Disc harrow blades: how to order the right one by size and part number
05.10.2026
Disc harrow blades
A disc harrow blade is specified by five dimensions at once, not by diameter. Diameter is the number everyone quotes first, but wrong deliveries almost always come from the other four: thickness, bolt count, bolt hole diameter and pitch circle diameter. This guide shows how to take those measurements off a worn blade and how to confirm fitment before paying for a set that will not mount.
In short: to order the right disc harrow blade you need diameter, thickness, number of bolt holes, bolt hole diameter and the pitch circle diameter (PCD), plus the manufacturer's part number where you have it. Worked example: the Lemken 620 x 6 blade, part number 349 0466, fits Rubin 9 and Gigant 10, while the AMAZONE 510 x 5 blade, part number 78201966, fits Catros, Catros+, Certos and Ceus.
Contents
- The five dimensions that must match
- How to measure a worn blade
- When a blade has stopped doing its job
- Notched or plain
- Heat treatment and why it matters
- The part number is the fastest route, not the only one
- Fitment by machine: two worked examples
- What to replace along with the blades
- Why you do not replace a single blade
- How to order so nothing surprises you on delivery
- Resetting the harrow after a blade change
- Storage between seasons
- Common mistakes when replacing blades
- Frequently asked questions
- Enquiries and availability
The five dimensions that must match
Five dimensions define a blade, and if one of them is wrong the blade either will not mount or will mount badly. Diameter alone is never enough, because two blades of the same diameter can differ in thickness and in bolt pattern.
| Dimension | What it is | What goes wrong if it is off |
|---|---|---|
| Diameter | outside size in mm, for example 510 or 620 | working depth and cutting geometry both change |
| Thickness | plate thickness in mm, usually 5 or 6 | affects how the blade seats on the flange, the clamping, the geometry and the strength |
| Bolt count | how many bolts hold the blade, for example 4 or 6 | the blade cannot be fixed to the stalk |
| Bolt hole diameter | for example 11.5 or 12.5 mm | the bolts will not pass, or they leave play |
| Pitch circle diameter (PCD) | diameter of the circle the hole centres sit on, for example 120 or 130 mm | the holes do not line up with the flange and the blade will not mount |
Two blades can share the same diameter and the same hole count and still be incompatible if the pitch circle differs. That makes it a dimension in its own right, not something that follows from the others.
Be careful with the wording in specification sheets. Some sellers list this figure as the centre bore, which is a different thing: the centre bore is the opening in the middle of the blade. Both blades in the examples below have a flat centre, so they have no centre bore at all, and the figures of 120 and 130 mm are the pitch circle. Where a sheet is ambiguous, ask the seller which dimension the number describes.
How to measure a worn blade
Measure the worn blade before taking it off the machine, because once the set is off the bench it is easy to lose track of which came from where. A vernier caliper and a tape measure are enough, and the job takes a few minutes per blade.
Diameter on a worn blade
Measure across the centre in two directions. If the two readings differ, the blade has worn unevenly and must be compared against the nominal size in the machine manual rather than against its current size. The nominal size is what you order.
Thickness
Take the thickness with a caliper near the middle of the blade, not at the rim. The rim has thinned in work and will give a misleading figure. A millimetre is not a detail here: thickness governs how the blade seats on the flange, how much thread the bolts have to work with and how well the blade takes an impact, so order exactly the figure the catalogue gives.
Hole count and hole diameter
Count the mounting holes and measure the diameter of one with a caliper. Take the diameter at the narrowest part of the hole, because the edge may have been chewed out by a loose bolt.
Pitch circle diameter
With an even number of holes the pitch circle diameter is measured directly: the distance between the centres of two opposing holes is the pitch circle itself. Both blades in the examples below have 4 and 6 holes, so direct measurement works on them.
With an odd number of holes there is no opposing hole and direct measurement does not give the right answer. Take the figure from the parts catalogue, use a pitch circle gauge, or calculate it from the distance between two adjacent holes: PCD = s / sin(180° / n), where s is the centre-to-centre distance between adjacent holes and n is their number. The angle is in degrees, so the calculator must be in degree mode; in radians the result is wrong. The tolerance is tight and an approximate figure will not do.
When a blade has stopped doing its job
A blade is replaced once it has stopped cutting and started pushing. Practice works to two indicators: a marked loss of diameter, usually in the range of 15 to 20 percent of nominal, and an edge that has lost its profile. The exact threshold for a given machine is in the manufacturer's manual, so check it before scrapping a set.
Loss of diameter
As the blade wears it runs shallower at the same machine setting. If you keep adding depth to get the same result, you are probably working with worn blades rather than a wrong setting.
Loss of edge profile
A new blade has a sharp edge that severs residue. A rounded edge presses and drags it instead, which shows up as wrapping on the stalks and as patchy coverage across the stubble.
Distortion and cracks
A blade that has hit a stone may be out of true without that being visible while mounted. Check it on a flat surface. A crack running from a bolt hole to the rim means immediate replacement, because that is where the blade comes apart in work.
Notched or plain
A notched blade suits heavy residue and hard surfaces; a plain blade suits cleaner and lighter soil. A coarsely notched blade bites more aggressively and severs stalks better, but leaves a rougher surface than a plain one.
When notched is the right call
Working after maize or sunflower, where residue is thick and plentiful, a notched blade cuts rather than drags. The same applies on dry, consolidated ground in late summer, where a plain blade simply skates.
When plain does the better job
For seedbed preparation on worked ground, a plain blade leaves a more even surface and seals moisture better. If the aim is a fine tilth rather than cutting residue, aggression does not help.
Mixed sets
Some machines leave the factory with notched blades on the front row and plain on the rear, combining cutting with levelling. If you replace only part of a set, do not mix the two types within one row, or penetration will vary across the width.
Heat treatment and why it matters
Heat treatment decides how long a blade holds its edge and how well it survives an impact. A properly hardened blade wears more slowly at the rim, which directly extends the interval to replacement.
Wear versus brittleness
Hardening is a balance. Too hard and the blade wears slowly but cracks on a stone; too soft and it takes impacts but abrades away. Serious manufacturers harden to the middle of that range, which is why on stony ground the origin of the part matters as much as the hardness figure.
Origin, and why it is stated
Both blades in stock are listed as made in Germany. Origin is not a guarantee in itself, but on a wearing part whose chemistry you cannot verify, a documented manufacturer is the most accessible proxy for quality.
The part number is the fastest route, not the only one
A part number identifies the component unambiguously and removes every question about dimensions. With the number from the machine manual or from an old blade, ordering takes a minute.
Where to find the number
It is in the machine's parts catalogue, and often stamped or labelled on the original component. On blades the stamped number usually disappears with wear, which makes the catalogue the more reliable source.
When there is no number
If the number is illegible, work from the five dimensions plus the make and model of the machine. That is entirely sufficient when the measurements are taken properly. The logic of ordering by part number, and what to do when you do not have one, is covered separately in the guide to ordering spare parts by part number.
Original and alternative
Blades exist both as original parts and from manufacturers of alternatives. What matters with an alternative is whether the five dimensions and the heat treatment match, not whose name is on the box. Ask for the dimensions in writing before you order.
Fitment by machine: two worked examples
The two blades in stock show clearly why diameter is not enough: across a 110 mm difference in diameter, thickness, bolt count and pitch circle all differ as well.
| Dimension | Lemken 620 x 6 blade | AMAZONE 510 x 5 blade |
|---|---|---|
| Part number | 349 0466 | 78201966 |
| Type | coarsely notched | coarsely notched |
| Diameter | 620 mm | 510 mm |
| Thickness | 6 mm | 5 mm |
| Bolt holes | 6 | 4 |
| Bolt hole diameter | 12.5 mm | 11.5 mm |
| Pitch circle diameter (PCD) | 130 mm | 120 mm |
| Centre | flat, no centre bore | flat, no centre bore |
| Weight | 12.5 kg | 7.5 kg |
| Treatment | heat hardened | heat hardened |
| Origin | Germany | Germany |
| Fits | Lemken Rubin 9, Gigant 10 | AMAZONE Catros, Catros+, Certos, Ceus |
Full details and current availability are on the product pages for the notched blade for Lemken 620 x 6 and the notched blade for AMAZONE 510 x 5, where the part numbers and dimensions are listed in full.
Why weight is information too
12.5 kg versus 7.5 kg per blade looks like a detail until you count the set. On a harrow with 24 discs that is 120 kg, which affects both haulage and what the tractor is asked to pull. When ordering a full set, ask for the total weight so the delivery can be organised.
One number, several machines
One blade often serves several models from the same manufacturer, as both examples show. The reverse is also true: one machine range may have been built with different blades depending on the year. That is why make and model are always paired with the year of manufacture.
What to replace along with the blades
The blade is rarely the only worn item in the assembly. Fit a new blade onto a worn bearing or a stretched bolt and the new blade will wear unevenly within one season.
The bearing unit
Turn each bearing unit by hand before removal and feel for roughness and sideways play. On most current harrows these units are sealed and are replaced rather than serviced; some machines do have grease points, so check the manual for yours. Changing them at the same time as the blades saves a second period off work.
Bolts and washers
Disc bolts come in a specific grade and are not substituted with whatever is on the shelf. Stretched or deformed bolts are always replaced, because with them the correct torque cannot be reached.
The stalk and its rubber suspension
On machines with sprung stalks, check the condition of the rubber elements. Tired rubber changes the blade's angle of attack, which shows up as uneven depth across the machine and is routinely mistaken for a blade problem.
Why you do not replace a single blade
A new blade among worn ones runs at a different depth from its neighbours and takes a disproportionate share of the load. The result is uneven work and faster wear on precisely the new blade.
Replace by row
If the budget will not carry a full set, it is sounder to replace a complete row rather than scattered blades. Depth then stays consistent at least within that row and the work stays predictable.
When partial replacement is justified
Partial replacement is justified only for mechanical damage: a cracked or bent blade is replaced at once regardless of the state of the rest. Note, though, that this blade will then be at a different wear stage from the next full set.
How to order so nothing surprises you on delivery
Ordering is quick and accurate when the data is gathered in one go. Work through this list.
- Write down the make, model and year of manufacture of the harrow.
- Find the part number in the machine's parts catalogue if it is available.
- Take the five dimensions off the existing blade as described above, and write them down rather than trusting memory.
- Decide whether you need notched or plain blades, and whether the machine runs different rows.
- Count the total number of discs on the machine and how many this order covers.
- Check the bearing units and the bolts so they can be added to the same order.
- Confirm the total weight of the order for haulage.
- Get the dimensions confirmed in writing before paying.
The rest of the wearing parts for tillage equipment are in the spare parts catalogue. Farms running a cultivator as well tend to order together, since cultivator spare parts follow the same principle of fitment by machine and number.
Resetting the harrow after a blade change
With a new set fitted, the machine runs deeper at an unchanged setting, because the new blades are at nominal diameter. Reset it on the same field and at the same speed, so the result is comparable with what went before.
The first few hundred metres
Travel 200 to 300 metres, stop and dig a cross-section. Measure actual depth at three points across the width, not only in the middle. A marked difference between the edge and the centre is a reason to check the frame levelling and the pressure in the support wheels before changing the depth setting.
Angle of attack
On machines with an adjustable angle, start from the middle setting and increase it only if residue is not being buried. A more aggressive angle mixes better but raises fuel use and accelerates wear on the new blades.
Forward speed
A disc harrow deals with residue through speed as much as through pressure. Too slow and stalks pass uncut even with new blades; too fast and the machine throws soil and leaves a rough finish. Set the speed before locking in the depth.
Storage between seasons
A new set of blades loses value fastest not in work but standing outside. Rust on the edge is not cosmetic: it eats precisely the thinnest part, the part that cuts.
End of season
Wash the soil off, dry the machine and coat the blades with a protective oil or grease along the edge. Residual soil holds moisture against the steel all winter.
Where to leave the machine
Under cover where possible, with the discs raised so they are not in contact with damp ground. If the machine must stay outside, put timber blocks under the frame and cover the bearing assemblies.
Common mistakes when replacing blades
Most problems after a blade change come not from the blade but from how it was ordered or fitted.
Ordering on diameter alone
The most common mistake. Diameter is the first thing anyone says on the phone and the last thing that determines fitment on its own.
Treating a worn measurement as nominal
Measure a worn blade at 580 mm, order 580, and you receive a blade that never existed in that size. What you order is the nominal size from the manual.
Tightening without torque control
An under-tightened bolt works loose and wrecks the hole in the blade; an over-tightened one stretches. Either way the blade is replaced long before the end of its useful life.
Changing blades without rechecking depth
After a full set is fitted the machine runs deeper at the same setting, because the blades are at nominal diameter. How depth and soil type govern the choice of harrow in the first place is covered in the guide to disc harrow types and correct tillage.
Frequently asked questions
Which dimensions do I need to order a disc harrow blade?
Five of them, and all five together. Diameter alone leaves the order unfinished: thickness decides whether the blade seats properly on the flange, while the hole count, the hole diameter and the pitch circle decide whether it can be mounted at all. Add the make, model and year of the machine. A part number replaces the measurements, though the dimensions are still worth checking against the delivery.
When should disc harrow blades be replaced?
Two indicators from practice and one absolute rule. The indicators: a marked loss of diameter, usually around a fifth of nominal, and an edge that presses residue instead of severing it. The absolute rule: a cracked or bent blade leaves the machine at once. The exact threshold for your harrow is confirmed against the manufacturer's manual, not from these figures alone.
Can I replace just a few blades?
You can, and it is a false economy in most cases. The new blade stands proud of its worn neighbours, so it carries more than its share and wears out first, which means paying twice. Replacing a complete row keeps depth consistent at least across that row. Mechanical damage is the exception: a broken blade comes out immediately.
What is the difference between notched and plain blades?
A notched blade severs residue and bites more aggressively into hard ground, which is why it is preferred after maize and sunflower. A plain blade leaves a more even surface and suits seedbed preparation on already worked soil.
Will a Lemken blade fit an Amazone?
No. The two examples in stock differ on all five dimensions: 620 versus 510 mm diameter, 6 versus 5 mm thickness, 6 versus 4 bolt holes, 12.5 versus 11.5 mm hole diameter, and 130 versus 120 mm pitch circle. Fitment is confirmed by machine and by number.
Should I replace the bearing units along with the blades?
Check first, then decide, because the answer is per bearing rather than per machine. Anything that feels rough or has sideways movement goes on the same order as the blades; the rest can wait. On most current harrows the units are sealed and replaced rather than serviced, though some have grease points, so check the manual. The reason to do both together is downtime: pulling the same assembly apart twice in one season is the expensive part.
Enquiries and availability
Both blades are in stock and can be ordered by part number or by the dimensions above. For a full set, send the make, model and year of the harrow together with the disc count to get an accurate quotation and lead time. The full range of disc harrows and their wearing parts is on the site, and non-standard sizes are checked on request.