Well with the Zat tensioner on the 12V engine, the instruction is to set the belt deflection between the two top pulleys a certain amount and then check and reset it at 15,000 miles. Hence they do not really stretch according to Zat and it has been my experience as well that they do not (deflection stays the same when at the same temp). I have also experienced (using fixed tensioners) on other cars like the 944 Porsche, where a mechanic over-tensioned the belt to the point of being able to hear the whine from it. Well, they will whine forever until properly retensioned, so here again, a very similar profile belt design to the 164, and it doesn't really stretch to speak of. The later 24V car has an even stouter t-belt and I seriously doubt it stretches much either. A car that is in my driveway with a similar profile tbelt is the 240 Volvo. Factory procedure is to set the fixed tension and run then the car for 200-300 miles and then reset the tension, which has seemed like a waste of time and effort whenever I have followed that instruction. I never seem to be able to notice any movement of the tensioner on the retension, but hard to tell as the cover is still on when you do this. Then again, modern tbelts are probably less prone to stretching than they used to be, just due to better manufacturing than when they first started making these things in the 1970s and 80s on a large scale. I would guess they are better than the belts made in the 90s as well. Just speculation on my part there. I really don't think they stretch, but if the guy feels like he is doing a better job letting it "stretch" overnight and he feels like it is making for a better tbelt change, than I wouldn't argue with him. It certainly won't hurt anything, but it is debatable whether it is really making any difference. I don't think the retension is necessary on the 240 either, but I follow the instruction just because the manuals say to do it.
Charles
