Automatically unadjusting valve clearance!!

[craigb]

On a recent country commute, my 69 280sl started making a very loud tapping noise. It was dark and under torchlight I figured it was coming from under the cam cover. I removed the cover and restarted the car and convinced myself it was coming from a sloppy cam chain. I limped into the nearest town (only one store that of course was closed) figuring the damage would be less than the potential of leaving the car on a quiet highway. My good wife drove 1.5 hours to rescue me with our young children asleep in the car and I rescued it the following day with a trailer.

Today under good light I easily spotted the problem of one very loose rocker. The cam surface looks fine but all that bouncing around has damaged the bit that sits in the valve retainer cap (and I guess I should also replace that cap) and the ball that adjusts the clearance.

Has anyone heard of this problem before? My understanding was that these were a tight thread and hence no lock nut, you just need a special or good spanner to do the adjustments. Currently I can spin that ball up and down by hand. Or is it the bit the ball screws into that causes the tightness or interfence, like a nyloc nut?

A fellow aussie Bob Smith put me in touch with this site and a post about cam chains. It looks like the tensioner only attains full force from hydraulic pressure from oil when the motor is running. I am trying to work out whether I did in fact see some play in the cam chain and whether I should be replacing anything in that area. The car has done 106k miles. I have heard the main wear is in the chain and cam gear normally. I removed the top chain guide to get a better look and noticed virtually no marking on this so the chain is obviously not slopping about much in this area.

Anyway, to focus this ramble of mine into questions:

1. Is it common to have the valve adjustment open up by itself and is there a trick to fixing this problem?
2. Is it the thread/nut bit that screws into the head that stops it coming undone and if so should I be looking at replacing all of mine?
3. What is a normal life for a cam chain, cam gear or tensioner?
4. Is there a definitive test as to whether these items are worn? Certainly if I rotate the crankshaft in the right direction the chain feels tight and the tensioner is extended to take up the slack, but if you back it off a bit in the other direction the tensioner pulls in and the chain goes slack.

'69 280sl

[enochbell]

Craig,

The ball stud has two sets of threads that have a slight offset, which is how the friction is created  that makes the stud keep its position once adjusted.  Over time, with repeated adjustment, the threads will wear and the stud will loosen, which is what has happened to you.  Replace the stud and you will fix this.  Check the others, as it is reasonable to assume that if one is bad others may be also.  The ball stud should be really hard to turn, sometimes I have to remove the rocker and put a socket on one or two to get them started, mine are new and have only been adjusted twice.

Sounds like you may also have to replace the crown and retainer on the offending valve.

Glad you made it home OK!

g

'64 230sl, fully sorted out...ooops, spoke too soon

[craigb]

Thanks for that. So that's how they do it! I adjusted the valves probably about a year ago and they were tight as tight from memory but will recheck.

'69 280sl

[hands_aus]

Hey Craig
Welcome aboard.
The search function (top right) works great. Type in something like 'valve adjust' and see what you find.
Search offers you the chance to search by username. Look for information by ja17 or BenzDr they are gurus.

Valves have been discussed many times with lots of invaluable information added.

here is a start http://index.php?topic=5608&whichpage=1,adjust

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[craigb]

Thanks Bob. This is like travelling overseas and meeting someone you know! .... but also having been told about a nice place to visit.

'69 280sl