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Valve replacement criteria.

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I've read alot here about valve replacement but don't see much about the actual failures. I'm working on a '05 RMZ450 but this applies to any model.

Why are valves replaced? What fails? Is it typically burning at the seat? Does the stem fail at the head and drop the valve? Is the stem worn out of spec? I read about intake valve failures. I would expect to see more exhaust failures  than intake as they are exposed to hot exhaust gas as oposed to cool intake air. Anyone have some insite to share?

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It depends somewhat on the particular engine, but in general, the wear occurs at the valve face.  If this were the old days when the valves were just steel, this wear would be dealt with by refinishing the valve faces on a valve grinding machine, after which the tips of the stems might be ground in the same machine to shorten the stem in compensation for the extra height gained by removing material from the face, so as to keep the valve lash in an adjustable range.  However, because of the use of titanium as a valve material, that cannot be done.

 

The reason is that titanium, for all its virtue, is not hard enough to serve as an engine valve.  Harding the metal itself is difficult, expensive, and results in material that would be too brittle to serve as a valve.  The solution commonly used is to finish the valve and then apply an extremely hard coating with very high bonding properties in a very thin layer.  Variants of titanium nitride are usually used for this, and the coating is normally less than .001" thick.  Done correctly, this produces a very strong, very durable valve in all respects, and if combined with the right seat material so as to reduce wear at the valve face, the valve will have a good, long service life. 

 

Eventually, however, the coating will wear through and the seat will start to wear into the softer underlying titanium at a vastly faster rate.  The valve may wear off center, causing it to scrub and twist at the stem/head junction as it closes, and that can lead to a broken valve and massive destruction if it happens at speed. 

 

Here's what the wear looks like:

 

Untitled-4_zpsb586445e.jpg

 

The valve in the foreground is a normal, healthy used valve.  The one in the back is worn well beyond usability.

 

As to criteria for replacement, when working with titanium valves, any reduction in clearance that requires more than a .05 mm smaller shim should be regarded as a bad sign, and anything that requires more than .10mm almost certainly needs work.  Also, if the machine will not stay adjusted at the correct clearance for more than just a few hours, this is an indication that the hard coat is gone.

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Thanks grayracer513. Good info. So is it high risk to reuse valves if they look fine? How do SS valves compare to Ti? Are they better or just lower cost?

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Hi, I am new to this site and need some advice regarding the inlet valve wear on my RMZ450 (06) as I am having to re shim every 2 hours to get the correct valve clearance. Clearance on one of the inlet valves goes to less than .005mm from the specified clearance in less than 2 hours and I am down to a 160 shim now. Is there a risk that the valve will fail and should I therefore be looking to replace the valves asap or do I not need to bother until I can no longer get a small enough shim?

 

The exhaust valves have not needed shim adjustment for over 20 hours remaining at a constant .018mm clearance and the bike runs as it should

 

Thanks

Kevin

 

Just reading the previous posts it looks like I need to change the inlet valves very soon, will I need a special tool to replace the cotter pins and is it just a case of removing the old valves and putting the new ones in or do I need to bed in the new valves and replace the guides or anything like that?

Edited by London27

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The intakes need immediate replacement.  The seats have to be professionally refinished to restore the stock angles and widths (a machine shop operation).  NEVER lap (or "bed in") a titanium valve with any sort of abrasive, as that can easily damage the hard coat to the extent that the valve life will be reduced.

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