Motorraum: Neuer Kettenspanner am M70 Motor

[Erich]

Cam shaft timing:
This may be the single biggest overlooked area of the V-12 as it ages! The timing chain stretches and as it does, the cam timing relationship to the crank is retarded, worse yet is that the (left) drivers side cam retards by a
smaller amount then the (right) passenger side cam resulting in uneven power between the left and right banks. M70 cams are timed at a rather mild 248 degrees so any retardation will further lower the Torque and HP!

How Much stretch v.s. degrees?
I have measured 3 degrees and 6 degrees respectfully left, right to the crank. This is around 1.5 (left cam) and 3 (right cam) crank shaft degrees which doesn’t sound like much but will effect HP and of course smoothness!
The latest measurements that were taken were on engines that were in mid life, around 120K-150K miles. I have also seen extreme chain wear on engines that had less then 50K miles! I don’t want to create mass hysteria
here but, IMNSHO cam timing should be checked anytime the valve covers are removed for any reason particularly on engines that have over 100K miles, visual evidence of chain wear, or suspect maintenance.
There is only one method to measure and correct the cam timing! Use the correct tools as recommended by BMW, any other method will result in poor results. The cam timing CANNOT be set by eye, one MUST use the
correct tools which consists of a crankshaft lock pin and two cam shaft tools. Proper procedure must be followed to insure engine is firmly locked at TDC (top dead center cyl #1) and all slack is taken out of the chain
ON THE TENSIONER SIDE ONLY before final tightening of the cam gear retaining bolts.
Does this suggest the timing chain should be replaced? Not necessarily! There are no reported incidences of timing chain failure on the V-12, it is a BIG job to replace the timing chain and cannot be done without removal of all front covers of the engine. The original and replacement chains do not have a removable links.
 http://www.mwrench.com/Whitepapers/M70S70%20info.pdf