Lapping valves

My friend tells me that lapping valves is a must when replacing them. I again have heard that lapping will destroy some special coating on valves and so valves won`t last for too long.

Some say that you can do lapping to stainless steel valves. Is this one true? Also I read from somewhere that lapping is acceptable for only stainless steel exhaust valves.

Is recuting seats the only right decision to make? If I would recut, would i still need lapping? My friend is sure that lapping is needed in one way or another.

I am really confused on this topic. I doubt I will need a valve job any time in near future, but it would be good to know the right answers when it comes down to it.

do not lap any modern valves.

you must have the seats cut by some one that actually knows how to do it.

Under no circumstances ever lap any titanium valve. Ti valves are not hard enough as they are to last for any realistic length of time. Steel isn't either, but steel can be hardened by a variety of heat and chemical processes that cannot be used on titanium. To get around this, a coating of one of a variety of titanium nitride compounds is applied to the valve. This coating is extremely thin, because it's too hard to be precisely machined as a valve must be, and is applied after the valve is finished. It is often less than .0005" (.01 mm) thick, and lapping a valve with abrasive paste can cut through it quite easily.

If the seats are properly re-cut, lapping will be unnecessary, even with a steel valve, but as Eddie said, someone who knows how to do it right and has the right tools should be the one to do it. It's very important.

This is a 2000 and I ordered parts from oem thumpertalk. Are the vavles I got steel or are they updated to titainium? Or in any case should I leave it alone anyway as they are both new, seat and valve?

The 2000 YZ 426 had steel valves. The '01 and '02 had titanium valves. If you ordered parts for a ' got steel.

Eddie is an authority in building 4 stroke singles and Grayracer knows all things YZf. DO NOT LAP THE VALVES!!! This ain't no mild chevy smallblock you're building.

To the OP. If your friend continues to beat the "lap the valves" drum, find a new friend :thumbsup:

New heads and valves should be usable as is. Both are precisely machined parts, and they shouldn't need lapping. But I have very rarely encountered new cylinder heads in which a valve did not seat, so it's prudent to solvent test the valves for sealing after assembly.

With the valves and springs assembled to the head, tap the top of each stem with a punch just heavily enough to bounce the valve on its seat once or twice. Tap only on the stem, and don't get carried away. Then pour the ports full and see how they do. (Some very slight seepage is acceptable in brand new parts).

"Re-cutting" valve seats, IMO, should only be done by grinding, not by cutting. The top and bottom angles on the seat that are used to establish the width and location of the seat may be cut if that is preferred, but the actual face of the seat should be ground. No cutting tool can match the finish applied by a properly maintained and used set of stones, and the finish of the seat is critical to the longevity of titanium valves, where they are used.

In my manual which is a 2004 yz250F original manaul it clearly states to lap valves.

In my manual which is a 2004 yz250F original manaul it clearly states to lap valves.
And the 1992 Chevrolet Cavalier manual said that the vacuum modulator could be blamed for low transmission pressure when no Cavalier was ever built with a vacuum modulator in the entire ten years of it's production up to that time.

The section of the manual you refer to was carried over from the 2000 models which do have steel valves and may be lapped. Do what you like. It isn't my engine, so it's of no concern to me. But do yourself a favor and read the 2nd and 3rd posts one more time.

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