chain stretch (was re: E28 valve noise)

[Bob Sutterfield <Bob@xxxxxx>]

Chris Apostol wrote:
> I noticed below mention of the chain "stretching."  
> How is that possible?  A rubber belt I could see, 
> but a steel chain stretching seems like a long 
> shot to me.

A chain is a mechanism of parts, and those parts wear over time, like any
other mechanism.  Between every pair of links is a pin, basically just a
rivet pressed through the side plates, encircled by a bushing on which the
cog's teeth bear.  As the side plate holes wear, the pin can shift,
permitting the links to move farther apart under tension.  The sum of those
movements is called stretch, because the chain becomes measurably longer
over time, even though the steel links themselves don't stretch in the same
sense a rubber belt stretches.

As the chain stretches, the pins become farther apart than they should,
causing them not to ride correctly between the cog's teeth.  This causes the
teeth to wear into a "shark fin" pattern.

If your bicycle chain has been used for more than a few thousand miles, you
can probably see these symptoms on its chain and sprockets.  Measure 12
links on a new bike chain (exactly 12 inches) and 12 links on an old chain
and you'll see quite a difference - maybe as much as a quarter inch.  I
don't know the link pitch on a timing chain, but you can look for signs of
shark fin wear.
- --
Bob Sutterfield
'87 E30 325iS http://bmwe30.net #1129
'88 E28 535iS http://m535i.org   #154
BMWCCA #169277 Rocky Mountain Chapter

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