Lawn Care
How to Sharpen Reel Lawn Mower Blades: Backlapping & Grinding Guide
The Mechanics of the Scissor-Cut: Why Reel Blades Dull
Unlike rotary mowers that rely on high-speed impact to chop grass, reel mowers utilize a shearing action. The rotating helical blades (the reel) sweep past a stationary horizontal blade (the bedknife), trapping and slicing the grass blade precisely between them. When this cutting edge degrades, the mower folds and tears the turf rather than slicing it. According to the University of Minnesota Extension, torn grass tips lose moisture rapidly and become highly susceptible to fungal pathogens like dollar spot and brown patch.
Restoring the cutting edge requires two distinct processes: backlapping (routine abrasive honing) and spin grinding (structural metal removal). Understanding when to deploy each method is the difference between a $20 DIY maintenance job and a $150 professional overhaul.
Diagnosing Dull Reel Mower Blades
Do not rely solely on visual inspection to determine if your reel mower needs sharpening. Micro-burrs and edge rounding are often invisible to the naked eye. Use the paper test to evaluate the shearing clearance.
The Paper Test Protocol:
- Cut a standard strip of 20lb printer paper (8.5 x 11 inches), about 2 inches wide.
- Hold the paper loosely and slide it through the contact point of the reel blade and bedknife.
- Push the reel forward (normal cutting direction). A sharp reel will slice the paper cleanly with zero resistance.
- If the paper folds, snags, or tears unevenly, the cutting edge is compromised, or the bedknife clearance is too wide.
Method 1: Backlapping (Routine Maintenance)
Backlapping is the process of running an abrasive lapping compound between the reel and bedknife while rotating the reel in reverse. This hones both the helical blades and the bedknife simultaneously, ensuring perfect mating surfaces. This method should be performed every 20 to 25 hours of mowing time.
Required Tools and Materials
- Lapping Compound: Danco 120-grit (for routine honing) or 80-grit (for moderately dull blades). Cost: ~$18 per 1lb tub.
- Acid Brush: 1-inch disposable bristle brush for compound application (~$3).
- Power Drill & Socket: A variable-speed drill with a socket matching the reel's drive gear or shaft nut.
- Safety Gear: Heavy leather gloves and ANSI Z87.1 safety glasses.
Step-by-Step Backlapping Procedure
- Secure the Mower: Elevate the mower on jack stands. Disconnect the spark plug wire on gas models or remove the battery on electric models. For manual push mowers, chock the wheels.
- Apply the Compound: Using the acid brush, paint a generous, even layer of lapping compound onto the bedknife and the leading edges of the reel blades.
- Rotate in Reverse: Attach your socket to the reel drive shaft. Set your drill to a low speed (300–500 RPM). Crucial: You must rotate the reel in the opposite direction of normal cutting. Rotating forward will grind the compound away from the cutting edge and damage the blade geometry.
- Monitor the Sound: Run the drill for 3 to 5 minutes. You will hear a distinct grinding noise. As the edges mate, the sound will transition from a harsh scrape to a smooth, high-pitched hiss.
- Clean and Test: Wipe away all residual compound with a rag and mineral spirits. Re-test with the paper test.
Warning: Never use a wire wheel or angle grinder on the bedknife or reel blades. Altering the factory relief angles will destroy the mower's ability to shear grass, requiring a complete cylinder replacement.
Method 2: Spin Grinding (Structural Restoration)
Backlapping only hones the existing edge. If your reel mower has struck rocks, roots, or debris, the blades may have physical nicks, or the cylinder may have developed a conical taper (wearing more in the center than the edges). In these cases, backlapping will fail. You must use a spin grinder to remove metal and re-establish the true cylindrical profile.
| Feature | Backlapping | Spin Grinding |
|---|---|---|
| Purpose | Routine edge honing | Reshaping and nick removal |
| Frequency | Every 20–25 hours | Annually or after impact damage |
| DIY Cost | $20 (Compound & Brush) | $80–$150 (Professional service) |
| Skill Level | Beginner / Intermediate | Advanced (Requires specialized jig) |
Because professional spin grinding equipment (such as the Foley Belsaw or Magna-Matic systems) costs upwards of $3,000, this process is almost exclusively outsourced to specialized small engine repair shops or turf equipment dealers. Expect to pay between $80 and $150 per reel for professional grinding and bedknife facing.
The Missing Step: Adjusting the Bedknife Clearance
The most common failure point in DIY reel mower maintenance is sharpening the blades but ignoring the bedknife adjustment. According to turfgrass management guidelines from Texas A&M AgriLife Extension, the physical gap between the reel blade and the bedknife dictates cutting quality just as much as blade sharpness.
"If the reel-to-bedknife clearance exceeds 0.003 inches, fine turfgrasses like Bermuda and bentgrass will fold over the bedknife rather than shear, resulting in a ragged, brown canopy regardless of how sharp the cutting edges are."
How to Set the Clearance
- Locate the adjustment screws on the bearing housings at both ends of the reel. These screws push the bedknife bar closer to or further from the reel cylinder.
- Loosen the locking nuts on the adjustment screws.
- Using a flathead screwdriver, turn the adjustment screws clockwise to bring the bedknife closer to the reel.
- Perform the paper test across the entire width of the bedknife (left, center, and right). You want the bedknife to just barely touch every helical blade.
- Once contact is achieved across all blades, back off the adjustment screws by exactly 1/8th of a turn. This creates the optimal micro-clearance (0.001 to 0.002 inches), preventing metal-on-metal friction that generates heat and accelerates wear.
- Tighten the locking nuts while holding the adjustment screws in place to prevent shifting.
Recommended Maintenance Schedule for 2026
Pro-Tip for High-Use Lawns: If you are maintaining a high-traffic Bermuda or Zoysia lawn and mowing frequently at low heights (under 1 inch), invest in a dedicated backlapping kit and perform the process monthly during the peak summer growing season. Frequent light honing preserves the bedknife geometry far better than annual heavy grinding.
Frequently Asked Questions
Can I use automotive valve grinding compound instead of Danco lapping compound?
While automotive valve grinding compound contains similar silicon carbide abrasives, it is often suspended in a heavier grease base that is difficult to clean from mower bearings and drive chains. Danco or specialized turf lapping compounds use a water-soluble or lighter oil base designed specifically for easy cleanup on turf equipment.
Why is my reel mower leaving uncut strips of grass in the middle of the cut?
This phenomenon, known as "ribbing" or "windrowing," usually indicates that the reel cylinder has worn into a cone shape (smaller diameter in the center than the edges). Backlapping will not fix this. The reel requires professional spin grinding to restore its true cylindrical profile.
Does the Fiskars StaySharp reel mower require backlapping?
The Fiskars StaySharp series utilizes a proprietary InertiaDrive reel and a specialized non-contact bedknife geometry designed to reduce friction. While the cutting edges will eventually dull, standard backlapping is generally not recommended by the manufacturer for these specific models, as it can disrupt the engineered clearances. Instead, utilize the Fiskars-specific sharpening kit or send the unit to an authorized service center.
