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jasonlion54

Overheated exhaust cam

6 posts in this topic

So I ran my 2008 YZ450F without water for a day or two (:thumbsup::cheers: I know :bonk::thumbsup:).

I took the motor apart and the exhaust cam shows some significant heat discoloration where it rides against the aluminum of the head. The camshaft has a bluish color to it centered around the cam journal but the discoloration does extend into both cam lobes.

My question is do I need to replace the exhaust cam? A friend of mine suggested that whatever heat tempering process was done at the factory has been ruined and the cam will soon fail. Basically, he says that the heat has weakened the metal. Is he right?

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Have you had the cam caps off to inspect the bearing surfaces? Let's have a look at that first.

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Yes, the aluminum surface that the cam rides on also shows some discoloration, but not very much. It is still completely smooth to the touch, and the cam still spins smoothly and freely when bolted down. As far as the actual cam bearing (next to the gear part of the cam), it does not appear to have been affected by the heat at all.

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Is the discoloration yellow and/or blue, or is it more like a blackening from something burning on the metal surface?

If your cam journals look good, then your oil came through for you, although if it got that hot, my concern would be for the condition of the cylinder wall and piston.

If the discoloration is blackish, it's a matter of oil "burning" or oxidizing on the cam. Polish that off the shaft journals with oil and fine scotch-brite or steel wool, or "crocus cloth" (emery cloth with a grit of 1000 or finer). Don't use abrasives on the cam lobes.

If the discoloration on the steel of the cam is blue, it might have slightly degraded the metal, but it's not likely to have been hot enough to do that without causing some significant damage to the head that you would have been able to see.

Check your camshaft clearances in the saddles with Plasti-Gage, and run it if it checks out. Have a look at the cams now and then to see how they're doing.

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Thanks a ton for the help, Grey.

The piston and cylinder are being replaced, so no worries there. The discoloration is blue.

What clearances should I be looking at with a plastigage?

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Camshaft to head. See page 4-17 of the manual for the procedure. Specs are .0011 to .0026"

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