19 Mar 2018
Category: Articles, Company News
Excessive valve stem and guide wear can occur on inlet and exhaust valves. Whilst there is a large number of potential causes for failure, some are actually symptoms of another problem. Whilst some causes are easier to spot than others, the most likely causes for excessive valve stem and guide wear include:
Too much stem-to-guide clearance can lead to the bell-mouthing of the guide. Misalignment of valve-guide-to-seat results in high-side loading. Incorrect valve height after an overhaul can lead to excessive side thrust on rocker-operated valves. The valve will be heavily recessed because too much metal has been removed from valve seat and face. Incorrect fitting of special high-lift cams may also be a cause, as they affect the rocker geometry.
Trapped foreign particles cause abrasive wear. This can occur particularly after an overhaul, when residual caborundum particles are present. Too much carbon packing at the guide’s port-end results in stem scuffing.
Lack of oil or a breakdown of the oil film can result in stem scoring and scuffing. Temporary poor lubrication coverage when cold starting in sub-zero temperatures can be similarly problematic.
Bent valve stems due to slipped or incorrect valve timing, or piston-to-valve collision due to over-speed in valve bounce.
The above issues can lead to other problems. For example, a bent valve stem will lead to stem seizure. However, this can also be caused by lack of lubrication, too little stem-to-guide clearance, or a damaged stem.
A valve stem tends to stick in extreme temperature conditions – due to gumming deposits when it is cold, or carbon packing between the stem and guide when the engine is hot. Either way, the sticking of a valve stem is a symptom of another issue that can lead to bigger problems if the root cause is not identified.
Root causes include:
A badly worn valve tip can be a symptom of various potential problems. Installers are advised to lookout for:
Troubleshooting valve train issues requires the installer to be aware that a valve train problem is likely a symptom of the root cause. Thus, it is imperative to resolve the core issue in order to ensure long-lasting results upon repair. More than 1,600 OE specification valve train parts by AE valves are available from FPS via same/next-day delivery. For more information, please speak with your local FPS representative.
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