OT: timing belts

From: Gordon JC Pearce <gordon_at_gjcp.net>
Date: Sat Dec 25 03:43:59 2004

Joe R. wrote:
> The '68 chevy didn't use a timing belt, it used a timing chain. Just
> like all the REAL American cars! Personally I wish they all still did. I
> hate these stupid rubber timing belts! All they're good for is generating
> revenue for the delaers when they slip or break and you end having to buy
> half of a new engine or a new car.

Well, that's just down to stupidity. A new timing belt costs a tenner
at the most and even on an absolutely evil b*st*rd of an engine to work
on (step forward, Citroen XM 2.5TD, not nearly as nice an engine as the
CX 25DTR) it takes at worst a couple of hours to fit. I can do Volvo 2-
and 3-series belts in about half-an-hour...

It's cheap, it's important, and it's something you can easily do at
home. Got to be a bit careful with diesels, though - sometimes you need
a locking tool to stop the pump moving. I change them on any car I buy
as a mere matter of course - think of it as (and here is where we swerve
briefly ot ON-topic) similar to pulling all the boards from a classic to
make sure the power supply isn't putting out the wrong voltage, the
first time you power it up.

As to why we use timing belts, well... The problem with timing chain
designs is that the chains wear and go slack. This means you either
need complicated hydraulic tensioners (like on the Peugeot/Renault/Volvo
B27/B28 V6 as fitted to Volvos 260s, Renault 30s, Deloreans and the
like, or Maserati V6es as fitted to Citroen SMs, or Triumph Stag V8s -
all notorious for timing chain failures), or just allow the chain to
slop about once it's worn (pretty much every other OHV chain-driven
engine). The problem with that is that if there is a lot of slack in
the chain, the position of the cam shaft (and hence the valve timing,
and on most OHV engines the ignition timing) can be out by as much as 5
degrees. If you've ever fitted a timing belt or chain to a car one
tooth out (ummm, nope I've *never* managed to do that, then wonder why
the engine was really quiet but down on power) you'll know how much of a
difference that can make. Now imagine that instead of being "out" by a
fixed amount, it's randomly flapping backwards and forwards plus or
minus 5 degrees...

Of course, the Ford Essex 3 litre V6 in my Scimitar solves the problem
by having no belts or chains at all - just two big gears. The large
gear uses fibre teeth which wear after a few hundred thousand miles, but
that's another story....

Gordon.
Received on Sat Dec 25 2004 - 03:43:59 GMT