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http://www.alfamen.com.au/timing-belts-explained/
One of the most frequent questions we’re asked at Alfa Men is ‘why do the Alfa Romeo belts need to be changed so often when those belts fitted to other engines last twice as long?’ The answer to the question is different depending on which engine you have and how you drive your vehicle but we’ll attempt to answer it below.
For the purpose of our discussion, we’ll imagine that we’re talking about the timing belts fitted to 147’s, 156’s and GTV’s as these engines require timing belts most frequently and prompt the above question most often.
First, we will make one big generalisation about Alfa engines fitted to the above models…. These engines we generalise as either 4 cylinders or V6’s. In each group there will be multiple engines but the basics are the same for each main group. Group 1 – ‘4 Cylinder’ will cover 2.0 Twin Spark and 2.0 JTS engines. Group 2 – ‘V6’s’ will cover 2.5, 3.0 and 3.2 V6’s.
GROUP 1 – 4 CYLINDER TIMING BELTS (2.0 TWIN SPARK AND 2.0 JTS ENGINES)
In these engines, we must consider that the engine actually contains 2 belts, 1 timing belt and 1 balance shaft drive belt. The failure of either belt is critical as even if the balance shaft belt breaks, the broken belt usually gets caught in the timing belt and results in timing belt failure. These two belts run side by side within the timing cover and because of space constraints both belts are relatively narrow in width. I’ve underlined the previous comment because the width (or lack of it) is the number one reason why these belts fail.
The wider the belt is, the stronger it is and in the case of ‘4 Cylinder’ belts… the lack of width results in a belt that is only just strong enough to handle the tension that it’s placed under every day. These belts have a tendency to snap as the belt weakens with age…. not because they wear out. But why would the belt ‘weaken’ you may ask? Well it’s a case of numerous influences deteriorating the rubber / materials of which the belts are made. Like a rubber band, the belt weakens with mainly age, chemical attack and heat. While timing belts are made of the best materials available (much better than a rubber band), the belt still loses a percentage of its strength each year.
To calculate the point in time that the belt is likely to snap, we must put some figures to this discussion; the timing belt when new can handle about 150% of the load they’re subjected to daily. Some things accelerate the ageing process like engine oils / coolant on the belt and high engine bay temperatures, which, is at its highest when a car spends time idling in hot city traffic. In average conditions however, most timing / balance shaft belts will lose around 10% strength per year due to mainly age (I’m being slightly simplistic here… the real figure is probably 6% in the first year, a further 8% in the second year, a further 10% in the third, 15% in the fourth… you get the idea).
Anyway, by the time the belt is about 4-5 years old, it’s likely to break because where the belt once had a strength 50% greater than the load it was subjected to, that margin is reduced over time, to the point where no margin exists and the belt snaps. The result… catastrophe.
Upon breaking a timing belt the ‘4 Cylinder’ engines almost always bend all the inlet and exhaust valves and always require a cylinder head replacement or recondition as part of the repair. The other main aspect of the repair process is that the load exerted on the pistons following timing belt failure (valves hitting pistons) usually causes damage to the conrod bearings and if these are not replaced as part of the repair, it’s common for the conrod bearings to fail only weeks after a replacement / reconditioned cylinder head has been fitted. For these reasons, the cost of a repairing an engine after timing belt failure often reaches the $3500 – $4000 mark.
To conclude, ‘4 Cylinder’ timing belts break mainly due to age. While some people justify putting off a timing belt change with the argument “Even though the belt is 3 years old, I’ve only done XXX km’s”, this is a huge mistake and the number one reason why owners of 4 Cylinder engines get caught out. Alfa Men advises all owners to closely monitor the age of their timing belt and as per Alfa Romeo’s advice have it replaced at 3 years of age or 60,000 km’s – whichever comes first. If the car has only done 10,000 km’s and it has been 3 years since the last timing belt change, it’s due for replacement.
And as a side note to this discussion, if you have already had the timing belt replaced and there are small ‘white out’ marks on you timing belt cove, chances are your belt was not fitted correctly. Every time I see an engine with these marks, I’m willing to bet money on the fact that the timing is wrong and the timing belt replacement has been done incorrectly. I say this because with the correct Alfa Romeo tools there is no reason to place ‘white out’ marks on the engine cover. These marks tell me that a) the mechanic who did the job doesn’t have the right tools and b) the mechanic probably doesn’t do very many Alfa Romeo timing belts and is probably inexperienced at this task.
GROUP 2 – V6 TIMING BELTS (2.5, 3.0 AND 3.2 LITRE ‘BUSSO’ ENGINES)
Assuming you have read the above discussion on 4 cylinder timing belts, forget everything I just told you… almost. Yes, the V6 timing belts do age and yes they are affected by oil and heat such that they lose strength but the main difference between 4 cylinder and V6 timing belts is that V6 timing belts have a much greater margin of strength to start with. In the V6 engine, the timing belt is wide and strong. In fact it’s almost twice as wide as the 4 cylinder timing belt. When the timing belt is new it can withstand about 200% of the tension it is subjected to (remember this figure was only 150% for the 4 Cylinder). For this reason, the V6 timing belt will deteriorate with age but is unlikely to break before the 6 year mark – much longer than the 4 Cylinder timing belt which requires replacement at 3 years.
The main factor that leads to V6 timing belt failure is the fact that the belt wears out!
In the V6 engine there are other factors at work that wear the V6 timing belts – factors which are often more critical than belt age and factors which are far more pronounced that in the 4 Cylinder engines. These factors are called ‘cycles’, ‘wrap diameter’ and most importantly ‘alternative bending’. All of which are annoyingly complicated concepts to explain but why would I stop now.
Cycles are the number of times the belt has travelled around the engine pulley system and it is solely related to the number of revolutions that the engine does in the belts lifetime i.e. it is roughly proportional to km’s travelled. If we assume that the average belt lasts 100,000 km’s, it’s likely that the belt has done millions and millions of revolutions around the engine pulleys. More if the car has spent a long time in slow moving traffic. In 4 cylinder engines, the reduced belt life of 3 years, means that the 4 cylinder timing belts are subjected to fewer cycles in their life.
But Cycles wouldn’t damage or ‘wear out’ the timing belts at all if the belt didn’t need to ‘bend’ around the pulleys – this is where ‘wrap diameter’ comes in. The larger the pulley that the belt wraps around the less wear the belt endures as it wraps around that pulley and straightens again. This concept is a little abstract but everyone has witnessed a similar affect at home when attempting to break an old carrot (one that’s gone soft). Let’s image you bend the carrot over a large diameter such as your forehead. Chances are you’d look stupid and the carrot would just bend to match the large diameter curve and would not snap. Now we take that same carrot and bend it around a small diameter such as our wrist – the carrot is likely to break. An odd analogy I know but you can see how the smaller the diameter that the belt needs to wrap around the more damage is done to the belt with each cycle around the engine. Surprisingly, the V6 engine has smaller diameter pulleys than the 4 cylinder engines and thus the V6 belts must wrap around smaller diameter pulleys placing a greater importance on ‘Cycles’ as a wear factors in V6 engines.
Now for another odd concept… ‘Alternating bending’. Alternating bending is the condition where a timing belt is forced to bend in one direction and then back in an opposite direction. As with ‘bending diameter’ discussed previously, we’ve all experienced a similar affect when snapping the ring pull off a soft drink can. We all know that the best way to snap the ring pull is to bend it one way, then back the other, again and again (cycles combined with alternating bending). This style of repetitive bending back and forth leads to an increased amount of belt wear. In V6 engines, the belt is forced to bend backwards as it travels around the 2 x idler pulleys and 1 x tensioner and then forwards as it reaches each camshaft pulley. This condition is also seen in the 4 cylinder engines but it’s worth noting that the 4 cylinder water pump pulley is almost twice the diameter of the idlers found in V6 engines. That leads us back to the ‘bending diameter’ factor – smaller pulley diameters cause increased belt wear with each cycle, especially if the belt is bend back and forth (alternating bending).
CONCLUSION
So now that we’ve covered almost all the factors that either weaken or wear out timing belts it’s time to recap on the factors most relevant to each engine category and I’ve noted them from most important to least.
4 Cylinder Engines (2.0 Twin Spark and JTS engines)
– Belt age – by 3-4 years the timing belt (and balance shaft belts) have weakened to the point where they snap.
– Cycles / Bending Diameter / Alternating Bending – by 3 years, the belt is unlikely to be worn out due to these factors as the engine has larger diameter pulleys and the number of cycles achieved is likely to be lower due to the reduced time period.
V6 Engines (2.5, 3.0 & 3.2 litre ‘Busso’ engines)
– Cycles / Bending Diameter / Alternating Bending – by 6 years, the belt is likely to have completed a great number of cycles and worn out due to these factors as the engine has small diameter pulleys and the number of cycles achieved is significant given the cycles completed within the 6 year life.
– Belt age – by 6 years, the belt has aged significantly, however, its generous strength margin when new provides for a longer belt life.
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