RE: lower timing chain question (74 GTV)

[David Rubie <David.Rubie@domain.elided>]

> In V9 #62 Eric Lutkin comments: I have a 74 GTV with ~210K 
> miles on it.
> I recently had it smogged and in doing the work to pass the smog test
> the shop informed me that I
> have a loose lower timing chain and that this is a 
> significant potential
> problem that requires major engine disassembly to fix.
> 
> **************
> 
> My first reaction was, "How the heck did they diagnose that one?"
> 
> Fortunately in #63 Russ Neely also seems a bit dubious, but does admit
> that with that kind of mileage, it might be shot. He also suggests you
> might simply take the chance and drive it, with the 
> understanding should
> it break, kiss your valves good-bye.
> 

I wouldn't drive it much until I'd had a bit of a look at the state
of the chains.

It's easy to do once you can see how they work, although you might
need access to a car in better condition to check the differences.

The lower timing chain wraps around the crankshaft, and drives a smaller
gear mounted about half way up the timing case.  The gear can clearly be seen
if you take the valve cover off and look down the front of the engine.

             Top timing chain
          ---------------
          *             *     (* is camshafts)
          \             /
           \           /
         ==*\         /   (==* is chain tensioner.  Push into chain to tighten)
            |        /
            |       /
            \      /
             \  @ /     @ is reduction gear
              ----
              /  \
             /    \       Bottom timing chain
             |    |
             \  @ /    @ is crankshaft
              ----

OK - so ASCII art isn't my strong suit, but the diagram is meant to
demonstrate the relationship between the two chains.  The reduction gear drive
is where you'll pick up whether the bottom chain is slack or not.

Slack top chains are fairly common on high-mileage Alfa motors and easy
to diagnose:  They make a rattling sound when you back off, and start cutting
tracks into the cylinder head and the valve cover (most older Alfa motors
have these marks on them.  Simply pushing with your finger on the chain
between the two camshafts will reveal how slack it is.  It should be
tight, not able to be deflected much (if at all).

A slack upper chain can sometimes be alleviated by proper adjustment of
the timing chain (there is a bolt on the front of the timing cover
that released a spring loaded chain tensioner).  If the chain tensioner
has no adjustment left in it, it's time to replace the upper chain.
If you're careful, you can replace the upper chain without removing the head or
the timing case.

The lower chain is harder:  There is no chain tensioner (not required, the chain
is very short).  The timing case (and cylinder head and sump) have to
come off to replace it.  You can check if it's slack by carefully rotating the engine with
a breaker bar/socket on the crankshaft nose and by peering down into the
timing case.  Alternatively, put the car in 3rd gear and have an assistant
move it while you're looking at the chains.  Best to slightly rock back
and forth so you can see the take-up slack in the chains.

If the crankshaft can move too much before the lower chain
activates the gear that drives the upper chain, you have a problem.  This chain
can, and does, snap, leaving you with an even bigger problem (bent valves).
It should be tight i.e. no visible slack.  When assembling these motors, it's
actually quite hard to shove the assembly onto the front of the motor due
to the tightness of a new chain.  If it isn't tight, you can end up with all sorts
of valve timing problems anyway.

It's relatively easy to check the state of the lower chain by this method.
It's a bit of a pain to fix, although all the Alfa's I've worked on, you can
do it with the engine in-situ if you try hard enough.  Both chains should be
replaced on every rebuild, regardless of their apparent condition.

dave.
1978 Alfetta GTV "trusty"
1975 Alfa Spider Veloce "rusty"
1994 Volvo 850 T5 wagon

--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided