What is the king post on a backhoe loader, and when does it need replacing?

The digging end at the back of a backhoe loader, meaning the boom, dipper and bucket, is attached to the machine through a single casting. In the field this casting is called the king post, or sometimes the swing casting. The boom swings left and right with it. On sideshift machines, such as the JCB 3CX and 4CX, the king post also slides sideways along the rails of the rear frame and is held in place with hydraulic clamps.

What load does the king post carry?

Every time the bucket hits a stone or a root, the shock goes first to the bucket pin and then through the boom to the king post. When the arm swings sideways, the force of the slew cylinders also ends up on the king post pins. That is why trouble usually starts in the pins and bushes before the casting itself.

The first signs

• A knock as the arm swings or at the end of the swing: there may be play between the upper or lower swing pin and its bush.
• The boom moving sideways with the bucket on the ground: if you can see the boom move around the pin when you push it by hand, the pins and bushes should be measured.
• The king post sliding sideways while digging: on a sideshift machine, the clamps are not holding the king post. Check the hydraulic pressure, the seals of the clamp pistons and the clamp bodies themselves.
• Cracks: the areas around the pin bores and the weld seams are where cracks most often start. Wash off the mud and look at a clean surface. That is the only way to see a fine crack.

Replace or repair?

If the play is only in the pins and bushes, there is no need to replace the king post. The pins and bushes are renewed. If a bore has gone oval, some workshops build it up and re-machine it. Whether that can be done is up to the workshop that sees the damage.

If there is a crack running to a pin bore, or the casting has bent, trying to save it by welding is risky. In that case the casting is replaced.

The clamp bodies on a sideshift frame

On sideshift JCBs, the king post is clamped to the frame from above and below. The two parts listed in our catalogue as “JCB King Post” are the bodies of these clamps. 128/14725 is the upper two-hole body and fits the 3CX, 3CX Sitemaster and 4CX. 128/14726 is the lower single-hole body, used on the 3CX, 4CX and 214. The cross-reference numbers for both are on their product pages.

If a clamp body has cracked or the surface the piston works on is damaged, the clamp cannot grip the king post fully. That is often how sideways slip while digging begins.

Before and after the job

Replacing a king post or a clamp body means handling heavy parts and calls for lifting equipment. Before you start, park the machine on level ground, lower the bucket and the front shovel to the ground, stop the engine and release the hydraulic pressure. Never get under a raised boom. Support the arm with blocks or a stand.

Once the part is fitted, grease every grease point until clean grease comes out. Swing the arm to each side a few times at low revs and listen for noise or play. On a sideshift machine, engage the clamps and load the arm in the digging direction. The king post should not move.

Frequently Asked Questions