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YZ250F valve question..

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So my brother has an 06 YZ250f that I just replaced the valves on about 20hrs ago. I brought the head to a mechanic to have the seats cut. Got the head back and installed new valves, went to shim them, and they wouldn't come into desired clearance, even with the smallest size 1.20 shim. I knew then and there what happened, the mechanic cut the valve seats too deep. I brought the head back to him and he shimmed the valves into clearance by shaving down the already very thin 1.20 shims. Now go figure, he was riding the bike this past weekend, and the motor locked up. Took it apart and it seems one of the valves floated, bending 2 intake valves, 1 exhaust valve, and breaking the other exhaust valve at the head. Now my question is, do you all think it may have had anything to do with the mechanic shaving down the shims in order to compensate for his mistake of cutting the seats too deep? My thoughts were the valve shims only come so small for a reason but I would like to hear some other opinions too... thanks!

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The shims are case hardened to resist wear. The problem with removing the hardened material, is you invite more rapid wear and the possibilty of the shim breaking. I think your mechanic should be fixing this for you.

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Yes, your mechanic effed up bad. Clearly has no idea how to cut seats. I have heard of cheap people removing a tiny amount on shims with no issues, like .02mm, but how much did your mechanic remove?

This is exactly why when machine work is required on motors, I only send my stuff to the professionals (Williamsmotowerx, pro circuit, etc) and not billy bob the mechanic. Mechanics are parts changers, not machinists. Even machinists are no good with this kind of stuff unless they are moto junkies themselves and understand whats going on in these motors.

How bad is the head? Is it right effed up?

If not, you can send it to the professionals and get new seats installed along with new guides/valves.

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Yup the head is done for, by time I pay to fix it I could buy a new one on ebay for $390. I'm gonna mic the shims and see how much material he took off. It had to be a substantial amount because the valves were almost zeroed when I brought it to him..

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Wow you know I did not know that honestly... well that explains it then. Id be willing to bet that because of that reduced spring pressure the valve(s) floated and caused it to smash into the piston... wow. I guess Ill be having a talk with the mechanic then!

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I have a feeling if he cut the seats too deep, chances are good there was other machining problems, too. Like maybe they weren't concentric or square. I was talking to Mr. Williams on the phone the other day and he brought up the very valid point that you're average ape working on a Chevy engine might not understand -or care, that the tiny valve stems require tiny cutter guides, and tiny cutter guides are flexible! Perhaps these fancy pants self centering valve cutting machines are a double edged sword...

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