A king pin lock is a theft-deterrent device that covers a semi-trailer’s king pin, blocking the tractor’s fifth wheel from coupling to the trailer.
The king pin is the vertical steel pin on the front underside of a semi-trailer. Normally, it slides into the tractor’s fifth wheel and gets captured by locking jaws, connecting the two vehicles. A king pin lock physically encloses that pin so the fifth wheel cannot latch onto it until the lock is removed. This stops “drive-off” theft: a thief can bring a tractor but cannot couple to the trailer while the lock is installed. It’s a simple, mechanical layer of security used on detached semi-trailers, RV fifth-wheel trailers, and similar equipment. If you’re shopping for one, our roundup of the best fifth wheel king pin locks covers tested options.
How a King Pin Lock Works
Operation is straightforward. You place the lock over the king pin, secure it with the built-in lock cylinder or a padlock, and remove the key. The lock’s housing blocks the fifth wheel’s jaws from closing around the pin.
To install one, follow this sequence:
- Slide the lock fully onto the trailer’s king pin.
- Rotate the lock so the cylinder faces the rear of the trailer.
- Push the cylinder in until it stops.
- Turn the key counterclockwise about 3/4 turn to the locked position.
- Remove the key and attach the included “IN USE” tag.
Removal is the reverse: insert the key, turn it clockwise about 3/4 turn, pull the cylinder out until it stops, and lift the lock off the pin. The “IN USE” tag matters — it warns anyone approaching that the trailer is locked and prevents an accidental coupling attempt.
What a King Pin Lock Protects Against
A manufacturer describes the mechanism’s purpose as preventing unauthorized coupling by blocking attempts to hook up an unauthorized tractor. The security comes from the physical design. One manufacturer’s brochure lists a solid, cast aluminum alloy housing weighing 6.5 lbs (3 kg), a hardened steel bottom plate, no accessible shackle to cut, and countersunk rivets meant to resist physical attacks.
Other manufacturers use different approaches. One describes a heavy-duty lock with a nine-cylinder disk locking mechanism; another uses a solid steel welded collar secured with a stainless steel pin. Common variants include a single-cylinder lock, a two-cylinder lock, and a style made for Pullrite/Sidewinder hitches. These differences matter for fit, so compatibility with your trailer’s king pin and hitch design is the first thing to confirm.
| Lock Feature | What It Does |
|---|---|
| Housing material | Cast aluminum alloy or solid steel; blocks jaws from closing |
| Locking mechanism | 4-pin cylinder, 9-disk cylinder, or stainless pin |
| Security extras | Hardened bottom plate, countersunk rivets, no exposed shackle |
| Weight | Around 6.5 lbs for the cast aluminum version |
| Fit variants | 1-cylinder, 2-cylinder, and Pullrite/Sidewinder styles |
| Price range | Roughly $156 to $525 depending on the model |
King Pin Lock vs. Coupler Lock
The two are often confused, but they secure different connection types. A king pin lock fits the vertical pin on semi-trailers and fifth-wheel RVs, blocking the tractor’s fifth-wheel plate. A coupler lock fits the ball hitch on bumper-pull trailers, securing the trailer’s coupler over the tow vehicle’s ball.
One maintenance tip from the instruction sheet: slip a plastic grocery bag through the bore of the lock and fold it over the large end to keep grease off the lock during installation. The same sheet notes the lock requires little maintenance beyond keeping it functional.
Keep in mind that a king pin lock is mechanical deterrence, not absolute protection. Manufacturers emphasize resistance to tampering, cutting, drilling, and forced entry, but no product claims invulnerability. SurePlus’s king pin lock instructions cover installation and maintenance details. It’s one strong layer in a broader security approach — a parked trailer with a visible lock is a much harder target than one without.
References & Sources
- SurePlus. “King Pin Lock Instruction Sheet.” Covers installation, removal, and the “IN USE” tag.
- Sigma Truck. “Fifth Wheel Coupling Guide.” Explains the king pin and fifth wheel connection.
- Syndigo. “King Pin Lock Product Details.” Lists specifications, pricing, and security features.