not the disc, sounds like bad bearings.

Diagnosis

The loud grinding noise emanating from the gear head of the Makita XAG04Z is almost certainly caused by the ingress of metallic and abrasive dust into the gear housing. Because this is a high-RPM tool, the centrifugal force pulls fine particulate matter toward the seals. Once the seals fail or wear down, the grit enters the bearing races of the pinion shaft and the output spindle. This creates a "sandpaper" effect, where the steel balls of the bearing grind against the race, leading to pitting, flat spots, and eventual catastrophic failure. You can confirm this by disconnecting the tool from the 18V battery and rotating the spindle by hand. If you feel a "notchy" sensation or hear a metallic clicking/grinding even at low speeds, the bearings are shot. If the noise only occurs under load, the issue may be the gear teeth themselves, but a constant, high-pitched scream or grinding during no-load operation is the hallmark of failed 6900 or 6001 series bearings. In some cases, the noise is exacerbated by the loss of lubrication, as the grease becomes contaminated with metal shavings and loses its viscosity.

How to Fix It

To resolve this, you must perform a full teardown of the gear head and replace the compromised bearings. You will need a set of metric hex keys, a bearing press (or a vice with sockets), and high-temperature lithium grease.

1. Power down the tool and remove the 18V LXT battery. Remove the wheel guard and the flange nut using the provided spindle wrench.
2. Locate the four long machine screws that secure the gear head assembly to the main motor housing. Unscrew these completely and carefully pull the gear head assembly away from the motor.
3. Remove the gear head cover by unscrewing the perimeter screws. Be careful not to lose the small shims if your specific build version utilizes them for gear spacing.
4. Extract the pinion gear and the output spindle. If the bearings are seized, you may need to use a punch and a hammer to gently tap them out of the housing.
5. Press out the old bearings. Replace the pinion shaft bearing with a new 6900 sealed bearing and the output shaft bearing with a 6001 sealed bearing. Ensure you use high-quality, shielded bearings to prevent future dust ingress.
6. Clean the interior of the gear housing thoroughly using a lint-free cloth and a degreaser to remove all metal shavings and contaminated old grease.
7. Apply a generous amount of high-temperature lithium-complex grease to the bevel gears. Pack the grease into the teeth and fill the housing approximately 60-70% full; overfilling can cause the seals to blow out due to pressure buildup during heat cycles.
8. Reassemble the gear head, tighten the four long bolts in a cross-pattern to ensure even seating, and test the rotation by hand before inserting the battery.

If That Didn't Work

• Inspect the Pinion Gear Teeth: If the bearings are new but the grinding persists, inspect the bevel gears for "pitting" or chipped teeth. If the gear teeth are worn down to a point, the bearings won't fix the noise; you will need to replace the entire gear set assembly.
• Check the Motor Brushes: While the noise sounds like it is in the head, a severely worn carbon brush can sometimes create a high-frequency vibration that resonates through the gear head. Check the brush wear indicators; if they are below 6mm, replace the brushes.
• Verify Spindle Alignment: If the tool was dropped, the output spindle may be slightly bent. Place the grinder on a flat surface and rotate the spindle; if you see a "wobble" in the flange, the spindle shaft is bent and must be replaced entirely.
• Check for Housing Interference: Ensure the gear head cover is seated perfectly. If the cover is warped or a screw is over-tightened, the housing can pinch the internal components, creating a grinding sound.

Heads Up

• Avoid Over-Greasing: Filling the gear head to the brim with grease creates excessive internal pressure and heat, which actually pushes the seals outward and allows more dust to enter.
• Seal Maintenance: Every 50 hours of heavy use, use compressed air to blow out the vents around the gear head to prevent dust buildup from migrating toward the seals.
• Bearing Quality: Only use "RS" (Rubber Sealed) bearings. Open bearings will fail within hours of use in a grinding environment.
• Battery Safety: Always remove the LXT battery before opening the gear head. The XAG04Z is a high-torque tool and accidental trigger activation during disassembly can cause severe injury.