Crank Left Arm with Diamond Hole, 170mm Aluminium Alloy Bicycle Crank Arm Left Replacement for Mountain Bike Road Bike (Black)

170mm Aluminum Alloy Left Crank Arm (Diamond Hole)

Key Features

• Precision Diamond Hole Interface: Features a square taper interface oriented at a 45-degree "Diamond" angle relative to the crank arm, ensuring compatibility with specific square-taper bottom brackets.
• High-Strength Aluminum Alloy: Forged from premium 6061 aluminum alloy to provide high fatigue resistance and durability while remaining lightweight for improved pedaling efficiency.
• Standard 170mm Length: The most common crank arm length, providing an optimal balance between leverage for climbing and ground clearance to prevent pedal strikes.
• Universal Pedal Threading: Features standard 9/16-inch threading, making it compatible with almost all mountain, road, and indoor exercise bike pedals.
• Corrosion-Resistant Finish: Treated with a deep black anodized coating that protects the metal from oxidation, road salt, and moisture.

Replaces / Cross-Reference Part Numbers

• 170mm Left Crank Arm (Diamond Pattern)
• Standard Square Taper Replacement Arm
• Diamond-Hole MTB/Road Crank Arm
• (Note: Not compatible with "Square" hole orientations where the flats are parallel to the arm).

Compatibility & Fitment

This crank arm is a specific replacement part. Before purchasing, you must verify the hole orientation of your original arm:

• Hole Orientation: This is a Diamond Hole. If your current crank arm hole looks like a square (sides parallel to the arm), this will not fit.
• Bottom Bracket: Designed for standard Square Taper (JIS) bottom bracket spindles.
• Bicycle Types: Mountain Bikes (MTB), Road Bikes, Fixed Gear (Fixies), and many Indoor Spin/Exercise bikes.
• Position: This is the Left Side (Non-Drive Side) arm only.

When to Replace

• Rounded-Out Hole: If the crank arm was ridden while loose, the square hole becomes "rounded," causing the arm to wobble or fall off the spindle.
• Stripped Pedal Threads: The internal threads where the pedal attaches are cross-threaded or flattened, making it impossible to secure the pedal.
• Visible Cracks: Stress fractures appearing near the spindle interface or the pedal eyelet, often caused by high-impact jumps or metal fatigue.
• Bent Arm: The arm is visibly warped from a crash or a heavy pedal strike, causing an uneven pedaling motion that can lead to knee pain.

Installation Tips

• Verify Orientation Twice: Hold your old crank arm and the new one side-by-side. Ensure the square holes are oriented in the same "Diamond" direction. If they are different, your pedals will be 45 degrees out of sync.
• Clean the Spindle: Before installing, use a rag to wipe away all old grease and metal shavings from the bottom bracket spindle.
• Grease the Threads: Apply a small amount of bicycle grease to the pedal threads, but keep the square taper spindle clean and dry (or apply a very thin film of oil) to ensure a solid friction fit.
• Tighten to Spec: Use an 8mm hex wrench or 14mm socket to tighten the crank bolt firmly (typically 35–45 Nm). A loose bolt is the #1 cause of rounded-out crank holes.
• Retighten After First Ride: Check the tightness of the crank bolt after your first 10–20 miles. The aluminum "sets" onto the steel spindle and often requires a final snugging.