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What makes valves go oput of adjustment?!?!?

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Every time the valve makes contact with the seat and tiny bit of metal wears away, eventually enough wears away for the valve to move up in the head and get tighter.

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The valves and seats wearing from the valve hitting against them. If you don't clean your air filter properly and dirt gets in the engine then you get dirt between the valve and the seat. This acts like sand paper and makes the valves and seats wear much faster than normal.

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what are the seats? the shims?

The seats are rings in the head at the end of each port that the valve actually closes and seals against. The shims are little disks between the top of the valve stem and the bucket used as spacers to set the clearance between the cam and the bucket. You need a certain amount of space between the cam and the bucket when the engine is at top dead center (called the valve clearance), and changing the thickness of the shims changes how big this space is. As a valve and seat wear, this space gets smaller, and a thinner shim must be put in to get the valve clearance back to the right size.

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All true. And the purpose of the valve clearance at TDC when cold is the valve stems grow (lengthens) when hot, and when then do, you don't want them contacting the cam lobe and cause your valves to hang open.

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