Re: IML: Timing chains

["Pete Engel" <peter.engel@xxxxxxxxxxx>]

Robin,

The original setup on about 90% of Mopars was a Morse silent chain, NOT a
single or double roller style chain.  Technically there are no gears used
because they are SPROCKETS.  (Gears mesh directly with other gears;
sprockets are connected by an intermediate component - a chains.)

The cam sprocket was an aluminum die cast part with nylon teeth molded onto
the aluminum.  I've done internal engine work on cars built as early as 1961
and those cars all had the aluminum/nylon cam sprocket.

The supposed reason for using the setup was noise reduction but I've never
heard what frequency range was affected.  There's so much fan noise and
other stuff going on that it's hard to imagine the timing noise being
objectionable.  Still, all the "Big 3" did it so there must have been sound
logic behind it.

I know that GM used the aluminum/nylon sprocket until at least 1987 because
I've repeated seen 3.8L V6 engines that were killed by failed cam sprockets.
On the 3.8L engine the pistons slam into the valves when the timing chain
slips so the fix is very expensive.  My local Oldsmobile dealer (now gone,
sadly) recommended replacing the timing components at about 80k miles.  Woe
to the customer who failed to heed that warning and learned the expensive
lesson.

On Mopars I've routinely run the factory timing setup to 100k or 110k miles
but then you're definitely living on borrowed time.  Fortunately, Mopar
engines (at least the ones we're concerned about) don't have piston/valve
interference issues if the chain does slip.

I should add that neither timing chain/sprocket setups nor timing belts
usually fail while the engine is operating.  It's usually a startup issue.
When you start the car the crankshaft suddenly spins the crankshaft sprocket
which then tries to get the camshaft moving.  On chain type setups the chain
moves but shears the nylon off the cam sprocket.  On belt setups the
crankshaft sprocket simply shears off the timing belt teeth that it is
contacting.

Others mention erratic idle and other driveability woes as problems caused
by loose timing chains but that hasn't been my experience.  You will see an
unstable timing mark if you strobe the crankshaft but I sure can't feel the
problem.  I have seen one 440 that ran long after ALL the nylon had been
shed from the cam sprocket.  Even the aluminum nubs that hold the nylon in
place were severely worn.  One day the chain slipped and the engine simply
wouldn't start.  We installed a new timing set and the car was returned to
service.  Here's a pic of the worn out sprocket:

http://bos-engel.com/imperial/sprocket.JPG

Pete in PA



From: "Robin Giesbrecht" <robings@xxxxxxxxxxx>
Subject: IML: Timing chains
Date: Tue, 22 Feb 2005 11:58:32 -0700
Reply-To: mailing-list@xxxxxxxxxxxxxxxx
I have several questions on timing chains for the big block mopar. Were the 
original chains single or double row? I hear talk of nylon gears? What years

were these used, if at all? Are the 383 and the 440 timing chain and gears 
interchangeable? Which engines/years had the three bolt hole camshaft 
sprocket?

Robin Giesbrecht
72 Imperial



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