SUPERPRO Trunnion Bushes with Fitting Position Front and Rear, Front Axle

A trunnion is a pivot built around a pin, and a trunnion bush is the sleeve that pin turns in. In suspension, the term usually describes the joint at the bottom of the upright on certain older independent front ends, where a threaded or plain pin lets the upright swing with the steering and the suspension at the same time. It is a design that predates the modern ball joint, and it relies on its bushes and on regular lubrication to stay tight. Most current 4WDs use ball joints or king pins instead, so trunnion bushes are a specialist part. Where a vehicle does use them, worn trunnion bushes mean a loose front end, vague steering and knocks that no amount of alignment will cure. Use the vehicle search to find trunnion bushes to suit your 4WD.

What a Trunnion Does

On a trunnion front suspension, the upright, also called the vertical link, connects to the lower control arm through a trunnion. The trunnion carries a pin, and the bushes around that pin let the upright pivot. Depending on the design, the joint lets the upright move up and down with the suspension, and lets it turn for steering.

The bushes have to carry vertical load from the weight of the vehicle, braking and cornering forces, and the twisting load from steering. Any wear shows up as play at the wheel.

Trunnions, Ball Joints and King Pins

A ball joint does the same job with a ball and socket. A king pin does a similar job on many solid front axles, with the steering knuckle pivoting on a pin and bearings or bushes. A trunnion is a different arrangement again, often with a threaded pin running in a threaded bush so it can carry load and rotate at the same time.

The three are not interchangeable, and the parts for one will not fit another. The term "trunnion" is also used in other places, including heavy truck tandem suspensions and machinery, where the part is quite different.

Which Vehicles Use Trunnion Bushes?

Trunnion front suspension is mostly found on older passenger and sports cars, particularly some classic British designs. Very few bakkies or 4WDs use this type of front suspension. Most independent front 4WDs use upper and lower ball joints, and most solid axle 4WDs use king pins or ball joints at the steering knuckle.

Because trunnion bush kits are made to suit specific vehicles, the vehicle search is the reliable guide to whether your vehicle uses them. If your vehicle does not appear, it is likely to use a different type of joint.

Signs Your Trunnion Bushes Are Worn

Play at the front wheel is the main sign. With the vehicle safely supported and the wheel off the ground, rocking it top to bottom will show movement at the trunnion if the bushes or pin are worn.

Clunks or knocks over bumps are common, as the upright moves in the worn joint.

Vague or wandering steering can follow, because the wheel is not held at a consistent angle.

Uneven front tyre wear, particularly on one edge, can point to camber changing as the worn joint moves under load.

A stiff or notchy feel in the steering can come from a trunnion that has run dry or seized.

Ball joints elsewhere in the front end, tie rod ends, wheel bearings and control arm bushes can all cause similar symptoms, so a full front end inspection is worth doing first.

Greasing and Maintenance

Lubrication is what keeps a trunnion working. Many trunnion designs run a steel pin in a threaded bush with grease or oil between them, and when that lubrication runs out, the pin and bush wear quickly. Some designs have a grease nipple, and others are filled with oil.

Factory service schedules for vehicles with trunnions often call for regular lubrication, more often than most owners of modern vehicles are used to. Check the vehicle's service manual for the correct interval and lubricant, and follow the kit instructions for replacement bushes. Greaseable replacement bush kits are available for some vehicles, which can make the job easier.

A trunnion that has been neglected can wear its threads or bushes enough that the joint needs rebuilding with new bushes and a new pin.

Rubber, Polyurethane and Threaded Bushes

Trunnion bushes come in different forms depending on where they sit in the joint.

Threaded bushes carry the main pivot load on many designs. They are usually metal and run on a threaded pin with lubrication.

Rubber bushes are often used at the inner pivots of the arms around the trunnion. They isolate vibration but soften and crack with age.

Polyurethane bushes are generally firmer and hold the joint more precisely. Some replacement trunnion kits use polyurethane in place of the original rubber, and some are made greaseable. They may transmit a little more vibration.

The product details show which type a kit uses and which part of the joint it covers.

Load and Rough Roads

Trunnions are older designs, and the vehicles that use them are usually lighter than a modern 4WD. Extra weight at the front, rough roads and corrugations all add wear. On a vehicle driven on unsealed roads, dust and water can also find their way into the joint and break down the lubrication, which makes regular greasing more important.

What Comes in a Trunnion Bush Kit

Kits vary by vehicle and position. Some cover the main trunnion bushes only, some include a new pin, and some cover the upper and lower outer bushes or associated pivot bushes. Seals, washers and grease nipples may or may not be supplied. Check the product details for which positions the kit covers and what is included.

Replace the Bushes or the Whole Joint?

New bushes are the fix when the trunnion pin and the upright are in good condition and the wear is in the bushes.

A new pin is needed when its threads or bearing surfaces are worn, since a worn pin in new bushes will quickly wear them out again. If the upright itself is damaged or its threads are worn, it may need replacing or reconditioning.

Fitting Trunnion Bushes

Rebuilding a trunnion involves separating the front suspension and steering components, so it is a job for a qualified technician with experience on the vehicle. The vehicle is supported on rated stands, the wheel and brake components are removed as needed, and the spring is safely controlled before the joint is dismantled.

The old bushes are removed, the trunnion, pin and upright are cleaned and checked, and the new bushes are fitted with the correct lubricant. On threaded designs, the pin and bush threads need to be set up correctly so the joint moves freely without play. Bolts are tightened with the vehicle at ride height and torqued to specification. The joint is lubricated before the vehicle goes back on the road, and a wheel alignment is required afterwards.

How Formula 4x4 Can Help

Formula 4x4 works across the whole suspension, which helps on a part that only some vehicles use. If you are not sure whether your vehicle uses a trunnion, a ball joint or a king pin, the team can help identify it and point you to the right parts. Play in a front end can come from several joints, and looking at the whole front end helps make sure the right one gets rebuilt.

Find the Right Trunnion Bushes with Formula 4x4

Use the vehicle search, then confirm your vehicle uses a trunnion front suspension and which positions need bushes. Check the condition of the pin and upright, consider a greaseable kit where one is available, and check the product details for what is included.

Older trucks still running trunnion front ends can find their bushes in the Formula 4x4 suspension bushes range.

If you are unsure what your 4WD needs, contact the Formula 4x4 team.