Valve adjustment oddity, or not?
 

In adjusting my valves on my '81 300D nonturbo I am perplexed with an odd occurence. After adjusting a valve to the right gap and tightening the lock nut and adjusting nut, I check the gap again and all is good. But when I use a wrench to spin the valve stem using the two nuts, which turns the valve stem and the spring assembely, the gap changes, even though the nuts are tight. The gap will get either tighter or looser, but if I keep spinning it it will return to my original adjustment. That is, if it gets tighter, by spinning it further it will eventually loosen up. Is this the sign of bent valve stems or a need to overhaul the head? If not, then is there a trick to adjusting the valves that will eliminate this occurence? It seems that adjusting the valves correctly will only result in a change in the gap as the valves rotate when the engine is running?
Thanks for any help,
Rusty

I'm not one of the knowledgeable people around here, but I think you have to hold the valve spring still or it's a PITA to get it done right. There's even a special wrench for it I believe. Dieselgiant doesn't mention it in their tutorial.

..thats b/c its not necessary to hold the valve spring....

My mistake. I thought that the 3rd wrench they were talking about here was for that:
http://www.peachparts.com/shopforum/diesel-discussion/190113-any-luck-getting-valve-adjustment-wrench-cheap.html

The valve spring assembly and collar do not spin when I do the adjustment. That part is fine and dandy. It's after the adjustment is finished and the lock nut is tight that I have the problem. I use the nuts to rotate the nuts, spring, and collar (the valve itself does not turn) and the gap changes as described above. I have the wrenches and know about holding the collar steady when adjusting.
Thanks,
Rusty

The question that begs to be asked is why are you trying to turn the nuts after you have adjusted them?

Very good question that I had to ask myself. Had I not turned them I could enjoy not knowing. It actually started by accident when I had to turn them on one of the valves to get the wrenches out. After turning them I checked the valve clearance again to make sure and found it had changed. I then started messing around with turning the nuts after tightening them and found this problem. Maybe it's not a problem and I have decided to live with it until my next valve adjustment. I did go through and readjusted all the valves (practice makes for speedy work) and left them at that without messing with them. I'll check the valves on my next adjustment and see if anything has changed.
Rusty

if the engine runs good w/out smoke, blowby,misfire, or knocking, your valves are just fine.

That's an interesting observation. I'd be curious to know how much the clearance changes when you rotate the valve.

What's occuring is the fact that the valve seat and the valve are not anywhere near close to perfect anymore and the valve doesn't quite sit correctly on the seat. In one position, you'll have the valve sitting .004" high and in a different position, it will sit .001" high. This gives some solid data that the valve doesn't perfectly seal to the seat.........which is likely in an older engine. Note, however, that a gap of .003" is negligible in the total picture and the amount of air leakage is small through such a gap.

When the head is reconditioned, this gap is eliminated and, I conclude that it's one of the reasons an engine with a reconditioned head performs so well.

A final conclusion on this topic is that the same engine has significantly improved cold start capabilities..........again, because of perfect sealing of it's valves.

The exhaust valves get the most wear from what I've seen. They end up wearing a groove all around the valve face.

Rotation of anything below the cam follower. Other than adjusting the clearance nuts might change existing wear relationships to some extent.
I agree that with fresh guides properly reamed and valve stems with little or no wear it is not much of a factor. Certainly it is not advisable to rotate a valve spring on an engine with some guide and possible stem wear. Just a guess it would change the existing lateral pressure location on a valve stem to some extent.

How much is a worn valve guide? If there's .003 clearance..........that's heavily worn. This would allow the valve to move .0015" radially from center...........and the effect on the valve face would be about .0005" or less. So, unless the guides are seriously worn (.006" or more) and the engine is consuming a lot of oil, the guides cannot cause this issue.

If the valve stem seals are still inact they might mask a lot of wear in the guides. Unfortunatly a lot of mercedes engines have a lot of wear because of their accumulated milages.
I can only relate to their gas engines in this area. I have seen mercedes gas engines with no real amount of top cylinder ridge wear evident yet the bronze guides were worn to death. The inside of some guides where worn so bad the recessed projecting portion of the guide that the seal sits over was partially worn away.
Another thought was if the gentleman rotated the valve spring for example and the edge of a valve lifted off even a little the clearance should increase not decrease as he experienced.
I suspect the seats get worn in a complex way as well when the guides or guides and valve stems are poor. Almost an oblong seat enterance and exit ramp perhaps. This may have produced his results.
The valve almost has to relocate itself as it closes. It initially does not hit square on its ideal seating position. So really good sealing is also questionable. All the improvements you have seen with the head you had sent out may have been a combination of many factors. No doubt in my mind a valving system in really good condition is worth the effort..
For discussions sake. Say you have an older engine that runs pretty well with known good compression and lots of accumulated miles. You suddenly burn a valve at some point. During the repair you have all the guides, valves and seats returned to specs. The car almost always seems to run better afterwards and usually starts easier.
One of my guesses is that even if you have good compression prior to the failure with a compression gauge at very slow rotational speeds that sealing might not be duplicated at operational speeds with a lot of wear present. The deficiencey may even start to appear as soon as rpms elevate any higher than cranking speed during the start proccedure. The valve may almost have to slide into the best available closing position. There may be no consistancy of that either. An additional contributor is the old valve springs may have lost some of their strength as well. At higher rpms there may not be enough time for the valve to aquire the best available seat position. Mass/inertia problems accelerate.
One nice additional feature of the diesel is it might tollerate a minor leaking intake valve better. Since it is not back bleeding into the fuel input mixture. Of course that too may be offset by the need for greater basic compression.

I'll investigate the difference on my next valve adjustment. From the insights I've read here it sounds like I might be experiencing the first signs of valve wear. The engine does have 187K miles, smokes significantly when cold but very little when warm, is not eating any oil that I can tell, has no cold start problems (it's not very cold in CA where I live) and seems to have reasonable power for a non turbo. I will do a compression check soon. What type of readings would indicate valves not seating correctly or worn guides?

Thanks for the info,
Rusty

With the description of the engine that you provided, I'd doubt that the compression figures will be meaningful. Remember, there are very few "absolute" numbers unless they are very low. So, you'd need a history on the engine with the exact same test setup to determine if the valves are slightly off their seats. The compression might drop 20-40 psi (as a guess) during cranking speed. It would not be immediately apparent that this is a problem from the numbers, provided that they were all within proximity of each other.

I wouldn't concern myself with the "problem". Personally, I'm just curious as to the actual data.