Engine Valves not Rotating

Paul Sutton

Paul Sutton

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I have just finished reinstalling the engine in my SH following a +50 rebore. After 20 miles I checked the valve stem ends and the left side valves are not rotating. This is shown by a single scuff on the stem end rather than a circular pattern.
  1. The old pistons have no dents or marks from hitting the valves historically with previous owners.
  2. When the engine was removed both right hand valves and adjusters were in excellent condition and the stem end showed the expected circular pattern. However, both left hand valves had a linear wear mark and the adjusters had corresponding linear grooves worn into them. This all shows the valves were not rotating prior to my ownership.
  3. During the rebuild the stem ends were reground, one tappet replaced and the other repolished smooth.
  4. All 4 valves were relapped by hand and the valves moved freely in their guides. Each valve cut a pencil lead mark cleanly when rotated.
  5. Following the lapping both heads were filled with petrol and showed zero leakage and identical evaporation rates with time.
I realize that eventually the ends of the stems will get damaged again along with the adjuster screws.

Just how important is this valve rotation? (I do not see how rotation can be maintained with elephants feet)
Do I just wait until the stem/adjuster damage occurs again and then fit 8mm Kedo flat ball adjuster screws?
Is there something else to be checking to restore valve rotation?

I do not really want to pull the engine again but will if necessary. It will only cost me a head gasket. Time is quite plentiful for me at the moment.

Thank you for any thoughts and ideas - Much appreciated.
 
Hey, Paul. Just a couple wild guesses here.

- Valve springs possibly installed upside-down. They're progressive wound, and would have an influence on valve rotation. Search for "spintron".

- Valve keepers (collets) fitment with valve stem and spring collar. There is old-style, new-style, and aftermarket parts. That's at least 3x3x3= 27 combinations, some tolerable, others not (like the square vs round keeper grooves).

That's all I got...
 
Thank you TooMany. I believe I will have to take the head off again but will wait until my research is complete. When I took the valves out the springs looked to be symmetric and the ends were identical. The collets matched the valves with round grooves. I may have to compare in detail the rotating valves with the non to see if I can spot the difference. I would prefer to solve this mystery.
 
I have removed the valves and set each in a lathe and used a dial gauge to check for straightness. All four valves show the same degree of deviation so I conclude the lack of rotation with the left hand valves is no due to a bent stem. I see no obvious reason for the valves not rotating when comparing left with right. Perhaps I should lightly polish the spring ends to ensure they are moving freely to allow rotation???

Edit: The progressive springs were all set the correct way around.
 
Well, maybe the next thing to check would be the adjuster tips. The original OEM tips have a spherical radius of about 8mm. I advocated using 19mm as the radius, as this would potentially reduce valve clatter by allowing for a broader oil hydrodynamic cushion. GggGary tried out a set of these reground adjusters, and noticed a similar straight-line scuff pattern on top of one of his valve stems. It appears that the broader 19mm radius makes initial contact with the valve top outside the normal torroid groove pattern, and may not be applying its scuff load at the right place to induce rotation.

http://www.xs650.com/threads/yamaha-xs650-valve-train-geometry.40042/

http://www.xs650.com/threads/xs650-valve-and-clutch-adjusters-resurfacing.41639/

Especially this pic:

http://www.xs650.com/threads/xs650-valve-and-clutch-adjusters-resurfacing.41639/page-2#post-461038

So, this adjuster tip radius thing is still an experiment and open to debate...
 
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following this , Very interesting ! Perhaps after measuring the valve stems for straightness (run out) and determining that in fact the valve stems are "straight" , have you considered using gauge pins to "feel" the valve guides for size ? Or even harder, alignment ? just curious how this is going to turn out , thx -RT
 
I have bored a couple of pieces of brass on the lathe to allow me to get a very close approximation to the radius of the adjuster screws. The two giving rotation on the right are not damaged at all and look to be bang on 9mm radius. The left hands ones are damaged on the tips but look as if they too were 9mm. I have a replacement VW adjuster and it is approximately 11 - 12mm radius. I must regrind the damaged adjusters and the VW one to 9mm radius this weekend once I get my lathe setup for the job.

I have measured the stems using a micrometer at three points (Topish, middle and Bottomish) and they are not worn oval and they are within specification. Have also put a straight edge along the stems and I am satisfied they are not bent.

Unfortunately I do not have a way to measure the bores of the guides. I did notice that there is some carbon in the bores but the left side ones look as sooty as the right side ones.

When I originally stripped down the engine the good adjusters did not have a mirror finish, more of a 600 Grit appearance like the VW adjusters. The damaged adjusters did however have a mirror finish.

Must keep up the search and also finish reading all of 2M's links above - Thank you.
 
I'm assuming the rotation of the valve is achieved by the contact path of the rocker being offset from the centre line of the valve stem.
Therefore if the rockers are not offset no rotation will occur.
Have you checked this is the case?

This may be of interest.
 
Sounds like I need to do an update to the "valve train geometry" thread, addressing the "in motion" relationship between the adjuster tips and the valve stem. In that thread I show the total 'scrub' values as 0.41mm, but the scrub rate isn't a constant, and where those incremental scrubs occur relative to the stem center may be revealing. Gotta cogitate on this for awhile...
 
Max Midnight, I read the article and have checked the position of the marks on the stem ends and they seem to be in the right places when comparing both cylinders i.e. off center. On another forum relating to diesel engines it was said that valve rotation is really only important for engines that produce a lot of soot. I wonder if the importance of rotation may not be too high for the XS in terms of valve face wear considering the successful conversions to elephant feet. Certainly the lack of rotation seems to result in more severe wear marks in the absence of rotation and I say this because my wear marks are already noticeably after 20 miles on my left side valve stem ends. When rotation occurs the stem end/adjuster will probably get better lubrication as the adjuster strikes a freshly oiled surface more often.

In my original entry above I raised the idea that if I cannot solve the rotation mystery then perhaps I should just switch to those Kedo flat ball adjusters???

Unfortunately I am tied up with work this weekend so cannot do anymore exploring.

Thank you everyone for your thoughts and suggestions.
 
Last edited:
2M, your suggestion of an update regarding valve train geometry would be interesting as I have learnt so much from your other threads which I am still discovering more of as I dig deeper. I must measure the scrub values next week and set my lathe for grinding those 9mm radii.

Thank you.
 
Been doing a lot of reading on valve rotation, lots to dive thru.
But, one thing caught my attention:

Valve rotation occurs at the higher RPMs.

I take it that you're in break-in mode, and keeping revs low?
 
2M, thanks for that. On a diesel site it was stated that rotation does not occur in the bottom 25% of performance. On the running in I have been varying my revs and the right hand side is showing rotation marks for both inlet and exhaust. It has also been suggested by a friend of a friend that the valve faces may not be perfect on one side and this may be inhibiting rotation. I will check this tonight in the lathe with a dial gauge.
 
Update: I gave the valves an extra lapping then thoroughly cleaned the stems and the guides. I needed to use some Autosol polish on a cloth wrapped around a pencil to clean the residues out of the guides. I then reassembled the bike and rechecked the valve gaps and the end of the stems for rotation after approximately 400 miles. The good news is:
  • The genuine VW lash screws are holding up and the gaps have remained unchanged.
  • The end of the valve stems show that rotation is occurring!
Perhaps it was all the cleaning and the use of higher revs that got things working correctly again........

...................Thank you for all the advice given.
 

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