I race a Corvette in the vintage road race classes and I have had a rash of broken rocker arms. The rules prevent me from converting to roller rockers. Is there any way to prevent these rockers from breaking? The failures usually occur right in the middle of the rocker arm but the pushrod also does cause a failure.
Jeff Smith: There are actually several things you can do.
The first thing would be to qualify all the rocker arms for thickness in the cross-section of the rockers where they normally fail. You didn’t mention it, but it’s possible that the failures have been on the exhaust side. Exhaust rockers run hotter than their intake counterparts because of heat transferred through the valve stem. If the rockers are breaking early in a race, you could try using broken-in intake rockers on the exhaust side. This will help.
It’s also essential that you use high performance balls that come with several oil-retaining grooves to help lubrication. It’s also worth mentioning that it is essential to use a high-quality synthetic race oil with higher levels of anti-friction additives like ZDDP. Plus, synthetics are much more stable at very high temperatures. If your engine’s oil temperature is over 225 degrees, that means that at the rocker arm the oil temperature will be much higher. This is where improved lubrication can make a huge difference. If I were to make a recommendation, I’d suggest looking into the lubricants offered by Driven Racing Oil. You didn’t share your chosen oil viscosity, but Driven offers several choices in performance lubricants that are of the highest quality. If I were to hazard a guess, I’d suggest Driven’s XP-9 full synthetic that is the equivalent of 10w40 race oil. This oil offers excellent thermal stability necessary for a road race engine and uses the new standard in synthetic oil blending stock. This is expensive stuff, but what is the price of protecting your engine while improving its durability? It’s also worth mentioning that you should not restrict oil to the top of the engine. These rockers, especially the exhaust side, need to be bathed in oil to stabilize the operating temperature.
You might also consider having the balls and rockers treated to the below zero cryogenic treatment. This treatment process can be viewed as another version of mechanical stress relieving. I’ve listed a firm that does this kind of work at the end of this answer. Another idea that I know works well is metal surface treatment. One process is called ISF SuperFinish (Remchem.com). If you magnify the surface of any metal, you will see a series of peaks and valleys. A rough surface means deeper valleys and taller peaks. The REM concept burnishes the metal – essentially polishing the surface to minimize the peaks. The result looks spectacular with an almost polished finish, but that’s just the cosmetic result. The benefit is that the metal surface now offers a greater load-bearing area and friction is reduced. Together this reduces heat and allows the lubricant to do a better job. Another idea is to shot peen the rockers first, then have the surface finished. None of these processes are cheap, but they will improve the durability.
Depending upon how diligent the tech inspectors are at demanding a “stock” part, there is another alternative. Crane makes a small-block stamped steel rocker arm called the Nitro-Carb rocker. Crane says they have tested this rocker arm to open loads of 350 pounds with excellent wear characteristics. Even better, these rockers are available in both 1.5:1 and 1.6:1 ratios. A long time ago, I tested these rockers and found that Crane has moved the position of the pushrod cup compared to a stock rocker. This bumps the ratio even higher at low valve lifts compared to a stock rocker arm. The part numbers for these two different rocker ratios for a small block Chevy are 11801C-16 (1.5:1) and 11802C-16 (1.6:1).
A 1.6:1 ratio adds about 0.030-inch to the 1.5:1 valve lift, which can be a benefit for power but the ratio also accelerates the valve faster at any given rpm, so valve float might become an issue. In my experience, small blocks with near-stock cylinder heads benefit the most from a 1.6 rocker on the exhaust side since without the benefit of porting, the exhaust ports are far more restrictive. Since the exhaust valve is smaller and lighter, float is less of an issue.
More info: Controlled Thermal Processing