How To Measure A Plow Bottom For Size?

Measuring a plow bottom for size comes down to two numbers: the width of cut and the size of the share and moldboard that fit the frame. The width of cut is measured from the outer edge of the share point to the outer edge of the moldboard, taken perpendicular to the direction of travel. That single measurement, usually stated in inches, tells you what the bottom is designed to do and what size replacement parts it needs.

This is a farm equipment question, not a medical one, and the honest answer is that most of the confusion comes from people measuring the wrong thing. A plow bottom is not sized by the length of the beam or the height of the frame. It is sized by how wide a furrow it cuts and by the part numbers stamped on the wear components. Get those two things right and everything else falls into place.

How To Measure A Plow Bottom For Size

The width of cut is the defining measurement. On a mounted or semi-mounted moldboard plow, each bottom is built to turn a furrow of a set width, and the frame spacing between bottoms is set to match that width so the plow runs true in the ground.

To measure it, set the plow on level ground or a concrete pad. Find the share point, which is the leading tip that enters the soil. Find the rear outer edge of the moldboard, which is the curved surface that flips the soil. Measure straight across from the share point to the outer edge of the moldboard, at a right angle to the line the plow travels. That distance is the width of cut.

Common widths of cut on older and current moldboard plows include 12, 14, 16, 18, and 20 inches. The 16-inch bottom is probably the most common size found on general-purpose plows in North America. A plow with three 16-inch bottoms cuts a total swath of 48 inches, which is why you will sometimes see a plow described as a “3-16” in equipment listings.

Two practical notes. First, measure a bottom that still has its original share and moldboard. A worn share can be several inches shorter than a new one, and that will throw off your reading. Second, if the plow has been rebuilt with mismatched parts, measure the frame spacing between bottoms instead. Center-to-center distance between adjacent bottoms equals the width of cut when the plow is set up correctly.

Why the Width of Cut Matters More Than Any Other Number

Width of cut determines how the plow behaves in the field. It sets how much soil each bottom turns, how much draft the tractor has to pull, and whether the plow matches the tire spacing of the tractor.

There is a relationship between plowing depth and width of cut that experienced operators use as a rule of thumb. A bottom is generally worked at a depth of roughly one-third to one-half its width of cut. A 16-inch bottom is typically run 6 to 8 inches deep. Go much deeper and the moldboard cannot lift and turn the soil cleanly. Go much shallower and you get poor burial of residue.

This is a rule of thumb from field practice, not a published engineering standard, so treat it as a starting point rather than a specification. Soil type, moisture, and residue load all shift the sweet spot.

The width of cut also has to match your tractor. A plow that is too wide for the available horsepower will spin the wheels or stall the engine. A plow that is too narrow wastes fuel and time. Matching draft to horsepower is a separate calculation from measuring the bottom, but the width of cut is the number you plug into it.

How to Identify a Plow Bottom by Its Share and Moldboard

If you cannot measure the width of cut cleanly, the wear parts will tell you. Shares, shins, and moldboards are made in sizes that correspond to the width of cut, and most carry a casting number or part number.

Look for stamped or cast numbers on the back of the share or on the frog, which is the heavy bracket the share and moldboard bolt to. Manufacturers have used different numbering systems over the decades, and a number that means one thing on a John Deere plow means something else on an Oliver or a Massey Ferguson. The reliable path is to match the number to a parts book for that specific plow model.

Physical clues help too. A 12-inch bottom has a noticeably tighter curve and a shorter moldboard than an 18-inch bottom. Lay a straightedge across the face of the moldboard and compare the arc to a known bottom if you have one. It is not a precise method, but it separates the small sizes from the large ones quickly.

Be careful here. A share that has been worn down or replaced with a generic aftermarket part may not match the original. Do not assume the part number on the plow today reflects what left the factory.

Measuring Frame Spacing and Beam Dimensions

Frame spacing is the backup measurement, and on a multi-bottom plow it is often easier to take than the width of cut. Measure from the centerline of one bottom to the centerline of the next. On a correctly set up plow, that distance equals the width of cut.

If the spacing measures 14 inches, you have 14-inch bottoms. If it measures 16, you have 16-inch bottoms. This works because bottoms are mounted at intervals equal to their cut width so each one runs in the furrow left by the one ahead of it.

What frame spacing will not tell you is the size of an individual bottom on a plow that has been cobbled together from mixed parts. It also will not help on a single-bottom plow. In those cases go back to the direct measurement from share point to moldboard edge.

Beam length, beam height, and the number of bottoms are not size measurements. They describe the plow, not the bottom. A common mistake is to assume a bigger frame means bigger bottoms. It does not. A five-bottom frame can carry 14-inch bottoms just as easily as 18-inch ones.

Common Mistakes When Measuring a Plow Bottom

Most errors come from measuring along the wrong axis or from measuring worn parts.

• Measuring the length of the moldboard instead of the width of cut. The moldboard is longer than the bottom is wide.
• Measuring at an angle instead of perpendicular to the direction of travel. This inflates the number.
• Measuring a worn share. A share worn back by an inch or more will read short.
• Measuring from the frame to the moldboard edge. That is a clearance measurement, not a size.
• Assuming the number stamped on the beam is the width of cut. It is usually a model or serial number.

The fix for all of these is the same. Put the plow on flat ground, use a tape or a folding rule, and measure share point to moldboard outer edge at a right angle to travel. Do it twice.

Does Bottom Size Affect How the Plow Performs?

Yes, and the effect is well understood in agricultural engineering. Wider bottoms turn more soil per pass and require more draft. Narrower bottoms turn less soil and pull easier, which is why small tractors and rocky ground often call for narrower bottoms.

Width of cut also affects residue burial and seedbed condition. Wider bottoms tend to bury crop residue more completely because they invert a larger slice of soil. That can be an advantage in heavy residue and a disadvantage where you want less soil disturbance.

What the evidence does not support is any claim that one width of cut is universally better. The right size depends on your tractor, your soil, your residue, and your depth target. Anyone telling you a single size is correct for all conditions is selling something.

When to Replace Rather Than Measure

If a share is worn to the point that it no longer scours clean or the point has lost its shape, measuring it accurately is pointless. Replace it first, then measure the new part or measure the frame spacing.

Wear limits vary by soil type and by manufacturer. Abrasive sandy soils wear shares far faster than heavy clay. There is no single hour figure that applies across conditions, and any specific number would be misleading. Inspect the point and the cutting edge before each season and judge by shape, not by a stopwatch.