Harrow Disc Center Hole Guide: Square, Round and Bolt Patterns Explained

The harrow disc center hole is the detail buyers most often get wrong. A replacement disc with the right diameter and thickness is useless if the hole does not fit the gang shaft or hub. This guide explains the main hole types, how to measure them, and what to send us so your replacement harrow disc fits the first time.

Why the Harrow Disc Center Hole Matters

On a harrow, discs are stacked on a gang shaft with spools between them. The center hole must match the shaft so the disc turns with the gang and does not slip or wobble. A loose fit wears the hole, the spools and the bearings; a hole that is too small simply will not fit.

Square Hole Disc Blade

The square hole disc blade is the most common type on harrow gangs. The square shaft drives all discs together. When ordering, measure across the flats of the hole (side to side), not across the corners. Square holes are used on light, medium, heavy offset and folding harrows.

Round Hole Harrow Disc and Bolt Patterns

A round hole harrow disc is usually mounted on a hub and fixed with bolts. Here two measurements matter:

• Center hole diameter.
• Disc blade bolt pattern: the number of bolt holes, their size, and the diameter of the circle through their centers (bolt circle).

Round holes with bolt patterns are common on disc ploughs, some ridgers and certain harrow designs. Our plough disc blades guide explains plough fitting in more detail.

How to Measure an Old Disc: 5 Checks

1. Diameter – measure an unworn disc from the end of the gang, not a worn one from the middle.
2. Thickness – near the center, with a calliper.
3. Hole type and size – square across flats, or round diameter.
4. Bolt pattern – number of holes, hole size and bolt circle, if any.
5. Concavity – lay a straight edge across the disc and measure the dish depth at the center.

Also note the edge (plain or notched). If you are unsure, send a photo next to a tape measure. We can match most common harrows and also produce to your drawing. Size charts are in our harrow disc blade sizes guide.

Special Holes and Made-to-Drawing Discs

Some older or regional harrows use special designs: a round hole with keyways, a larger square for heavy gangs, or extra bolt holes for a scraper or hub. If your disc does not match a standard pattern, send us a drawing or a sample disc. We can produce discs to drawing in 65Mn or boron steel and heat-treat them to the same HRC 38–44 range as our standard blades. For dealers, it is worth listing the hole size of each machine model you sell, so that every replacement harrow disc order is right the first time and customers do not wait for a second shipment.

Spools, Bearings and Gang Assembly

When you replace discs, check the spools (spacers) and bearings too. Worn spools let discs move on the shaft and damage the center hole. We supply grease or oil-bath bearing assemblies to match, so a full gang can be rebuilt in one order. See the harrow discs page for details.

Common Ordering Mistakes

• Measuring a square hole across the corners instead of the flats.
• Forgetting the bolt circle on round-hole discs.
• Ignoring concavity – a flatter or deeper disc changes how the gang works.
• Not replacing worn spools and bearings at the same time.

Reliable spare parts keep machines working through the season, a point stressed in FAO guidance on sustainable agricultural mechanization.

FAQ

Which is more common, square or round center holes?

On harrow gangs, square holes are the most common. Round holes with bolts are common on ploughs and hub-mounted discs.

Can you make a special hole to my drawing?

Yes. Send the drawing or a sample and we will produce it.

Do you sell replacement harrow discs for other brands?

Yes. Send diameter, thickness, center hole and edge type, and we will match blades for most common makes.

Can I order bearings with the discs?

Yes. Matching bearing assemblies can be shipped in the same order.

Get a Quote

Send diameter, thickness, center hole size and type, bolt pattern, edge, quantity and destination port on WhatsApp +86 133 8469 2372. Compare complete disc harrows and our export options.