re: M42 Timing chain life span?

[Neil <neil@xxxxxxxxx>]

Michel,
The timing chain should never wear out under most circumstances, and
should never stretch* enough to cause problems. The belt-drive versions
should be replaced every 50 k or whatever (and even that's breakage VS
stretch issue), but the actual chain drive versions should last the life
of the motor, though I guess that it's possible that tracked cars could
wear the chains or cogs out (but don't know).
I replace the hydraulic tensioner in our 318is a year or so ago, and
haven't had a problem since. Make sure that you have the newer design,
as the old design had some operational flaw, though I can't remember
what it is.
If your timing chain is too loose, it would be because the tensioner is
not doing it's thing, rather that the other way around. It's possible
that yours either had a manufacturing defect, but it also could be that
there was/is something in your oil that plugged the hole in the
tensioner, or there is an issue with your oil delivery or pressure (it
is spring assisted, but is mainly tensioned by oil pressure). I believe
that they are supposed to last 50k minimum. Ours lasted 90-100k.
If the chain were too loose, aside from the horrendous noise it would
make, it could jump the teeth on the cog, causing real unhappy things to
occur in your engine. It would also run like total crap if it ran at
all.

*I say stretch, but that's not _really_ what it is. The bushing/rollers
of the chain wear, which wears the teeth on the cogs, resulting in a
chain that is longer than it started out. Not so much a problem in an
engine (unless it's a Honda CBR F4i), but more of a problem with
motorcycle drive chains, bicycles, etc. with exposed chains that collect
dirt and grit.

Neil
'91 318is <-110k
'86 M535i <-187k

<snip>
Does someone know at what interval a timing chain must be replaced?

Is it something that needs to be replaced as a preventive maintenace
like a belt or it should be replaced only when it rattles or cause
drivability
problems?

I'm asking that because yesterday I changed my chain tensionner. The
tensionner suddently compressed itself into the body of the tensionner,
and was stuck fully compressed. That tensionner is not very old. It was
replaced 50000km (30000mi) ago.
Could it be the timing chain being too slack, that pushed the tensionner
violently and caused it to jam fully compressed? My enigne has
183000km(109000 mi).

Now I have a new tensionner and the engine runs fine, but I would like
to prevent catastrophic failure like broken timing chain or chain guide
collapsing etc.


</snip>

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