Measuring your tractor's three-point linkage category takes ten minutes with a tape measure and saves you a return shipment on the wrong attachment. This guide walks through checking pin diameter, arm spacing and mast height so you land on Cat 1, Cat 2 or Cat 3 before you order.
- Cat 1 lower link pins run 22mm, Cat 2 sit at 28mm, Cat 3 jumps to 37mm — measure before you buy.
- Mismatched cat pin size is the number one return reason on tractor implements bought in 2026.
- A Cat 1-2 conversion bush kit lets a Cat 1 tractor run some Cat 2 implements without new pins.
- Top link pin diameter and lower link pin diameter are different measurements — check both.
- Compact tractors under 45hp are almost always Cat 1; anything over 80hp usually needs Cat 3.
Why this matters
A flail mower rated for Cat 2 will not drop onto a Cat 1 tractor without an adaptor, and forcing it damages both the pin and the lift arm bushing. Implements Direct sees this most often with lifestyle block owners upgrading from a sub-compact tractor to something in the 45-80hp range, where the category jumps without warning.
Getting the cat 3 pin size wrong on a heavier implement is worse than a Cat 1 mismatch — Cat 3 pins are 37mm, nearly double the diameter of a Cat 1 pin at 22mm, so there is no bodging it with washers or shims. Measure first, order second.
What you'll need
- Tape measure or vernier calipers (calipers give a more accurate pin diameter reading)
- Torch or phone light for underneath the tractor
- A notepad or phone to record three measurements: lower pin diameter, top link pin diameter, lower link spacing
- Your tractor's model and horsepower rating, if you have it
- Access to the implement's linkage brackets, if you already have one to fit
The steps
1. Find your tractor's horsepower rating first
Horsepower gives you a starting guess before you touch a tape measure. Tractors under 45hp are almost always Cat 1, machines from 45-100hp typically run Cat 2, and anything above 80-100hp moves into Cat 3. This isn't gospel — some 50hp tractors ship Cat 1 from the factory — but it narrows the search before you crawl under the linkage. Expected outcome: a rough category guess to confirm with the next four steps. Common mistake: assuming brand determines category; horsepower and factory spec decide it, not the badge on the bonnet.
2. Measure the lower link pin diameter
The lower link pins are the two pins that sit at the bottom of the three-point linkage, connecting the lift arms to the implement. Use calipers across the pin's widest point, not the hole it sits in — holes wear oversize over years of use. Cat 1 reads 22mm, Cat 2 reads 28mm, Cat 3 reads 37mm. Expected outcome: one clean diameter reading in millimetres. Common mistake: measuring the worn bore instead of the pin, which reads 2-4mm larger than true category size and leads to ordering the wrong pin.
3. Measure the top link pin diameter
The top link connects at the top of the linkage triangle and uses a smaller pin than the lower links. Cat 1 top link pins run 19mm, Cat 2 sit around 25mm, Cat 3 goes to 32mm. This measurement matters separately because some tractors run a Cat 2 lower linkage with a Cat 1 top link bracket — a genuine mismatch that only shows up if you check both ends. Expected outcome: a second diameter figure to cross-check against the lower pin reading. Common mistake: assuming top and lower pins match category automatically.
4. Measure lower link arm spacing centre to centre
Measure from the centre of one lower link pin hole to the centre of the other, with the arms in their resting position. This spacing widens as category increases and confirms your reading from steps 2 and 3 if the numbers don't line up cleanly. A narrow Cat 3N linkage uses the same 37mm pins as standard Cat 3 but a tighter arm spacing, which matters if you're fitting a wide implement to a narrower tractor chassis. Expected outcome: a spacing figure in millimetres that either confirms or contradicts your pin diameter reading. Common mistake: measuring with the arms hanging loose rather than at rest height, which skews the figure by 20-30mm.
5. Check the mast height for the top link mount
Mast height is the distance from the ground to the top link mounting point on the tractor, measured with the linkage at working height. This affects how an implement sits level when hitched — get it wrong and a flail mower or box blade tilts forward or back at the working depth you actually need. Expected outcome: a height figure that tells you whether the implement will sit level once mounted. Common mistake: skipping this step entirely because pin size feels like the only thing that matters — a correctly sized pin on a badly matched mast still leaves the implement fighting gravity.
6. Cross-check against the implement's spec sheet
Every tractor implement sold by Implements Direct lists its category on the product page. Compare your three measurements — lower pin, top pin, arm spacing — against the listed category rather than relying on a single number. If your tractor reads Cat 1 on pins but Cat 2 on spacing, you're looking at a non-standard or older linkage, and a Cat 1 hydraulic top link with adjustable length can sometimes bridge minor spacing differences without a full category change. Expected outcome: confirmation your tractor and intended attachment share a category, or clear evidence they don't. Common mistake: buying on horsepower guess alone and skipping the spec sheet comparison.
7. Decide whether a conversion or adaptor solves a mismatch
If your tractor is genuinely Cat 1 but the implement you want is built Cat 2, a conversion bush kit adapts the lower pins and top link pin down in diameter without replacing the whole linkage. For loader-style implements — pallet forks, bale grabs, brush grapples — a 3-point linkage to Euro hitch adaptor sidesteps the category question entirely by moving to a quick-attach system. Expected outcome: a clear next purchase — either a bush kit, an adaptor, or confirmation you need the correct category implement outright. Common mistake: buying an implement first and hoping an adaptor will exist later; check compatibility before the implement ships.
8. Test-fit before you torque anything down
Once the implement arrives, hitch it without fully tightening any pins first. Pins should slide through with light hand pressure, not require a hammer. A pin that needs force is either the wrong category or the bore has worn oversize on an older tractor. Expected outcome: a smooth hitch-up with pins seated fully through both brackets. Common mistake: forcing an undersized pin through an oversized bore and calling it fitted — it will work loose within the first few hours of PTO use.
Troubleshooting
- Pin won't slide through the bracket bore — you've likely measured the worn bore rather than the pin itself. Re-measure the actual pin with calipers and compare against the 22mm/28mm/37mm reference.
- Implement sits crooked once hitched — mast height is off for your tractor, not the category. Adjust top link length before assuming a category mismatch.
- Lower pins fit but top link pin doesn't — you have a mixed linkage, common on older or modified tractors. Check both pins separately every time, never assume they match.
- Bore has worn oval over time — a genuine wear issue, not a category problem. New pins will still slide loose in an oval bore; the bracket itself needs bushing or replacement.
- Implement was built for Cat 3N but tractor is standard Cat 3 — pins fit but arm spacing binds. Check spacing measurement from step 4 before assuming pin diameter solved the problem.
- Quick hitch doesn't lock — often a spacing tolerance issue rather than category. Confirm the quick hitch is rated for your measured category, not just your horsepower guess.
Tools and resources
- Vernier calipers for accurate pin diameter readings, more reliable than a tape measure on round stock
- Implements Direct's 3-point linkage to Euro hitch adaptor for loader-style attachments that sidestep category matching
- A written record of your three measurements — lower pin, top pin, arm spacing — kept with your tractor's paperwork for future implement purchases
- The category spec listed on every implement's product page before you order
What to do next
Once you've confirmed your category, read the guide on fitting a Category 1 quick hitch to your tractor if you're planning to run multiple implements through 2026 and want faster changeovers than pin-and-clip every time.
FAQ
What is the cat 3 pin size?
Cat 3 lower link pins measure 37mm in diameter, with the top link pin at 32mm. This is roughly double the diameter of a Cat 1 lower pin at 22mm, so Cat 3 implements do not fit Cat 1 or Cat 2 tractors without a full linkage change.
How do I know if my tractor is Cat 1 or Cat 2?
Measure the lower link pin diameter with calipers: 22mm is Cat 1, 28mm is Cat 2. Tractors under 45hp are almost always Cat 1, while 45-100hp machines usually run Cat 2, but always confirm with a physical measurement rather than horsepower alone.
Can a Cat 1 tractor run a Cat 2 implement?
Yes, with a Cat 1-2 conversion bush kit that adapts the pin diameter down. This works for lighter Cat 2 implements but isn't recommended for heavy attachments like box blades or ploughs where the linkage needs to handle full Cat 2 loads.
Is Cat 3N the same as Cat 3?
Cat 3N uses the same 37mm pins as standard Cat 3 but with narrower lower link arm spacing. Pins from a Cat 3 implement will physically fit a Cat 3N tractor, but the arm spacing may bind or require adjustment.
What size are top link pins compared to lower link pins?
Top link pins run smaller than lower link pins in every category: 19mm versus 22mm for Cat 1, 25mm versus 28mm for Cat 2, and 32mm versus 37mm for Cat 3. Always measure both separately since some tractors run mixed linkages.
How much does a category conversion kit cost in 2026?
A Cat 1-2 conversion bush kit runs around NZ$10 in 2026, making it one of the cheapest fixes for a minor category mismatch. Full linkage replacement costs considerably more and is only necessary for major category jumps like Cat 1 to Cat 3.
Do all tractors of the same horsepower use the same category?
No. Horsepower is a rough guide, not a guarantee — some 50hp tractors ship Cat 1 from the factory while others run Cat 2. Always measure the actual pin diameter rather than relying on horsepower alone.
What happens if I force the wrong size pin into a linkage?
Forcing an undersized pin into an oversized bore causes the pin to work loose within hours of PTO use, and forcing an oversized pin risks cracking the lift arm bracket. Measure first and only fit a pin that slides through with hand pressure.
One last thing
The most overlooked measurement isn't pin diameter at all — it's arm spacing on a Cat 3 versus Cat 3N tractor, because the pins will slide in either way and the mismatch only shows up once the implement binds mid-operation. Measure spacing every time, even when the pin diameter checks out clean.