Securing Trailers With Master Lock's Universal Trailer Coupler Lock

April 1, 2007

Master Lock's new Universal Trailer Coupler Lock is designed to protect trailers from “tow away” theft by securing the coupler. The coupler is the front end portion of the trailer that accommodates the ball attached to the rear end of the tow vehicle. The Master Universal Trailer Coupler Lock, part number 379TPY, is designed to accommodate UFP, rolled lip, surge brake, straight lip and hammer blow couplers having 1-7/8”, 2” and 2-5/16” diameter ball openings. The Master Lock Universal Trailer Coupler Lock is manufactured of cast metal alloy, powder coated in a silver gray flake.

The main portion of the Master Universal Trailer Coupler Lock is hook shaped to fit over the top of the coupler, and has a base that is positioned beneath the coupler. The sliding portion of the lock fits inside of the coupler's ball opening. To secure the lock onto the coupler, the sliding portion is unlocked, mounted onto the coupler and slid towards the front end of the lock. The sliding portion can be locked at any position to accommodate the different styles of couplers and the different diameter balls. The hook front is curved and recessed to accommodate different shaped coupler faces and lips, while preventing the coupler lock from being pulled off.

With the key turned to the unlocked position, the hook shape is slid over the front of the coupler and the sliding portion inserted into the ball opening of the trailer coupler. Once completely within the coupler, the sliding portion is slid towards the front of the coupler. At this point, the key is rotated to the locked position. The Master Lock Universal Trailer Coupler Lock is secured and locked.

To remove the coupler lock, unlock the lock mechanism and push the sliding portion away from the front end of the coupler.

The Master Lock Universal Coupler Lock is equipped with a four-disk tumbler, dual side bar lock mechanism. When no key or an incorrect bitted key is inserted into the lock plug, the disk tumblers and the two side bars extend from the body of the plug, crossing the shear line and into the cutouts in the housing, preventing the plug from rotating. When the correct cut key is inserted, the disk tumblers and the side bars retract within the body of the plug, permitting the key to be rotated. The key rotates 180 degrees to the locked or unlocked position and can be removed in either position.

The key code series for the Master Lock Universal Trailer Coupler is probably 3201-4000. The plug is equipped with four spaces for the operating disk tumblers. There are seven depths of cut (0-6). The sample lock had a key code of 3460, which decoded to 2213 for the depths of cuts. The code number is stamped into the keys. There is no code number stamped in the face of the lock cylinder or anywhere on the coupler lock.

The code series 3201-4000 is a common code series for Master Lock. Spacings start at the shoulder to the center of the first cut. The Cut-to-Cut is .125”. There are seven depths of cut (0-6) and the drop is .0155.” The original Master Lock single-sided key blank is the 1K.


1- .187” 2-.312”

3-.437” 4-.562”


0- .272” 1- .257” 2- .241”

3- .226” 4- .210” 5- .195”

6- .179”

This single-throw lock mechanism has two side bars and the four wafers. The three locking points resist picking.

To service this lock, a Cylinder Retaining Disk Tumbler is in the fifth space. The plug can be removed for servicing using the Master Service Key, part number 8401-04004. The Master Service Key has deep cuts in spaces one through four and a no cut in the fifth space.

To remove the lock plug:

Step 1. Insert operating key and turn lock plug to the unlocked position.

Step 2. Insert remover key.

Step 3. Slide the plug out of the lock. A little persuasion may be required to release the plug from the lock.

The Master Lock Wafer Rekeying Kit, part number 8491, contains 0-6 depth disk tumblers, cylinder retaining disks, driver springs, key gauge, rekeying tools, service key, and wafer sidebar plug assemblies.

For more information, contact your local locksmith wholesaler or Master Lock, 137 W. Forest Hill Ave. , Oak Creek , WI 53154 . Telephone: 414-571-5625. Fax: 414-766-6333. Web Site: .