| I think you are trying to sound intelligent, but here are my responses, which I elaborate upon further down: 1) I agree with you if we were talking about new belts just lyin' around. I doubt they would go bad in any time frame that we are interested in. But we aren't talking about new belts just lying around gettin' polluted. We are talking about installed belts which are under stress and undergoing temp cycles, which fail before the mileage interval because they are old. Factor in that they may be subject to oil or coolant leaks over that timeframe, which probably accelerates the aging. It is reasonable to think that some belts at the tails of the distribution of the population of installed belts which are short of the mileage interval are in some bad conditions. (A little 4-sigma reference for ya. You think you're smarter than everybody else don't ya?)
Summary: It seems your technical explanation is for something different than what is being discussed. 2) Your "data" is what is written in the manual, but you expand on what is stated in the manual. However, you have no proof for your embellishments, but only your technical opinions about fibers and how they age, or maybe something else. The manual does not say that anything other than what is written in the manual is wrong. That's what you said, not the manual. So you say that you are basing it all on the manual, but you are actually going a little bit further, relying upon your own judgment. What you say "follows from the manual" does not actually, it comes from you.
Summary: No, your position is not completely based on data, it contains pieces of your opinion. 3) I find that others in the industry seem to agree that it makes sense to replace the belt ahead of scheduled intervals in some cases. The suggestions are based on balancing cost/risk/etc, and the knowledge that you can't diagnose a belt's condition by looking at it. I copy a quote from the first link I found in a quick search, from folks in the industry. True enough that it happens to be about Honda/Toyota, but it's not a stretch that it applies. Here is a quote, which is proceeded by a statement that you can't look at the belt and be sure that it is good, therefore: "Belt replacement is also recommended anytime the cylinder head has to come off of the engine, or when replacing a water pump that is driven off the timing belt. In both cases, the belt has to be removed anyway to make the other repairs so there is no extra labor required to replace it." Here's the link:
[ http://www.aa1car.com/library/2003/us70343.htm ] This particular quote suggests that the mileage intervals are not the last word, that some degree of judgment related to cost/risk is useful. That also implies that it's worth going through the analysis of that balance, regardless of what the manual says, and based upon cost/risk/etc, and not theoretical musings about how fibers age. Summary: There are clearly cases in which it is not a waste of time to replace belts ahead of mileage intervals.
Philosophically:
You seem to be operating under the assumption that the data (manual in this particular case) will tell the complete story in all instances, and that there is no need to doubt it or imagine that something might have been missed.
Folks who choose to replace the belt ahead of time are simply operating under the assumption that something might have been missed (like an oil or coolant leak on an engine which was stored in a garage in Phoenix), and that it would be wise to put a new one on. Is such wisdom "common sense"? Perhaps. Sorry, that was long. I am travelling on business.
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20:48:40 12/11/09
