Tractor linkage categories explained
Cat 1, 2, 3 and 4 — pin dimensions, lift capacities, which tractors use which category, and how to adapt implements when categories don't match. If your implement doesn't fit your tractor, the answer is almost always a category mismatch, and there's usually a straightforward fix.
Category dimensions at a glance
Cat 1
19.3 mm
Up to 45 hp
Cat 2
28.7 mm
45–120 hp
Cat 3N
37.4 mm
120–300 hp
Cat 3
37.4 mm
120–300 hp
Cat 4
51 mm
300 hp+
Store linkage category, pin sizes and key specs against every tractor and implement — free.
What are tractor linkage categories?
The three-point linkage (3PL) connects mounted implements to the rear of a tractor through two lower links and one top link. The category defines the physical dimensions of the connection — specifically the pin diameter and the lower link spread (pin-centre to pin-centre distance).
Categories are defined by ISO 730, ensuring any implement of a given category fits any tractor of that same category regardless of manufacturer. There are four categories in common UK agricultural use.
Cat 1
Up to 45 hp
Cat 2
45–120 hp
Cat 3
120–300 hp
Cat 4
300 hp+
Up to 45 hp · Up to 1,500 kg
Lower Link Pin
19.3 mm (¾")
Top Link Pin
19.3 mm (¾")
Link Spread
685 mm
Lift Capacity
Up to 1,500 kg
Typical Power
Up to 45 hp
Typical tractors using Cat 1
45–120 hp · 1,500–4,000 kg
Lower Link Pin
28.7 mm (1⅛")
Top Link Pin
25.5 mm (1")
Link Spread
825 mm
Lift Capacity
1,500–4,000 kg
Typical Power
45–120 hp
The Cat 2 top link pin (25.5 mm / 1") is smaller than the lower link pins (28.7 mm / 1⅛"). They are not interchangeable — this catches people out regularly when ordering replacement pins.
Typical tractors using Cat 2
120–300 hp · 4,000–8,000 kg
Lower Link Pin
37.4 mm (1-7/16")
Top Link Pin
37.4 mm (1-7/16")
Link Spread
1,010 mm (Cat 3) / 825 mm (Cat 3N)
Lift Capacity
4,000–8,000 kg
Typical Power
120–300 hp
Cat 3N (Narrow) uses Cat 3 pin diameter (37.4 mm) but with an 825 mm spread — matching Cat 2 width. This allows large tractors to use Cat 2 width implements while retaining Cat 3 pin strength. Some tractors have switchable Cat 2/3N lower link ends.
Typical tractors using Cat 3
300 hp+ · 8,000 kg+
Lower Link Pin
51 mm (2")
Top Link Pin
51 mm (2")
Link Spread
1,010 mm
Lift Capacity
8,000 kg+
Typical Power
300 hp+
Cat 4 implements are uncommon on most mixed or livestock UK farms. Parts are sourced through specialist agricultural dealers rather than general trade suppliers.
Typical tractors using Cat 4
Compatibility — what to do when categories don't match
Category mismatches are common and usually straightforward to resolve. The most frequent situation in UK farming is a Cat 2 implement on a Cat 1 tractor.
Cat 2 implement on Cat 1 tractor
Reducing bushesSteel sleeves that fit into the Cat 2 holes on the implement and reduce them to accept Cat 1 pins. A set of three (two lower links, one top link) is inexpensive. Always check that the implement weight is within the Cat 1 tractor's rated rear linkage lift capacity — Cat 2 implements are built for significantly heavier machines.
Cat 1 implement on Cat 2 tractor
Adapter pinsSlimmer pins with a wider head that fit through the Cat 2 lower link end and into the smaller Cat 1 implement hole. Sometimes called "Cat 1 cross pins" or "Cat 1 adapter pins for Cat 2 tractor". Common where older Cat 1 implements are used on a modern Cat 2 machine.
Cat 2 implement on Cat 3 tractor
Cat 3N ends or reducing bushesIf the tractor has Cat 3N (Narrow) ends, the lower link spread already matches Cat 2 width (825 mm) and adapter sleeves handle the pin size. Cat 3 to Cat 2 reducing bushes work on the same principle. Some tractors also have removable lower link ends that swap between Cat 2 and Cat 3 configuration — check the operator manual.
Cat 3 implement on Cat 2 tractor
Not practicalCat 3 implements are built for tractors with significantly more power, ballast and lift capacity than Cat 2 machines. Even if pin adapters could be fabricated, the implement size and weight would likely exceed the tractor's rated capacity and make it unsafe to operate.
Quick hitches — do they change the category?
Quick hitch systems — Walterscheid, Dromone, JCB Q-fit and others — attach to the three-point linkage and allow implements to be connected without the operator leaving the cab. They do not change the underlying linkage category. A Cat 2 quick hitch fits a Cat 2 tractor and connects Cat 2 implements.
Quick hitches add height to the linkage connection point. Some mounted implements with low headstock clearance may not be compatible even if the pin size is correct. Always check headstock dimensions and clearance before purchasing a quick hitch.
How to identify your tractor's linkage category
Measure the lower link pin diameter
Remove one lower link pin and measure its diameter with callipers. ~19 mm = Cat 1 · ~28 mm = Cat 2 · ~37 mm = Cat 3 · ~51 mm = Cat 4
Measure the lower link spread
Measure pin-centre to pin-centre across the two lower links in their standard working position. ~685 mm = Cat 1 · ~825 mm = Cat 2 or Cat 3N · ~1,010 mm = Cat 3 or Cat 4
Check the operator manual
The linkage category is always stated in the specifications section. This is the most reliable method if the manual is to hand.
Ask your dealer or agricultural factor
Any tractor dealer will know the linkage category from the model and year of manufacture. Always verify before ordering parts.
Top links and check chains
Top link pin sizes by category
| Category | Top Link Pin Diameter | Common Thread Size |
|---|---|---|
| Cat 1 | 19.3 mm (¾") | ¾" UNF or M20 |
| Cat 2 | 25.5 mm (1") | 1" UNF or M27 |
| Cat 3 | 37.4 mm (1-7/16") | 1-7/16" UNF or M36 |
Cat 2 reminder: The top link pin (25.5 mm / 1") differs from the lower link pins (28.7 mm / 1⅛"). They are not interchangeable. Ordering the wrong size is the most common mistake when replacing Cat 2 linkage pins.
Sway chains / check chains limit lateral movement of the lower links when driving with a mounted implement. Hook and pin sizes are category-specific, though chain length and grade vary more by tractor model than by category.