engine timing puzzle

[vox_incognita]

Gentelemen,here is a baffling one:
You have an engine that was rebiult(1982),probably the head shaven to some extend(you don`t know how much).I aligned the cam marks and it read between 10 and 6 degrees BEFORE TDC. Above all,the chain still incorporates the c-clip (20+ years).I found an old cam sprocket and chain tightener sprokcet with badly worn teeth in the basement,the ones installed look o.k. though..
What should I do first to establish the chain elongation? Would the 2mm intake valve travel still give an accurate reading in this case?Or the radical decision-change evrything(chain,new master link,guides etc.).And no,I do not know if it`s still the original chain in there Perhaps some woodruff key installed in the rebuild that gives the before TDC readind?...Too many unknown things here I really need the sequence of steps...
I did the search,believe me
Vengerov, w110 200D om 621.918
Thanks!!!

[t walgamuth]

but are you sure you know how to determine btdc from atdc? it can be a little tricky.

tom w

[Ralph69220d]

The fact you have circ-clips means that chain has been replaced. The 2mm protocol is the most accurate, but you can line up the cam mark with the cam tower and then look at the crank pulley to see if you need a new chain. Make sure the chain tensioner is functioning properly as insufficien tension is going to retard the cam a bit. No need to replace the chain if cam agrees with crank. But, if chain shows obvious wear marks then replace it.

[vox_incognita]

Originally Posted by t walgamuth

but are you sure you know how to determine btdc from atdc? it can be a little tricky.

tom w

The big line between O&T letters on the harmonic ballancer determines TDC,I believe Pointer seems ok to me.Looks like chain/rails/tightener replacement time.I wouldn`t like to take a chance on those parts with unknown history.

Ralph,I alligned the cam tower/cam washer marks several times(yes,a fracion of a mm there gives couple a degrees change on the crank ballancer),but it still read BTDC.

Thanks for the replies.
And if ,after the replacement of the abovemantioned parts,the the prescribed degrees ATDC-using the 2mm.method -are not reached(due to sprocket wear or head height),I shall correct it by the correct woodruff key...Right?And religiously observed OCI`s

[Wes Bender]

Originally Posted by Ralph69220d

The fact you have circ-clips means that chain has been replaced. The 2mm protocol is the most accurate, but you can line up the cam mark with the cam tower and then look at the crank pulley to see if you need a new chain. Make sure the chain tensioner is functioning properly as insufficien tension is going to retard the cam a bit. No need to replace the chain if cam agrees with crank. But, if chain shows obvious wear marks then replace it.

I concur except for the chain tensioner bit. Insufficient tension due to chain tensioner wear or malfunction would not retard the cam. (Unless the slack becomes so great that it can jump a tooth, then all bets are off.) If you analyze the chain path, you can see that the chain tensioner is located on the "loose chain" side of the camshaft sprocket. The camshaft sprocket is being pulled by the "tight" side of the chain as it comes from the IP and the crank sprocket.

Chain wear (or stretch as many prople call it) will cause the IP to be retarded and the camshaft to be somewhat more retarded.

[Brian Carlton]

Originally Posted by vox_incognita

Gentelemen,here is a baffling one:
You have an engine that was rebiult(1982),probably the head shaven to some extend(you don`t know how much).I aligned the cam marks and it read between 10 and 6 degrees BEFORE TDC.

If you are postitive that the reading is BTDC, I'm going to guess that the alignment between the crankshaft and camshaft is off by one tooth. IIRC, it's about 20° of crankshaft rotation for one tooth of the camshaft. So, if the installation of the head went badly, the indivudual probably lost the chain and ended up one tooth off. If you are reading 10° BTDC, moving the camshaft by one tooth would result in timing that is approx. 10° ATDC. This would be an approximation, of course, and you should then check it via the 2mm valve lift method. If the head has been milled a bit, it's possible that the valves would be late by 10°. You could then add a proper Woodruff key to correct this.

If you choose to move the camshaft by one tooth, please post again with the new figures for timing at the crankshaft.

[Ralph69220d]

I was wrong on the tensioner contributing to the problem. I had it in my mind that the chain guides between cam sprocket and crank sprocket were on a pivot, which I believe I'm wrong on.

[vox_incognita]

Thanks once again...The valve travel method on om621 seems to be different than on om 616(zero experience),but i`ll give it a try...
Brian-a tooth off without valve kiss and no starting/running problems-maybe the start of delivery is off too..it`s been 20 odd years...Anyway,the chains goes out.Thanks!