Hi, looking at the valves of my motorbike I noticed that the exhaust stem, where the tappet works, has a convex surface facing upwards. I wonder how it is possible to adjust the valve clearance since we would be working on the edge with the feeler gauge.
I only have this photo but the problem is visible.
Has anyone who is more experienced ever noticed this?
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Based on the fotos above, this is what happened to my 2022 twice within 6,000km. Once on the original valves and cam and then on the Kent cam and Hitchcock valves. I have pretty much ruled out hammering damage as I have used the 0.18mm lash spec from the moment I installed the new valves and cam. So the only other cause is valve stem overheating and a loss of its heat-treated temper.
An exhaust valve stem tip can lose its heat-treated hardness (temper), a failure often caused by extreme over-temperature conditions, resulting in severe wear, deformation, or breaking. The stem tip is usually hardened (often 50-59 HRC) to withstand the high-impact hammering from the rocker arm or lifter, and if that temper is lost, the metal softens, leading to "mushrooming" or rapid wear.
Here is a breakdown of why and how this occurs:
Causes of Hardness Loss
Extreme Overheating (Over-tempering): Exhaust valves operate in high-temperature environments (1,200°F–1,450°F+). If the engine runs too hot, or the valve fails to seat properly and cool, the tip can exceed its designed tempering temperature, causing the hardened martensitic structure to over-age, soften, and lose strength.
Excessive Valve Lash (Hammering): If the valve clearance (lash) is too high, the rocker arm slams against the valve tip, creating severe shock loading. This "hammering" effect combined with high heat can destroy the hardened tip.
Poor Lubrication/Friction: Lack of oil to the top end can cause frictional heat to build up at the tip, breaking down the tempered surface.
Improper Heat Treatment: During manufacturing, if the induction hardening process for the tip was incorrect, it may not have achieved or maintained proper hardness.
Consequences of Lost Hardness
Mushrooming: The tip of the valve stem flattens and spreads outwards, which can make it impossible to remove the valve from the guide.
Galling/Wear: The soft, un-tempered metal galls against the rocker arm, accelerating wear and creating debris that can damage the engine.
Valvetrain Noise: A worn or deformed tip often leads to a "ticking" or "popping" noise as the valve lash becomes excessive.
Fatigue Failure: Loss of hardness drops the fatigue properties of the alloy, allowing cracks to form and the tip to eventually break off.
Prevention
Maintain Correct Valve Lash: Regularly checking and adjusting valve clearances to prevent excessive hammering.
Proper Cooling: Ensuring proper engine cooling and heat management.
Use Quality Components: Using high-quality stainless steel (e.g., 21-4N) or Stellite-tipped valves, which are designed to hold their hardness better at high temperatures.
My reco is to use the RE recommended oil Castrol 20W50 Actevo, also check and change oil by the RE procedure designed around the oil cooler or you will have low engine oil that can damage the exhaust valve tip. Install a motorcycle cylinder head temperature gauge and use it. The valve tappets on 411 can be replaced with better quality Honda Unicorn valve tappets. All else being said, our main challenge with the air cooled 411 is keeping the exhaust valve from overheating and starting a chain reaction of failures. I guess that's why the 450 is water cooled.
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- off to do them the "turns" method.
