Key, WDRF 5/16 X 1.00#1008

What It Is

This is a shear key (Woodruff key, WDRF 5/16 x 1.00#1008) that locks the flywheel or blade pulley to the crankshaft on small engines, preventing rotational slip. It’s typically used on walk-behind mowers, riding mowers, and other power equipment with horizontal-shaft engines (Briggs & Stratton, Tecumseh, etc.).

Fitment & Compatibility

Before buying, verify three things:

• Measurements: 5/16" wide (key width) x 1.00" long (key length) x 0.1008" thick (key height). This is a #1008 Woodruff key size. Use a caliper—don’t guess.
• Crankshaft Keyway: Must match a 5/16" wide slot. Common on engines with 1" or 7/8" diameter crankshafts (e.g., Briggs & Stratton 8-12 hp, Tecumseh HMSK110, Kohler Courage). Check your engine model number.
• OEM vs Aftermarket: This is a generic replacement, not OEM. It fits any engine using that key size, but if your mower has a tapered shaft or special keyway (e.g., Honda GXV), it won’t work.

Key Things to Know

• Shear key purpose: It’s designed to break (shear) if the blade hits an obstruction, protecting the crankshaft from bending. Always replace with the same size—never use a stronger key or bolt.
• Installation difficulty: Easy (10 minutes). Remove the blade/flywheel, tap out the old key, insert the new one, and torque the bolt to spec (typically 40-60 ft-lbs for blade pulleys).
• Common gotcha: If the old key sheared, inspect the crankshaft keyway for burrs or damage. File it smooth before installing the new key—otherwise it won’t seat flat and will shear again quickly.
• Price note: $9.63 is reasonable for a single key, but you can often buy a pack of 5-10 for the same price online. Check if you need extras.

Should You Buy It

Buy this if you’ve confirmed the 5/16" x 1.00" x 0.1008" size matches your engine’s keyway and you need a quick, budget-friendly replacement for a sheared key on a standard small engine mower. Skip it if you have a Honda or Kawasaki engine—they often use metric or proprietary keys.