Engine Failure - What Happened?

Everybody makes mistakes, everyone experiences failure, and we all have had things go unexpectedly wrong at the worst possible time. But the smartest among us learn from those setbacks so that they can minimize the chances of them happening again. After all, "live and learn" isn't a saying that everybody knows by accident.

And that certainly holds true when it comes to racing, and race engines in particular. But the problem when it comes to presenting an article on engine-failure analysis is that talking about blown engines is the last thing many engine builders want to do. While researching this article we had more than one engine builder refuse to offer help by saying something along the lines of "I wouldn't know. My engines never blow up." Of course, if that's true, either you aren't pushing hard enough or you aren't selling your engines to real racers.

We all know that if you want to see how quickly something can be broken, just give it to a racer because he will push everything to its limits. Engine builder Keith Dorton of Automotive Specialists knows that. If you plan to win races you have to plan to push the limits of durability and performance with your race engines. Dorton says competitive engine building is a continuous cycle of pushing the limits of engine power until it becomes too much for a component (or system of components) to withstand, strengthening those components and then pushing the power limits even further. There is no shame in breaking something, as long as you aren't breaking that same component again and again and not learning from it. Because of that, Dorton keeps a library of exploded pistons, bent rods and other assorted pieces of engine shrapnel that he's collected over the years. Every piece, he says, he's learned something from. And he's agreed to share some of that with us.

This is a case of where you sometimes simply have to make compromises. This aluminum shaft-mount rocker broke at the adjuster. Dorton believes the crack started at a stressed riser created by the threads in the rocker body for the adjuster and grew from there until the rocker split in two. Of course, the threads cannot be eliminated so a different solution was required.

To find out exactly what was going on, Dorton paid to test his entire valvetrain combination on a Spintron assembly. What he found out was a loss of valve control at high rpms were simply putting too much stress on the rocker. The solution? "You can't just give up power," he says. "So when we couldn't make it work with the aluminum rocker, we had to switch and go with a steel piece that is holding up for us a lot better. It's heavier, but the results were better than backing off on the cam lobes."

This spaghetti valve stem is an extreme example, and it is astonishing that it did not break. Broken valves are a dreaded engine failure, because they often lead to a grenaded engine with damage throughout. There are several ways a valve can be broken, and determining exactly what happened in an engine that has turned itself into scrap metal can be difficult.

But Dorton says the key is to think through how the failure could have happened. "Certain things always have to happen first," he says. "So sometimes the parts that aren't damaged will tell you a lot about what is." For example, if you have a broken valve, but the spring and lifter are still okay, then the source of the problem lies with the valve itself. Next, check the clearance between the valve stem and the guide. Excessive clearance between the stem and the guide will allow the valve to meet the seat unevenly which will eventually cause the head to break off the valve. If the clearance between the guide and valve stem is good, then it may simply be a problem with the actual valve.

Another common reason for broken valves is contact with the piston. This can come from insufficient piston-to-valve clearance or from over-revving the motor. Dorton says the check for this is simple. If you have banged one valve into a piston hard enough to break it, then you have probably had contact between pistons and valves in other cylinders as well. Check the piston tops in the other cylinders for tell-tale marks from piston-to-valve contact.

Finally, the last reason for a broken or bent valve is a valve that gets stuck in the guide. This is easy to spot from the marks on the valve stem that you can see in the photo. It is possible that this can happen from poor machining and too little clearance between the stem and the guide. But it is much more common to see this as a result of engine overheating. Also, Dorton says that teams that pump ice-cold water into their cooling system to aid qualifying speeds also suffer from stuck valves more often.