How to Identify PTO Shaft Size

Three measurements identify PTO shaft size: spline count, stub diameter, and required torque class.

Identifying PTO shaft size requires three separate measurements: the spline profile at the tractor end (which determines compatibility), the torque class required by the implement (which determines the series), and the correct shaft length for the tractor-implement combination. Getting all three right ensures the replacement shaft fits, transmits the correct power, and lasts through the season.

Step 1: Identify the Spline Profile

Count the spline ridges on the tractor PTO stub — the short rotating shaft protruding from the rear of the tractor gearbox. Six splines: you have a 1-3/8 inch Z6 stub (540 rpm standard). Twenty splines: you have a 1-3/4 inch Z20 stub (1,000 rpm standard). Twenty-one splines: you have a 1-3/8 inch Z21 (less common, some European tractors).

Counting splines takes 30 seconds and completely determines which yoke profile you need.

Step 2: Measure the Stub Diameter

If the stub condition makes spline counting difficult, measure the major diameter (the outer tip of the splines) with a digital caliper. A 1-3/8 inch (34.9 mm) measurement indicates Z6 or Z21. A 1-3/4 inch (44.5 mm) measurement indicates Z20. If the measurement reads 34.9 mm, count the splines to distinguish Z6 from Z21 — both have the same outer diameter but different spline counts and are not interchangeable.

Step 3: Determine the Required Torque Class

The torque class is determined by the implement power demand. Calculate: Torque (Nm) = Implement power (kW) × 9,549 ÷ PTO speed (rpm). Add 20% safety margin. Select the lowest T Series or L Series model where the Nm@PTO speed rating meets or exceeds your adjusted figure. See the T Series torque table or L Series torque table for complete data.

Use Nm@540 for standard PTO. Use Nm@1,000 for high-speed PTO. Always add 20% to the implement demand before selecting.

Step 4: Measure the Correct Shaft Length

1. 1

   Connect the implement to the tractor at normal working height.

2. 2

   Measure the coupling centre distance: tractor PTO stub face to implement input shaft face.

3. 3

   This is your reference dimension. Stock shaft compressed length should be 50 mm shorter.

4. 4

   Raise implement to maximum height. Verify at least 50 mm tube overlap remains.

5. 5

   Lower to minimum height. Verify tubes do not bottom out and 80 mm engagement remains.

6. 6

   If stock lengths do not fit, order a custom OEM length (MOQ 50 pcs, 25–30 days).

Reading the Existing Shaft Markings

Many shafts carry a series marking or part number stamped or labelled on the tube yoke or the shield. Look for labels such as T5, L5, Series 5, or a cross-kit dimension (e.g. 30.2×80). The series number directly corresponds to the T Series or L Series designation. If the shaft has a part number from another manufacturer, cross-reference using the cross-kit pin diameter and journal length — these are universally standardised dimensions.