Here is a set of generic poly locks used on stock rockers. We prefer to use poly locks over pinch nuts as the poly locks maintain their position far better than pinch nuts. (Image/Jeff Smith)

I just discovered a rocker arm on my small block that loosened up and popped off the stud. It kinda mangled the threads on the stud, so I’m going to replace the stud, poly lock and the rocker just to be safe. I think I may not be applying enough torque on that Allen lock nut, but I’m afraid it will just strip the inside of the Allen lock nut if I lean on it. Is there a trick to getting this tight enough?


Yes. The poly lock nut for roller rocker arms is a great little invention. I learned the little secret to locking these nuts tightly from an engine builder probably 40-odd years ago and the technique is simple and works extremely well.

The poly lock nut is merely a threaded nut with a small Allen screw placed inside the female threads. With the large adjuster positioned to set the lash or lifter preload, then the Allen nut can be tightened. But sometimes the Allen won’t tighten against the top of the stud tight enough and the large adjuster can then work its way loose—as you’ve discovered.

The technique is to set the large adjuster nut to just shy of the final position of the poly lock. With the large nut in this position, tighten the Allen wrench to contact the end of the stud. Then tighten the large poly lock nut the last 1/16th of a turn or so. This places more load on the locking Allen plug than can be accomplished with the Allen wrench alone.

This does require some practice to get the setting correct, especially when setting lash on a mechanical lifter engine since you’re working to within 0.001-inch. This requires a technique where you can estimate the amount of change that last 1/16-turn will affect the actual lash. It can be frustrating but with some experimentation, you can come up with a technique that will produce the desired lash.

One other little tip we’ve learned is that ARP makes a great adjust nut called a Perma-Loc with a simple yet very helpful addition. All other poly locks use a six-point hex that allows a box end wrench to fall past the hex, requiring you to hold the wrench in position on the hex. The ARP Perma-Lock uses a 12-point nut with a shouldered ledge on the bottom of the points that prevents the box end wrench from falling past the hex. Plus, the Allen set screw is machined flush on the bottom so it offers full optimal contact—as opposed to other adjusters that use a tapered face. This might seem like a small thing, but once you’ve used the ARP Perma-Locs you will more fully appreciate these features.

The ARP Perma-Lock rocker arm adjusters feature a 12-point head with a small shoulder stop at the bottom that prevents the wrench from slipping off the hex. This allows you to place a standard box-end wrench over the adjuster that will remain in place. This is really nice when setting lash as it’s far less clumsy. (Image/Jeff Smith)

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.