Good morning. Two brief questions for our resident experts:

First, I rotated a stud on the valve cover (forward starboard side). The stud threads seem good and the head threads seem good, so am I in any danger if I spray them with PBBlaster, wire brush them as best as possible, and then turn the stud back in?

Second, what is this stuff on the inside of the valve cover?







After the adjustment, the car ran smoother but occasionally missed, so I pulled the valve cover and found a few intake valves had loosened out of tolerance. Miraculously, the car (81 300SD with 134k, good service history for suspension work, crankcase oil changes, coolant work, freon work, everything except valve adjustments, all from a "German car specialist.") had several intake valves that were right in spec despite having most of the exhaust valves at .20 mm.

Any thoughts before I put the valve cover back on?

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver

It looks like RTV Silicone. The High Temp Version of the RTV Silicon is often Red.
Why that end was plugged off would be a guess.

I have not seen mine for a long time but perhaps there was some sort of Sheet Metal Insert there that fell out or maybe the Silicon is being used to hold in a loose Sheet Metal Insert.

Or someone simply wanted the Blow-by gasses go in from the other end of that tube before it goes out to the Breather.

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84 300D, 82 Volvo 244Gl Diesel

Whatever it is it was put under the cover or left under the cover by the last mechanic that did a valve adjustment. Is it permetex or some other sealant?
Is the cover cracked? Does it block up a Pcv vent?

Cover isn't cracked at all; I couldn't find an obvious reason for it. I'm not too concerned... more concerned about a stuck valve. I need the mysterious third wrench (I just have the 14s); not about to risk a broken valve guide. If anyone has one for sale (or loan near Seattle), otherwise I might have to buy the whole Hazet set just for the big one.

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver

Upon checking all the valves before installing the cover, I found one loose one (cylinder No. 3 exhaust) so I went to tighten it up...

...and I can't get the locknut to lock.

No trouble dialing in whatever clearance I please with the upper nut, but no amount of turning the lower (middle) nut will get it to tighten with the upper nut set for .35mm.

Now I'm very worried that I "killed" a "brand new" (134K!) engine... I'm sure the nut can be replaced, but that might be beyond my capability... I imagine the camshaft will have to come off.

Ugh!

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver

Actually if you need more room to work you can remove the Rocker Arm Assembly Bolts and yank the whole assmbly out. I did that when I changed My Valve Stem Seals.
On mine I needed the prpoper Sized Allen Socket to do that.
I did not remove the Camshaft.

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84 300D, 82 Volvo 244Gl Diesel

wow... stripped the threads on the middle lock nut... that sucks. I hope the problem is the nut, not the valve... cause then the valve would need to come off.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I'm pretty sure it's the lock nut, since I can tighten the adjustment nut DOWN to grip the lock nut, it just leaves the tolerance far too big, bigger than my biggest feeler (.65mm).

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver