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Do you lap valves

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2003 YZ250F

The book says to lap the valves. I am also told the titanium valves have a coating on them which you should not touch. So do you lap them? I would say no due to the coating.

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If you had a new valve, or one with very low hours on it, and you were putting it into a low-hour valve seat in nearly new condition, it would be OK. But it isn't that simple.

Here's the deal. Your valves are made of titanium, which is a very light metal that is outstandingly strong, particularly on a per pound basis. It is not, however, very hard. So the faces are coated with titanium nitride, which is exceptionally hard, and is the gold stuff you see on some drill bits, etc. It's used on drills to harden the cutting edge beyond what is normally possible with tool steel, and it's cheaper and safer than carbide. The TiN coating enables the titanium to have a reasonable wear life (in a YZF, anyway) while banging around on a hardened steel valve seat.

When valves are lapped, a grinding paste is put between the valve and seat, and removes metal from the two parts as they are moved against each other. In any such situation, the softer part will loose the most material. In the case of a new YZF valve, the seat is the softer of the two, so at least at first, the bulk of what comes of will be removed from the seat. BUT, the coating on the valve face will also be ereoded.

So, if this is done, it should only be done when a very small amount of correction is required to restore the seat. Add to that that you should never lap a used valve, since therre is no way to tell how thick the remaining coating is, and you may grind through it.

Lapping valves is never the best choice, regardless of what the valves are made of, if any more than very minor wear is present at the valve seat. In that case, they must be cut or ground. If that's done correctly, lapping will be unnecessary.

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The Yamaha Wr450 manual says it is okay to lap the valves. They suggest two steps with different compounds. DR. DIRT in DIRTRIDER magazine says don't touch them with lapping compound because it removes the coating on the titanium. I'm not lapping.

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I test the valve seal by spraying brake cleaner into the head with the valve installed. If the liquid leaks thru, I'll lap it a little more than test it again.

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