Joe: GM P/N 12557390

[Joe C.]

Are these rocker stud nuts standard use for most GM smallblock engines, regardless of whether the lifters are mechanical or hydraulic?
Thanks in advance.

Joe

[Joe L.]

Re: Joe: GM P/N 12557390

Joe-----

Yes, these nuts are for use with ALL 1955-96 small blocks except LT4 (which used a posi-lock due to the roller rocker arms). The type of lifter used has no impact on the type of rocker stud lock-nut used.

During the pre-73 period, the GM #3744341 rocker stud nut was used for all small block applications, regardless of the type of lifter. The GM #12557390 is the current GM replacement for the 3744341.

[Joe C.]

Re: Joe: GM P/N 12557390

Joe:

I bought them yesterday at a Buick/Pontiac/GMC dealer about 20 miles from home. Most dealers were OOS. They're "tried and true" after being used for all of those years, so it's off with the Posilocks!

Thanks again.

Joe

[Joe L.]

Re: Joe: GM P/N 12557390

Joe-----

I don't think you'll ever regret it. The only problem with the stock type lock nuts is that they will eventually wear and lose preload. However, this only occurs as a result of rotation of the nut and not spontaneously. For a hydraulic lifter engine, this, essentially, means that they're good for the life of the engine. For a mechanical lifter engine, which sees more nut rotation due to the need for valve adjustment, there may be a need for replacement of the nuts about once during the life of the engine. For your situation, even with mechanical lifters, I think that replacement time will likely occur "after it doesn't make any difference to you". The nuts should always be routinely replaced at engine rebuild time, but many shops don't do this and VERY FEW problems are ever encountered.

I'm using posi-locks on my "ZL-1" but only because I'm also using roller rocker arms. I tried mightily to figure out a way to use self-locking nuts with the roller rocker arms, but I came up "dry". I even considered foregoing the roller rocker arms and using stock type rockers just so I could use stock-type lock nuts.