Broken valve springs cause excessive valve noise and may cause erratic exhaust gas temperatures. The actual breaking of the valve springs is not always the most serious consequence. Actions following the breaking cause the most serious damage to the engine. When a spring breaks, it may collapse just enough to allow the valve to drop into the cylinder, where it may be struck by the piston. In addition, the valve stem locks or keepers may release the valve and allow it to drop into the cylinder, causing severe damage to the piston, cylinder head, and other nearby parts.

You can take a number of precautions to prevent or minimize corrosion and metal fatigue, which cause valve springs to break Be reasonably careful when you

Figure 3-14.Excessively band-ground valve.

assemble and disassemble a valve assembly. Before you reassemble a valve assembly, be sure to thoroughly clean and inspect the valve spring. (Use kerosene or diesel fuel for cleaning. NEVER use an alkaline solution; it will remove the protective coating.) The condition of the surface of a valve spring is the best indication of impending failure. (Use magnafluxing to help find cracks that would otherwise be invisible.) The free length of a valve spring should be within the limits specified in the manufacturers technical manual. If such information is not available, compare the length of a new spring with that of the used spring. If the length of the used spring is more than 3 percent shorter than that of the new spring, replace the used spring immediately. Remember, however, that loss of spring tension will NOT always show up as a loss in overall length. Springs may be the proper length, but they may have lost enough tension to warrant replacement.

Figure 3-15.Facing a valve on a lathe.

Do not reinstall springs with nicks, cracks, or surface corrosion. Replace them. To minimize corrosive conditions, use clean lube oil, eliminate water leaks, and keep vents open and clean