TM 5-3805-255-14
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VALVE RECONDITIONING CONTINUED
3. Discard severely burned valves (metal behind burn has lost its original properties). Valves which show
indications of "necking" must also be discarded. Necking is evidenced by a reduced diameter of valve stem
above port end of guide and is result of hot corrosion (Figure 4). Necked valves are susceptible to breakage, a
most expensive type of failure, since engine may be badly damaged when a valve head breaks off.
Figure 4. Necked Valve Stem.
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4. Discard valves with badly scuffed stems (Figure 5). Rough stems cause rapid valve guide wear. If there are
only slight indications of scuffing at extremities of guide contact area and there is no appreciable reduction in
stem diameter, valves will continue to give satisfactory service.
5. Inspect for worn keeper grooves and damaged valve tips. If present, discard valve. Worn grooves allow
cocking of spring retainer which tends to tip valve in guide, increasing guide wear and in extreme cases
causing leakage across valve face. Similar results can be expected from use of valves having badly damaged
tips. Refacing machines usually have an attachment for reconditioning valve tip, however, where tip is badly
damaged, grinding may remove all case-hardened metal on tip. Valves with deep notches or grooves in tip
must be replaced.
Figure 5. Scuffed Valve Stem.
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