What is the length of a link?
The distance between the pin centres, measured with the link at the middle of its adjustment: from the centre of one rod end bore to the centre of the other, with the assembly set to its nominal, mid-adjustment position. This C–C length is different from the overall length, the length of the body, or the length of the rod. Say which one you mean on a drawing or an order.
How far does the link move with each turn?
By the thread pitch, or twice the pitch with opposing threads.
One full turn of a thread moves it one lead: 1 ÷ TPI (threads per inch) for an inch thread, and the pitch for a single-start metric thread. In a turnbuckle the body is a double nut with opposing threads, so the two end pieces move towards or away from each other without turning, and each of them moves one lead. The take-up per revolution is twice that of a single thread.
| Thread | Pitch or TPI | One end only | Right-hand and left-hand ends |
|---|---|---|---|
| M6 | 1 mm | 1 mm | 2 mm |
| M8 | 1.25 mm | 1.25 mm | 2.5 mm |
| M10 | 1.5 mm | 1.5 mm | 3 mm |
| 1/4-28 | 28 TPI | 0.036 in | 0.071 in |
| 3/8-24 | 24 TPI | 0.042 in | 0.083 in |
| 1/2-20 | 20 TPI | 0.050 in | 0.100 in |
(Metric rows use the coarse pitch, and the inch rows use UNF. Fine pitches give less travel per turn and finer adjustment.)
How do you work out the length from the parts?
Add up the body and the two rod-end shanks, less the thread that is screwed in.
For a turnbuckle with a male-thread rod end in each end:
C–C = B + (H₁ − e₁) + (H₂ − e₂)
B is the length of the body, H is the distance from the pin centre of a rod end to the tip of its threaded shank (given in the maker's table), and e is how far the shank is screwed in. Turning the body changes both e values by one pitch each, so the length changes by 2 × pitch per turn.
Example, with illustrative numbers: a body of 100 mm, two rod ends with H = 40 mm, and M10×1.25 threads. Suppose you want at least 12 mm of thread engaged at full extension, and the thread in the body is 24 mm deep.
- At full extension, with 12 mm screwed in at each end: C–C = 100 + 2 × (40 − 12) = 156 mm
- Fully screwed in, with 24 mm in at each end: C–C = 100 + 2 × (40 − 24) = 132 mm
- Adjustment range: 156 − 132 = 24 mm
- Travel per turn: 2 × 1.25 = 2.5 mm, so 24 ÷ 2.5 = 9.6 turns from one limit to the other
How much thread must stay engaged?
Enough that the screw breaks before the thread strips. K05-Q1-04 derives it: with the same material on both parts, the thread strips at an engagement of about 0.7 times the thread diameter (d). If the softer part has half the strength, it rises to about 1.4 d. In steel, 1 d is a common starting figure, and it sits between the two. Use a longer engagement in softer metals or special alloys, and take the final value from the maker (K04-Q1-23). In the example, 12 mm on an M10 thread is 1.2 d.
The minimum engagement sets the longest length, because you cannot back the thread out any further. The depth of the thread in the body sets the shortest length, because the shank bottoms out (or the length of thread on the rod end, if that is shorter). Keep a margin at both ends.
How much adjustment range should you specify?
The range you need to cover, plus the spare to take up manufacturing tolerances. Add up the tolerance on each part in the chain, the mounting positions' tolerances, and any later wear. If you need a range larger than the body can give, use a longer body, or add a second adjuster. Aim for a working setting near the middle of the range, so you can adjust in either direction.
How do you set the length when you replace a link?
Measure the old link centre to centre if it is still straight and undamaged, otherwise measure between the mounting points with the mechanism at the middle of its travel. Then set the new link to that length, lock it, and run the mechanism through its full travel to check for binding (Q1-10).
How does adjuster travel turn into toe on a steering link?
On a steering tie rod, the adjuster changes the length of the link, and the length change becomes an angle at the wheel through the steering arm. For a small change, with the arm roughly square to the rod, the wheel angle in radians is the length change divided by the arm length. Illustrative numbers: a 120 mm steering arm and an adjuster with opposing 1.5 mm pitch threads, which change the length by 2 × 1.5 = 3.0 mm per turn (see "How far does the link move with each turn?" above). One turn on one side turns that wheel by asin(3.0 ÷ 120) = 1.43°, a quarter turn by 0.36°, and a quarter turn moves the edges of a 600 mm tyre by about 3.75 mm relative to each other. So a single turn is a very large step for toe, which is why a steering link is adjusted in small fractions of a turn, with the sleeve marked before you start and the same amount taken on each side to keep the steering centred. The toe value itself comes from the vehicle maker or the builder's data, never from this example. The tie rod adjustment calculator does this conversion, and the tie rod pages cover the parts.