How to Set Cutting Height on a Mower Deck to the Grass Type

Why this matters

The number stamped beside the deck pin is not the height the blade cuts. On most commercial decks it is a nominal figure that drifts with tire wear, tire pressure, deck pitch and how the lift linkage has stretched, and on a fleet of three machines the same stamped "4" can cut three different heights. The customer sees it as a lawn that looks scalped one week and shaggy the next, or as a visible step where two mowers met. This card owns the shop's method for turning a grass type into a true blade height, and for proving it on the bench and on the turf.

The shelf rule it serves: cutting height is measured on a level pad from the ground to the blade tip, and mowing follows the one-third rule.

Start state: what has to be true before you measure

Measuring height on a machine that is not ready gives you a precise wrong number. Before you put a ruler under the deck:

• The deck is level side to side and pitched front to back as the maker specifies. Leveling is its own job; see related: Level a Mower Deck. Height setting assumes it is done. If the left and right tips differ by more than 1/8 in when you measure below, stop and level first, because no pin setting fixes a tilted deck.
• Tires at the maker's pressure. A rear drive tire a few psi low drops that side of the deck. Set pressures from the operator's manual or the sidewall, every tire, before measuring.
• Blades sharp, straight and correctly installed, sail up. A bent blade gives a different tip height on each end. See related: How to Remove, Sharpen and Balance a Mower Blade.
• A level pad. A shop floor or a slab you have checked with a 4 ft level in both directions. A trailer deck, a driveway or a garage floor that drains to the door is not level, and the error lands directly in your reading.
• The tools. A steel rule or tape reading to 1/16 in, a blade-holding block, and a turf height gauge (a prism gauge or a ruler with a sliding foot plate) for the on-turf check.

Find the target height from the grass type and the season

Start from the grass, not the machine. The species sets the range and the month sets where in the range you sit.

• Cool-season turf (tall fescue, Kentucky bluegrass, perennial ryegrass, fine fescue) runs higher in summer heat and lower in spring and late fall. See related: Cool-Season Mowing Heights Through the Year for the month-by-month card.
• Warm-season turf (bermudagrass, zoysia, St. Augustine, centipede, bahia) runs from under an inch on hybrid bermuda to 3 to 4 in on St. Augustine and bahia. Some of it is better cut with a reel than a rotary; see related: Warm-Season Mowing Heights and the Reel Versus Rotary Question.
• The species ranges are listed on Mowing Patterns + Height by Species. Tall fescue sits at 3 to 4 in, Kentucky bluegrass 2.5 to 3.5 in.

Write the target on the property card as one number, not a range. "Tall fescue, 4.0 in, June to August" is an instruction a second-year tech can follow; "3 to 4 in" invites everyone to pick their favorite.

Check the target against the one-third rule before you commit. No single cut should remove more than a third of the leaf. So the tallest the grass can be at the next visit is the target times 1.5: at a 4.0 in target, grass up to 6.0 in can be cut in one pass (6.0 minus a third, 2.0 in, leaves 4.0). If the property is on a weekly route and the lawn is putting on more than 2 in a week in a spring flush, the target is fine and the frequency is wrong, or the crew needs a two-step cut: one pass high this week, back to target next week.

Set the pin and measure the true blade height

Lock the machine out before a hand goes under the deck. Engine off, parking brake set, key in your pocket, spark plug boots pulled and taped back on a gas machine; on a battery machine pull the safety key and disconnect the pack. On a walk-behind the blade is bolted to the crankshaft, so turning it by hand turns the engine through a compression stroke, and a hot engine restarts more readily than a cold one. Wear cut-resistant gloves to rotate the blade, because a sharpened edge cuts.

1. Set the deck pin at the stamped number nearest the target. Lower the deck onto the pin as the manual describes, with the lift pedal or lever fully released so the deck hangs on the pin and not on your foot. Reason: a deck partly held up by the lift linkage reads high, and the reading changes the moment the operator lets go.
2. Turn each blade so its tip points straight forward. Reason: decks are pitched slightly nose-down, so the front tip is the lowest point in the blade's circle and it is the height that actually cuts. A blade measured sideways or at the rear reads high by the deck pitch.
3. Measure from the pad to the cutting edge at the tip, not to the sail. Hold the rule vertical against the pad and read where the sharpened edge crosses it. The sail (the upturned back edge of the blade) sits higher and reading it overstates the height.
4. Measure every blade. On a three-blade deck, all three tips forward, three readings. Side-to-side spread over 1/8 in sends you back to leveling.
5. Compare the average to the target and note the offset. The offset is target minus measured. Move the pin up or down by holes until the true height lands within 1/8 in of the target, re-measuring each time.
6. Write the offset on the machine. A label on the dash or the deck reading "pin 4.25 cuts 4.0" turns this into a thirty-second job next time and stops every operator from relearning it. Record it on the equipment record too.

 SIDE VIEW ON A LEVEL PAD, BLADE TIP TURNED FORWARD

 REAR DRIVE TIRE     DECK SHELL           FRONT CASTER
   .----.    +---------------------+
  (  ()  )   |       spindle       |         .-.
   '----'    +----------+----------+        ( o )
             sail -> ===+=========== tip     '-'
                                     |
                              ruler  |  4.03 in
 ====================================+========= PAD
 pad checked with a 4 ft level both ways

 DECK PIN PLATE (stamped figures are nominal)
    3.50   3.75   4.00   [4.25]   4.50
                           ^ pin here = 4.03 true

Worked example: a fescue route, one mower, two corrections

A 60 in zero-turn goes onto a tall fescue route in June. The cool-season card puts the summer target at 4.0 in.

Bench, first read. Tires checked: both rear drive tires are low against the sidewall figure. The shop tech measures anyway, to see the starting point, with the pin in the 4.00 hole: left tip 3.66 in, center 3.62 in, right tip 3.70 in. Spread is 3.70 minus 3.62, 0.08 in, inside the 1/8 in leveling gate. Average is (3.66 + 3.62 + 3.70) divided by 3, which is 10.98 divided by 3, 3.66 in. Offset from target: 4.0 minus 3.66 is 0.34 in low.

Correct the cause before the setting. Both rear tires inflated to the sidewall figure. Re-measured, pin still at 4.00: 3.78, 3.75, 3.80. Average 11.33 divided by 3, 3.78 in. The tires were worth 3.78 minus 3.66, 0.12 in, and the deck still cuts 4.0 minus 3.78, 0.22 in under its stamped number. That remainder is the nominal plate, worn caster bushings and the lift linkage; none of it is fixed today.

Move the pin. This deck's holes step 1/4 in. Pin to 4.25: readings 4.03, 4.00, 4.05, average 12.08 divided by 3, 4.03 in. That is 0.03 in over target, inside the 1/8 in band. Label goes on the dash: "pin 4.25 = 4.0 true, tires set."

Why fix the tires first: had the tech moved the pin to cover the full 0.34 in, the deck would have cut right until someone aired the tires, then jumped high by 0.12 in with nobody touching the pin.

On the turf. The first property's lawn measures 5.5 in before the cut. Removing 5.5 minus 4.0, 1.5 in, is 1.5 divided by 5.5, 27 percent of the leaf, under the one-third limit. After a test strip the tech reads the cut with the height gauge at five spots: 3.9, 4.0, 3.8, 3.9, 4.0 in. Sum 19.6, mean 3.92 in, 0.08 in under target and inside the 1/8 in band. The turf reads lower than the bench because the tires sink slightly into a soft, thatchy surface and carry the deck down with them; on firm, dry turf the two agree more closely.

Verify you got it right

The bench number is the setting; the turf number is what the customer sees. Both get checked.

• Bench: every tip within 1/8 in of the target, side-to-side spread no more than 1/8 in, tires at pressure, offset label on the machine.
• Turf: five or more height-gauge readings across the first lawn of the day, mean within 1/8 in of target and no single reading more than 1/4 in off. Read on flat ground away from wheel tracks; a reading taken in a tire rut is measuring the rut.
• Leaf tips: a clean cut under a hand lens. Frayed, torn tips mean a blade problem, not a height problem.
• Second machine on the same property: cut a strip beside the first machine's strip and look across both at a low angle. A visible step means the two offsets do not match, whatever the plates say.

What changes the setting

A soft or thatchy lawn reads low on the turf by more than the band. Do not chase it by raising the pin for the whole route; set to the bench target and note the property. Persistent sinking is a thatch or soil problem for the fall program.

Reel mowers are set on a bench plate, from the bedknife edge, with a different gauge and a different idea of "height". This card's ruler method is for rotary decks.

The season moves the target, not the method. When the card says drop fescue to 3.5 in for fall, re-run steps 1 to 6 for the new pin. The offset you found in June is a good starting guess, not a substitute for measuring, because the linkage behaves differently at the other end of its travel.

A heavy accessory changes the deck load. A mulching kit, a bagger frame or a striping roller can change how the deck hangs. Measure with it fitted the way it runs on the route.

Where this goes wrong in the field

Setting the fleet by the plate. Three mowers, all on "4", on a route of fescue lawns. One cuts 3.6, one 4.0, one 4.3, and the crew lead cannot explain why Tuesday's lawns look better than Thursday's. The fix is one label per machine, not one number per route.

Measuring on the trailer. Trailer decks are pitched to drain and flex under load. A reading off the trailer can be out by more than the whole tolerance.

Leaving the offset in someone's head. The tech who measured leaves, and the next operator sets the pin to the target number. Every height correction that is not written on the machine has to be discovered again.

References

• See related: Level a Mower Deck; Mowing Patterns + Height by Species.
• See related: Cool-Season Mowing Heights Through the Year; Warm-Season Mowing Heights and the Reel Versus Rotary Question.
• See related: How to Remove, Sharpen and Balance a Mower Blade; Mower Blade Sharpening and Balance Standard.
• Manufacturer operator's manual for the mower, for tire pressures, deck pitch, height adjustment procedure and lockout steps before working under the deck.