Valve adjustment nuts, how tight is right ? - PeachParts Mercedes-Benz Forum

Jimmy Joe · #1 05-07-2008, 03:14 PM

Reading through the many valve adjustment threads, I came across this a few times:
"The most important part of the valve adjust job is to get the lock nut tight enough so the nuts never come loose and let the valve drop into the cylinder."

Sounds potentially scary, no?

So, I don't see any reference to exactly how tight is enough. Can't get a torque wrench in there....

#2 05-07-2008, 03:25 PM

Guten tight. About as hard as you can squeeze the two wrenches together with one hand. Probably equivalent to ~10lb/ft.

rrgrassi · #3 05-07-2008, 03:54 PM

Since these valves tend to tighten, I do not see how they could come loose and fall in. If the tended to become loose, then I could see that happening.

#4 05-07-2008, 03:56 PM

They don't tighten, the gap gets more narrow.

rrgrassi · #5 05-07-2008, 03:59 PM

Quote:

Originally Posted by ForcedInduction

They don't tighten, the gap gets more narrow.

Is that because of valve face and seat wear? If so, I wonder how long they can actually last.

#6 05-07-2008, 04:03 PM

Quote:

Originally Posted by rrgrassi

Is that because of valve face and seat wear?

That would be the logical explanation, I can't think of anything else that would cause it.

toomany MBZ · #7 05-07-2008, 05:31 PM

Agreed, no torque values from MB.

1983/300CD · #8 05-07-2008, 06:29 PM

The two nuts need to lock together, pushing against the threads of the valve stem. Tight enough so they stay put.

Since there's no way of getting a torque wrench in there, it's pointless to give a value. Tight enough so they stay put.

nhdoc · #9 05-08-2008, 07:13 AM

Quote:

Originally Posted by Jimmy Joe

Reading through the many valve adjustment threads, I came across this a few times:
"The most important part of the valve adjust job is to get the lock nut tight enough so the nuts never come loose and let the valve drop into the cylinder."

Sounds potentially scary, no?

So, I don't see any reference to exactly how tight is enough. Can't get a torque wrench in there....

I guess I don't see what is meant by "...so the nuts never come loose and let the valve drop into the cylinder". I mean, what happens if the cap nut and locknut is loose? The valve can't "drop into the cylinder" under these or any circumstances that I can fathom by the locknut being left loose.

The fact is if there was a chance of something catastrophic happening as a result of leaving the locknut loose we'd probably all have heard about it by now and there would be countless tales of trouble from it...I just think this may be another urban myth but if someone can explain how it is possible for a catasptrophic problem to result from this I would like to hear it?

Also, the shop manual contains no such dire warnings...it simply says once the adjustment is made "secure cap nut with locknut and recheck clearance".

1983/300CD · #10 05-08-2008, 09:46 AM

Yea, you'd have to be able to remove the cap and lock nuts with the camshaft still in place.

vox_incognita · #11 05-08-2008, 09:48 AM

I`ve found a hint only in om617.950 manual on this topic,saying "secure nuts with torque reference of 20-30Nm"
...last page on:
http://handbook.w116.org/Engine%5C617%5C05-210.pdf
...height of counternuts also mentioned ...

vstech · #12 05-08-2008, 09:57 AM

... aren't the springs holding the valves up?... the locknut just sits on top of the valve end right?

rrgrassi · #13 05-08-2008, 10:31 AM

So, based on the dire warning, I guess the valve retainer is like a politician, just taking up space...

nhdoc · #14 05-08-2008, 10:56 AM

I think it is reasonable to conclude that if the locknut is left too loose that the worst that can happen is that the particular valve in question's clearance will not be maintained correctly but I really don't see it being all that big a deal.

Having said that I can also say that the concept of a lock or jamb nut is very sound and even with minimal torque it would probably keep the adjusting nut from moving out of spec. The only way I think the cap nut would move is if someone completely forgot to tighten the jamb nut, which can certainly happen but would not result in anything too serious other than a valve clearance being wrong, and, most likely preventing the valve from opening all the way or at all. Any of us anal-retentive owners would probably notice this condition immediately once the engine's performance started to be affected.

Diesel911 · #15 05-08-2008, 08:41 PM

Since the Valve Spring pushes on the Valve Spring Retainer that pushes on the bottom Lock Nut the chances of it turning are slim as there is always spring pressure on the nut. I guess it would be possible for a loose top Cap Nut to vibrate/rotate upwards and take up the clearance; but I don't think it can go further.