Originally Posted by rmbuilder (Post 4109712)

There is significant data regarding the well defined, finite levels of fatigue (and the specific cycle value) incurred using aluminum as a body material in a rocker arm construction. Durability testing (T & D) of shaft rocker systems has shown the point of failure of an aluminum rocker to be ~ 11 million cycles. That cycle rate, at an average RPM of 3500, is ~ 105 hours run time. That may not seem excessive, but to give a frame of reference, that is the cycle equivalent of running a Cup car 8500 RPM at Daytona for 43 hours. Lets emphasize here that is the point of first failure, not an average or sampling. As happened in your case, it is not uncommon to see closely sequenced batch failures when the arms are not cycled out at the end of their usable lifespan.

Fatigue is not necessarily a failure in design or manufacture; it is primarily inherent to the material property differences between Alum and steel. That figure represents the count the component reaches a cycle value that Alum becomes fatigued. The relatively low modulus (Ealum30.103 psi) of aluminum, in conjunction with the repeated cycling of the arm, through extension and compression of the spring, is the primary cause. From that point foreword potential failure becomes a possibility. There are a number of parameters that will influence the rate of degradation, but the rockers will, at some safe point, need to be cycled out or risk a failure.


What are the advantages of a steel rocker? First and most importantly are the stiffness advantage 4130 represents in comparison to aluminum. The elastic modulus of the materials is:

E steel 30.106 psi
E alum 30.103 psi

That 200% increase in stiffness is a huge advantage in minimizing deflection within the system. Any reduction in compliance nets a more accurate valve path, increasing lift at the valve while increasing the natural frequency of the rocker element of the system. I would also be wary of taking literally (and introducing into practice) the idea that compliance introduced into the system (by aluminum arms) is a valid way to damp unwanted “harmonics”.
What are the downsides to steel? There aren’t any. Steel does provide increase in mass due to the density of steel compared to Al. The good news is: with advance design and manufacturing capabilities it is possible to mitigate the mass penalty incurred using steel. A properly designed steel arm will incur only a small amount of mass increase over AL when properly executed. That increase is primarily at the pushrod side of the lever and therefore minimizes increases to the mass on the valve side. Any mass reduction on the valve side of the fulcrum, including valve diameter and mass of the spring itself pays far greater dividends than mass reduction on the PR, lifter system. The continued use of aluminum is driven largely to maintain a competitive price point in the market.

As far as value is concerned, it’s no contest. While steel has higher upfront costs, the perceived saving in aluminum is false economy. Steel has virtually infinite cycle value. With the exception of an occasional tip or bearing replacement steel will provide safe, reliable performance for the life of the engine. Aluminum will offer upfront cost reductions, however the renewal of the cycled parts will quickly eradicate the initial value.

Bob

Originally Posted by FIXX (Post 4109889)

So bob which are better to use, the jessel pro steel or crower SS shaft rockers..?

Originally Posted by mrmalo32 (Post 4109932)

+ 1

Jesel steel @ $220 each rocker $ rails

Crower @ 1640 each motor complete with rails

Jesel get up ther in price, will be nera $8k to do this

Originally Posted by throttleman (Post 4110187)

we had a problem like this on a pr of sc 540s in our race boat . the push rod tip was incorect for the jessel system and would move the push rod out of the cup on the rocker , causing the valve to tag the piston slighty , breaking the valvehead off !, the fix to the problem was a pushrod change .

Originally Posted by sutphen 30 (Post 4110009)

unfortunately,your gonna have to pay to play.especially at the rpm's you want to run.

btw,what are you doing w/ this boat?

Originally Posted by mrmalo32 (Post 4110231)

I get asked this a lot
We just say " just go FAST ! "
We have a few boats that get out together, stop in the middle oth the lake, swim, eat lunch, then go fast to the other end of lake.
While it's going it just fantastic, and then we have these problems,
It's worth fixing just to get back out and do it all again
Cross my fingers to sort this issue out and get some hours up.
Over there you guys have many of these boats but here you can only imagine what people think of them
Love it !