tool gets crazy hot right behind the guard after like 30 seconds of use, way hotter than my old one.

Diagnosis

The localized heat buildup immediately behind the guard on the Makita XAG04Z is a classic symptom of mechanical resistance within the bevel gear assembly. While the motor itself generates heat, the specific area you're describing is the gear head. When the lubricant degrades, separates, or becomes contaminated with metallic shavings, the friction between the drive pinion and the ring gear increases exponentially. This manifests as a rapid temperature spike—often reaching a point where the casing is too hot to touch—long before the motor's thermal overload protector kicks in. This issue is frequently caused by "grease starvation" or the use of an incorrect lubricant that cannot handle the high RPMs (up to 11,000 RPM) of the XAG04Z. If the tool was dropped or subjected to heavy side-loading, the gear mesh may have shifted slightly, causing the teeth to rub against the sidewalls of the gear housing rather than rolling smoothly. You can confirm this by spinning the spindle by hand while the tool is off; if you feel a "gritty" sensation or a rhythmic catch, the gear lubrication has failed or the gears are damaged.

How to Fix It

To resolve this, you must perform a full gear head service. You will need a high-temperature molybdenum disulfide (Moly) grease and a solvent like brake cleaner.

1. Power down the tool and remove the 18V LXT battery.
2. Remove the wheel guard and the locking nut/flange to expose the spindle.
3. Unscrew the four hex bolts securing the gear housing to the motor body. Carefully pull the gear head assembly away from the armature.
4. Open the gear case by removing the perimeter screws. Use a lint-free rag and solvent to completely remove all old, blackened, or hardened grease. Pay close attention to the corners of the housing where sludge accumulates.
5. Inspect the teeth of the drive pinion and the ring gear for pitting or chipped edges. If the metal is gouged, the gears must be replaced.
6. Apply a small amount of high-temp Moly grease to the teeth of the gears. Fill the gear housing approximately 50% to 60% full. Warning: Do not pack the case to the brim; overfilling creates "fluid churn," which actually increases heat and can blow the seals.
7. Reassemble the gear head, ensuring the gaskets are seated flush to prevent grease leakage.
8. Tighten the four housing bolts in a criss-cross pattern to ensure even pressure.

If That Didn't Work

• Inspect the Spindle Bearing: If the heat persists, the front bearing may have failed. Remove the spindle and check for play or a "crunchy" feel when rotating the bearing. If the race is pitted, replace the bearing (typically a shielded ball bearing) to eliminate friction.
• Check for Armature Rub: Remove the motor armature and inspect the commutator and the brushes. If the armature is slightly bent or the brushes are dragging unevenly, it can create heat that radiates forward into the gear head. Ensure the brushes are seated squarely in their guides.
• Verify Disc Compatibility: Ensure you are using a disc rated for the 11,000 RPM maximum of the XAG04Z. Using an oversized or unbalanced disc creates excessive vibration and axial load, which forces the gears to work harder and generate heat.
• Test for Electronic Overload: If the tool is brand new, check for a "binding" feel. A defective electronic speed controller can sometimes cause the motor to run "lean" or erratic, creating heat that mimics a mechanical gear failure.

Heads Up

• Grease Selection: Never use general-purpose chassis grease or lithium grease from a hardware store; these will liquefy at high RPMs and leak out of the head. Only use high-pressure, high-temperature gear grease.
• Seal Integrity: When cleaning the gear head, be careful not to damage the rubber oil seals. A compromised seal allows dust and grit to enter, which turns your lubricant into an abrasive paste.
• Heat Cycle: After regreasing, run the tool for 30 seconds, then let it idle for 30 seconds. This allows the grease to distribute evenly across the gear teeth before applying a heavy load.
• Cleaning Habit: Use compressed air to blow out the vents behind the guard every few uses. Dust buildup blocks the airflow that helps cool the gear housing.