TDC when rebuilding the head

[johnk]

I am about to pull the head off on my 280sl to send it in for a rebuild but want to make sure I am doing the right thing in regards to the timing position. I have the marks lined up between the cam washer and the cam bearing casing, but the timing mark on the crank is about 5 degrees past zero. Also the notch on the distributor shaft is pointing slightly before the mark on the distributor. The small mark lines up with the beginning of the notch on the shaft but not the center of the 1/4 inch notch.

Am I OK to leave it at these settings before I pull the head? Is there anything I should do now or later to correct the differences?

I greatly appreciate the input.

Thanks
John K

[stickandrudderman]

As long as you know what you're expecting to see when you re-assemble it then you don't need to be overly concerned.
I would rotate the crank to TDC, observe and record the timing mark position on the cam and dizzy and then just go ahead and strip.
If you're particularly worried about re-assembly you can position the crank so that no piston is at TDC before installing the head/cam. That way you know you have no chance of bending a valve.
Once the head/cam is installed you can gently rotate the cam/crank to get them aligned before installing the cam gear.
After installation you can verify your timing marks and compare them to what you had when you started. You will then know how the cam timing is different (better) for your work.

[Novamonte]

This sometimes happens when the cam chain gets a bit stretched. There are offset woodruff keys for the cam sprocket to compensate for this if needed. Usually not necessary though if the engine runs well, but make sure that the cam chain tensioner does not bottom out. If it does you should replace the cam chain now when the head is off.

[johnk]

Thank you gents! This was a big help. I aligned the crank to TDC and the cam is only off a very little. On to deal with the chain now.

John K

[alchemist]

I agree with Novamonte, your chain has stretched and this is why your timing is 5 degrees off. If you are going to re-build, it is best to replace the timing chain and the tensioner all together. This should allign the cranck with the cam marks. I hope this helps.

[johnk]

Thanks Zalsaigh. This has been very helpful. I had planned on replacing the chain based upon what I have read so far. I will look into getting a new tensioner as well. I have enjoyed pulling the head off so far, but I don't like having to do anything twice just to save a few bucks.

[Eminent]

I allways thought that this offset woodruff key is used when the head is often decked (this makes the chain "longer").
Why should you put an offset key in and keep an stretched chain in place?
And i believe that the original chain has to be grinded to make it loose so it has to be replaced also when replacing this woodruff key.

[ja17]

Hello Keith,
One chain link is equal to 18 degrees.  Correcting the chain timing using off-set keys is seldom done since it makes no noticeable difference in most cases. 5 degrees of late vavle timing is negligible. Chains usually are fine for 100,000 miles. Replace them if you have a lot of miles or you in there doing work and the time is right for you.  Just comparing the timing marks is not always a good indication. For example... Someone's original timing chain has worn or stretched 24 degrees, so instead of just replacing the chain they advanced it one link (18 degrees). So now the timing marks are only six degrees off, but the chain is dangerously stretched (worn). The timing chains get longer with usage. We often refer to them as being "stretched".  In actuality the chain lengthens and weakens as each single pin or link is slightly worn. You multiply this slight wear by 134 links and you have a longer chain. Cutting the cylinder head increases the length of the chain (amount cut off the head doubled) since the chain travels up then down the cylinder head thickness. Catastrophic chain failure can occur when the chain becomes so long that the chain tensioner no longer takes up the slack in the chain. At this point the chain can jump a sprocket or slaps a chain rail until it fails and falls into the gears. The loose chain then jumps timing and all kinds of destruction can occur.

You can remove the sprocket without "breaking" the chain if you wish to install an  off-set key.  You can even remove the head without breaking the chain. If you wish to replace the chain, a link is ground and removed so a new chain could be threaded into place. A standard chain master link is used and usually comes with the new chain. Original chains were endless (no master links). I have never seen a master link fail when installed correctly.

I check timing chains by inspecting to make sure that the chain tensioner is still taking up all the slop in the chain. Also I automatically replace them at 100,000 miles or when major repairs are made. Many of the original chains had a figure "8" shaped end plate used for side plate of the chain links. If you find this configuration the chain is old and possibly the origina.  Later and replacement timing chains have  oval shaped end plates for the links, which is an improved and stronger design.

Another bad thing about the off-set keys is that correcting the valve timing with off-set keys makes the timing exactly correct, but the off-set keys make the notch on the camshaft washer more out of sync. so that visual inspection is no longer accurate. (the notch in the cam washer stays in the same place but the off-set key advances the camshaft) not a good situation.

These days part of the new chain replacement procedure should be removing the cam sprocket and check to make sure no off set key has been used on the engine before installing a new chain. If an off set key is found a standard key should be installed with a new chain and then the valve timing should be visually checked using the scale and notch. Check with a dial indicator for perfect results,  but usually not necessary.