147/156 Cam Belt Change

[Garibaldi]

For anyone thinking about leaving their cam belt change any longer than 3 years/ 60,000 klms I have a word of advice, DON'T. My cam belt was last changed just under 3 years and 35,000klms ago. I have just had it changed and it was starting to break.

[colcol]

Good advice there Mr Garibaldi, and do the waterpump every second change or if unknown age of waterpump, just change it, waterpump is easy to change with the timing belts off and the pumps are not that expensive.
And do your balance belt and tensioners as well.
Pick up the belt and have a look at it and think about what a sterling jobs these belts do, turn a few billion times in the heat, cold, dust and oil and do what is expected of them within the time and distance, Colin.

[Beatle]

And then look at the timing chain on a Nord that's been rebuilt four times and and done a million KMs and forty years.......

[Garibaldi]

I hate you Beatle.

[Beatle]

It's called progress..................

[colcol]

When the timing belts are changed, the valve timing goes back to where it should be.
With a chain, it may not be broken or noisy, but how do you know that it hasn't stretched?
In the olden days of Nord motors, there were no camshaft position sensors to squark at you that the timing chain has stretched and the cam timing is out.
To be followed by it lacking performance and going into limp mode.
Unless you set up a top dead centre gauge on your piston and check the opening angles of the cams, you won't know how accurate the timing is.
Point taken though, the 3 years on the twin sparks and JTS is a bit short.
But i am not going to be the one who lets it go any further to prove a point, Colin.

[Tornado20]

Hi Folks,

Question without notice for the people here familiar with a cam belt and water pump change; would you recommend pressure testing the cooling system prior to installing the new belts ?

Reason I ask is I haven't got tooling to carry out a pressure test and a leak test would be done at the end of the job.

Would this be considered too risky ?

Regards .......

[dehne]

When I changed my belts and water pump I just made sure all surfaces were clean and marked so that new replacements went on exactly where the old ones were. No special tools just marked absolutely everything

[Tornado20]

When I changed my belts and water pump I just made sure all surfaces were clean and marked so that new replacements went on exactly where the old ones were. No special tools just marked absolutely everything

Thanks dehne,

I will be using JTS timing tools.

My question actually relates to cooling system pressure test prior to reinstalling belts, probably wouldn't be too difficult making up my own test set.

Regards .....

[johnl]

When the timing belts are changed, the valve timing goes back to where it should be.

From here: http://www.aa1car.com/library/ar594.htm

"Contrary to what you might think, rubber timing belts do not stretch with accumulated mileage and wear. They are reinforced with strands of fiberglass which makes them virtually unstretchable."

Which is also my understanding. 

With a chain, it may not be broken or noisy, but how do you know that it hasn't stretched?
In the olden days of Nord motors, there were no camshaft position sensors to squark at you that the timing chain has stretched and the cam timing is out.
To be followed by it lacking performance and going into limp mode.
Unless you set up a top dead centre gauge on your piston and check the opening angles of the cams, you won't know how accurate the timing is.
Point taken though, the 3 years on the twin sparks and JTS is a bit short.
But i am not going to be the one who lets it go any further to prove a point, Colin.

Yes, in use over time of course chains do significantly stretch, causing the valve timing to eventually become at least somewhat retarded. With some engines this will also affect the ignition timing, which is at least one reason why it's bad practice to trigger the ignition from a camshaft position sensor (or to run a distributor from a camshaft, though it was often done on the assumption that the ignition timing would be adjusted every now and then).

Of interest; chains are also less suitable (than belts) for high rpm engines, because the mass of the chain always 'wants' to move in a straight line, but is forced to move in fairly tight arcs as it wraps around the various sprockets. This causes the chain to 'throw out' as it leaves each sprocket, most significantly as the chain 'unwraps' from the cam sprocket.

Imagine looking at the engine from the front, at the cam drive sprockets and chains. The crank pulls and tensions the chain between the crank and camshaft sprockets, so at low rpm you could put a straight edge along the chain run between the crank and cam sprockets. However, at high rpm (also at low rpm, but a lot lot less) the chain no longer describes a straight line between the sprockets, but assumes some form of curved line (this affect is greatest on the longest unsupported run between the final cam sprocket and the crank sprocket). As the chain 'unwraps' from the camshaft sprocket, the centripetal force acting on it throws the chain away from the center of the sprocket arc. So, because the chain has significant mass, the chain then tries to move to the side (i.e. deviate from a straight run between sprockets), which is resisted by the chain tension. However this is not enough to prevent the tensioned chain deviating from being 'straight' between the two sprockets.

It's my understanding that the chain will tend to assume an 'S' shape (along its' longest run between the cam and crank sprockets), the exact form depending upon sprocket diameter, the rpm, the chain mass, chain length between sprockets, whether the rpm are rising or falling, harmonics, etc. This means that the effective chain length between camshaft and crankshaft sprockets erratically varies from the 'static' nominal length as rpm rise, causing the valve timing to erratically advance (beyond nominal timing) to whatever degree (though fairly small unless the rpm are substantially high).

Belts have far less mass than chains, so are far less affected (but not completely unaffected) by this than are chains, so are more suitable than chains for high rpm operation. This is also why purpose designed racing engines usually use a series of gears rather than a chain or belt for the camshaft drive, i.e. with a gear train the valve timing remains constant regardless of rpm, but gear drives are expensive (for OHV engines), can be very finicky to adjust, and tend to be quite noisy.

Regards,
John.

[bazzbazz]

Point taken though, the 3 years on the twin sparks and JTS is a bit short.
But i am not going to be the one who lets it go any further to prove a point, Colin.

Tut, tut, no-one has a sense of adventure anymore . . . . . 

[poohbah]

Anyone know where Colcol has got to? He was a frequent and informative poster on most Alfas, (especially 156s, Suds and 33s) but hasn't posted since mid last year?

[V AR 164]

Anyone know where Colcol has got to? He was a frequent and informative poster on most Alfas, (especially 156s, Suds and 33s) but hasn't posted since mid last year?

I have been thinking about that too and was going to start a thread asking where he has been.

I saw him at Spettacolo but he hasn't been on the forum since September last year.

Andrew.

[poohbah]

I hope all is well.

[Garibaldi]

Don't worry Colin is fine. I do know why he is not on the forum anymore but it is up to Colin to tell you if he chooses to, not me.