Motor Grip AOE

Before shimming any high-performance build, verify the angle at which the motor grip seats against the lower receiver. Stock grip + stock receiver is usually correct, but always confirm. Any aftermarket grip or aftermarket receiver almost always introduces a small angular offset — shimming on top of that offset produces a perfectly shimmed gearbox that still grinds in the lower because the pinion is canted.

• 80 grit sanding block (preferred) or a fresh razor blade
• Phillips screwdriver
• Hammer and pin punch for body pins
• Black fine-point marker

1. Screw the grip to the gearbox with the receiver removed. Use the marker to outline the flat areas of the grip where it would seat against the lower receiver.
2. Install the gearbox into the lower receiver with both the rear and gearbox body pins seated. With everything clamped at the correct geometry, mark on the grip where the lower receiver actually contacts.
3. Compare the two sets of marks. If they line up with no gap, the alignment is correct — you're done. Move to chapter 7.
4. If you see a gap, the grip is canted relative to the receiver. Sand down the flat seating surfaces of the grip with the 80 grit block, working evenly across the marked area. Re-check fitment frequently — 80 grit removes material slowly, which is what you want for a job this easy to overshoot.
5. Alternative: a razor blade can shave the seating surfaces faster, but is much easier to overshoot. Use only with a steady hand and a fresh blade.

When the grip seats flush with no gap, the pinion will rise to the bevel at the correct angle and your shim job will hold.

Reference video: https://youtube.com/shorts/9o3Gkzhj_EU