I have been advised that if I use aluminum rockers, I should check them periodically because they can fatigue and break. Or better yet, replace them at intervals. While I know very well that aluminum has a fatigue life that depends on the stress level on the part, I think (?) GM used aluminum rockers on the LT4, so they must be ok in that application for well over 100k miles. Also, I think the only visible sign of pending failure would be a crack. By that time, failure would be soon, so an annual inspection wouldn't cut it.

Since the stress in the rocker depends on cam/springs/rpms, etc, I would think somebody may have run their motor enough miles to have this happen. But maybe the problem gets blamed on something else. Or, the rockers are overdesigned enough that it 'never' happens. The fact that highly built engines tend to run low-mileage lives may offset the fatigue risk, too. I plan on Pro Magnums to avoid this issue, they are steel.

Has anybody ever found cracks or broken an aluminum rocker?
Do folks change them out for new ones when they get old, like valve springs? How old?

To help identify a fatigue failure as opposed to an overload failure: there would be very little deformation of the part, and the broken surface would have a scalloped appearance. There is a pretty good picture of the typical fracture pattern here, 2nd item down: http://www.testlabs.ca/tech-ref.html

 

Personally, I've never heard of an aluminum rocker breaking without some outside influence; ie. incorrect valve train geometry, incorrect valve lash adjustment, etc. I've never actually seen one that snapped in half. Most likely, with aluminum being as soft as it is, it would most likely bend or deform before it would 'snap'. If one did deform, you would hear it in the valve train and more times than not, it won't cause any harm to internal components (unless you were at 6k+ rpm when it decided to let go, but as long as the springs don't float, you'd probably be alright here as well). HTH

 

I've used them for years on street and race cars with no issues. If I had a setup with a huge SR using mega springs (750+lbs) I would consider something different. But otherwise, they are fine for general race or performance use.

Rich

 

Fatigue failure can occur without reaching stress levels that would typical deform the bulk material. Cyclical stress is the culprit- repetitively pulling on a pre-existing flaw (crack, pore, inclusion) can eventually lead to its growth into a macro-scale defect.

 

Is that english? LOL Those are some nice words....

 

That's the key word there. The thing is, it doesn't matter which material you deal with (steel or aluminum), if there is a pre-existing flaw, crack propogation from cyclical loading is a possibility (just depends on how much loading there is). I took Fracture Mechanics, so I know exactly what you're talking about. I don't have any reference material here, but does a crack propogate through aluminum faster and at a lower cyclical load than steel?

 

Well I'm no fracture expert, merely a self-proclaimed materials geek - I actually work with ceramics where there is no such thing as fatigue

However the material does matter; saying that it doesn't is like saying you could use either wood or titanium to make head bolts, because either will break if you pull hard enough on them. Back to fatigue, there are properties intrinsic to each material (fatigue/enduance limit is probably most relevant,
fatigue strength also) which would affect their performance, along with physical characteristics such as surface finish and of course design of the part. I can't tell you for sure why X-brand aluminum rockers are better than Y-brand steel ones, but it lies somewhere in there.

 

I have heard of the bearing giving up on some aluminums but never a body failure unless the bearings only failed because of body deformation but never saw anyone claim that.

I ran a set of ProForm 1.5 SA rollers for like 50K much of that with upgraded cam and complimentary springs, they survived just fine. For the new setup I went with ProMagnums though.

 

Steel parts can be designed to have infinite fatigue life if the part operating stress (material strain to be exact; think of it as how far the part flexes on each cycle) levels are low enough, below the endurance limit. The strain needs to be in the low, flat part at the right hand end of the S-N curve, as I recall from about 20 years ago. Aluminum has no such 'infinite life' design point, so aluminum parts will all fail eventually by fracturing if they run long enough, the question is how many cycles. That's why race parts and airplanes get retired after a certain amount of use, even if they are visually ok.

I am trying to get a feel for whether the aluminum rocker arm cycle life ever falls within the useful life of a motor, expecially in the more highly stressed applications. It is starting to sound like the answer is no.

 

JP95ZM6, your last post is dead-on. However, aluminum rockers do break sometimes. One of my friends owns an engine shop that builds dirt-track engines (Late Model SUPR mostly). He's broken lots of aluminum rockers and has switched to the Pro-Magnums as a result. However, he's using spring pressures of 275 on the seat minimum. I've heard him talk about going well over 300 also.

Mike

 

Thanks mike! Thats the kind of info I am looking for.

 

Haha, I haven't heard that since I was in my Metallurgical Engineering class

...but he is right..