what cause tappet clearance smaller ?

[Guest]

Hi . maybe is wrote somewhere but I didn,t find. I think I know what cause clearance bigger but smaller ....

[tkent02]

The valves wear at the contact point with the valve seat, as the valve wears it "recedes" into the head, so the end of the stem gets longer and takes up clearance. They can go the other way, but it's rare, as something else has to wear to do it such as the cam lobe or cam bearing. Usually those don't wear unless they run dry, then they burn up quickly.

Another common cause is that whoever set the valve clearances last didn't know what he was doing.

[Katarat]

I set mine on the high end of the specs because they never loosen, only tighten up

[Redman]

yep

yep

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[bwringer]

Yep.

The technical term is "valve recession". It's a combination of wear on the valve and valve seat where the valve basically recedes into the head.

I think we notice it more because the acceptable valve clearances on a GS are incredibly small -- most engines use a lot more clearance. But in all the motorcycle engines I've had my grubby mitts on, valve clearances tend to decrease very slowly with the miles (assuming the engine is running normally). Once in a while you'll see what looks like an increase in clearance that proves to be just heavy carbon buildup from a bike with other problems. Once the engine is running right again, the carbon quickly wears away again and clearances close up. That's why I always recommend double-checking valve clearances after running a resurrected bike for a few miles -- you don't really know what things look like in there.


Some light reading:

https://www.google.com/search?q=valve+recession&ie=utf-8&oe=utf-8