The 1.7-litre turbo diesel engine used in several models in the Vauxhall/Opel Astra and Zafira range, is extremely sensitive to the tension of its timing belt, which can easily be over-tensioned, with eventual belt failure as the likely result.
The reason that the belt can become over tight is because although it is straightforward to install the new spring loaded tensioner, it is also very easy for it to move while it is being secured and just a small movement can make a big difference on the tension on the belt.
To demonstrate the extent of this phenomenon, if during its tightening the tensioner moves by only four or five degrees, the tension on the belt will increase threefold from the 20kg it is designed for, to a catastrophic 60kg plus. This then causes the tensioner to wear excessively and ultimately, distort and fail.
To prevent this costly problem, Dayco recommend technicians follow its published fitting procedure designated for the specific engine code and make sure that they tighten the bolt to the manufacturers specified tension, which is also stated by engine code in the Dayco technical sheet.
This is particularly important as it seems more than a coincidence that the most common failures tend to occur on the Z17DTH and Z17DTL units, which both feature tensioners that have a lower torque setting than the majority of the other engine codes.