my saw makes a horrible grinding noise and vibrates like crazy, but only on the highest speed setting

Diagnosis

The grinding noise and excessive vibration occurring specifically at high speeds on the Makita XRJ05Z indicate a failure in the counterbalance assembly or the drive gear synchronization. In a reciprocating saw, the counterweight is designed to offset the forward momentum of the blade; when it loses its alignment or the internal bearings fail, the centrifugal force at high RPMs creates a harmonic imbalance. This manifests as a violent "shaking" sensation that is barely noticeable at low speeds but becomes disruptive as the motor reaches maximum velocity. This is typically caused by the degradation of the needle bearings supporting the counterweight shaft or a failure in the lubrication of the planetary gear set. If the grease has migrated or contaminated with metal shavings, the friction increases exponentially with speed, leading to the grinding sound. You can confirm this by removing the blade and running the tool at full speed; if the vibration persists and the noise is coming from the rear of the motor housing rather than the blade clamp, the internal counterbalance mechanism is definitely the culprit.

How to Fix It

To resolve this, you must disassemble the gear housing to inspect and replace the damaged components. You will need a set of Torx drivers and a replacement counterbalance bearing kit.

1. Remove the battery and all external accessories. Use a T15 Torx driver to remove the housing screws securing the motor casing to the gear housing.
2. Carefully split the housing shells. Be mindful of the trigger switch wiring to avoid pinching or snapping the leads.
3. Locate the counterbalance weight—the heavy metal piece positioned opposite the blade drive. Inspect the needle bearings for discoloration (blueing) or a gritty texture.
4. Remove the counterweight shaft. If the shaft shows scoring or flat spots, it must be replaced. If the shaft is smooth, press out the old needle bearings using a punch and a small hammer.
5. Clean all old, contaminated grease from the gear teeth and the counterbalance track using a lint-free cloth and a mild solvent.
6. Install new high-temperature needle bearings. Ensure they are seated perfectly flush; any tilt will cause the vibration to return immediately.
7. Apply a generous amount of high-pressure lithium-based gear grease to the counterweight shaft and the drive gears.
8. Reassemble the housing, ensuring the gaskets are seated correctly to prevent sawdust from entering the gear chamber. Tighten the housing screws in a cross-pattern to ensure even pressure.

If That Didn't Work

• Check the motor armature for "out-of-round" wear. If the armature shaft is slightly bent, it will create a centrifugal imbalance that mimics a counterweight failure. This requires replacing the entire motor assembly.
• Inspect the blade clamp mechanism for internal debris. A small piece of hardened metal or a broken blade fragment trapped in the slider can cause a rhythmic grinding noise that intensifies as the stroke speed increases.
• Examine the drive gear teeth for "chipping." If a tooth on the primary drive gear has sheared off, it will create a momentary drop in tension every rotation, which feels like a vibration at high speeds. Replace the gear set if any pitting is visible.
• Verify the integrity of the motor brushes. While brushes usually cause electrical sparking, severely worn or unevenly worn brushes can cause the motor to run "rough," creating an unbalanced vibration during high-amp draws.

Heads Up

• Never use WD-40 or thin oils to lubricate the internal gears; these will wash away the heavy grease and accelerate bearing failure. Only use dedicated gear grease.
• Avoid running the saw at maximum speed for extended periods when cutting through dense materials like thick steel, as this puts maximum stress on the counterbalance bearings.
• Always ensure your blades are straight. A slightly warped blade can put lateral pressure on the internal drive system, wearing out the bearings prematurely.
• Blow out the motor vents with compressed air every 10-15 uses to prevent sawdust from migrating into the gear housing, which is the primary cause of grease contamination.