Problem deciding

[hellofadriver75]

Thanks to everyone for advice re my problem deciding re timing belts...will consider replacing soon (when I ve saved up!!) as I do want to keep the car. Have done the pads and connecting sensor, and getting quote on the bodywork slight dent with scratch that appeared on it after parking at a car boot sale. hellofadriver75

[Avulon]

I am really surprised that no-one debating the issue has bothered to mention MTBF (Mean Time Between Failures) and how that applies to timing belts.

Shame on you

TL;DR - It's not just guesswork - while many will last longer than the 6 year period the chance of breakage rises steeply (as judged by the engine designers) after this time.

So basically what this means is that the replacement by time and mileage are both set depending on the predicted mean average between failures - i.e. if the mean average mileage between failures is say 100k miles we could make it that - but that would mean that half the cars on average would fail before being replaced wouldn't it? So where's a point where say (and I don't know the exact figure used which probably varies between manufacturers a little) no more than 15% of failures occur? 60k miles - ok we'll set the replacement mileage at that.

Now for time when that mileage isn't reached? 10 years on half average half will fail? but very few within 6 years? and a significant number after 7? then set the time period at 6 years.

Now when a car is first built it's all educated guessing and design driving these figures but as the cars age and some real world numbers are available these figures can be refined - and it's often happened that belt mileage periods have been quite significantly reduced on some engines in the past. As for time period when we are talking about the kv6 well rover didn't make this engine for long enough to get any real significant numbers for low mileage cars over 6 years old did they? So without those numbers being available what are you going to trust? If it were me (it's not, I have a diesel and an 1800t) then I'd trust that the designers weren't pessimistic, after all it's always partly a selling point to say that major items don't need replacement as often as the competitor....

[SD1too]

Thank you for thinking about this issue Mr. Avulon and using some very appropriate words.

Originally Posted by Avulon

... the replacement by time and mileage are both set depending on the predicted mean average between failures ...

You're possibly right. It's easy to test timing belts to destruction on mileage but as for the time period alone ...

... Rover didn't make this engine for long enough to get any real significant numbers for low mileage cars over 6 years old did they? So without those numbers being available what are you going to trust?

Exactly! The answer is that you trust the mileage, as I have argued and proven with my own vehicle.

Now when a car is first built it's all educated guessing and design driving these figures but as the cars age and some real world numbers are available these figures can be refined ..

A good example of this is the initial requirement to change all the airbag components after ten years; clearly a "cover our backs" policy with no scientific basis whatsoever. If this can be true for airbags, it can also be true for timing belts.

Simon

[hellofadriver75]

Many thanks, sarah (newton abbot garage advice)

[Ratty]

Exeter Road Garage are in Newton Abbot

[Avulon]

Originally Posted by SD1too

A good example of this is the initial requirement to change all the airbag components after ten years; clearly a "cover our backs" policy with no scientific basis whatsoever. If this can be true for airbags, it can also be true for timing belts.

Simon

And where it relates to serviceable components (you can't get new air-bags for our cars) I'd defer to the manufacturers schedule, at least it has a chance of being based on some facts: for all we know the airbag manufacturer said 'guaranteed 10 years' (or a manufacturer of one of the airbag components), so what should Rover do when setting a replacement schedule?. I don't think that comparing timing belts to airbags is a particularly significant argument. Rover designed the kv6 themselves - which gave them the opportunity of specifying the belts themselves.

Each owner should assess the risk/cost/benefit and make the decision for themselves. I certainly wouldn't advise anyone that exceeding the age limit on low mileage belts will be safe. I have taken the risk myself with a v6 2.0 : but as I only paid £100 for it and sold it 3 months later for a similar price to someone that was made aware that the belts were due for a change I wouldn't hold this as support for routinely exceeding the belt age. I was lucky that was all. As I already showed - most belts will last longer than 6 years - by design however the risk of breakage will increase significantly (according to Rover) past that age. I wouldn't take that risk on a nice example or one that I planned to keep.

It would be an interesting exercise to collate the age and mileage of failed timing belts that are below the mileage limit.
e.g.

1. Age in years
2. mileage at time of failure
3. location (i.e. where the car has spent most of it's life - climate might have some bearing on belt age).

Let's see how many fail before 6 years and how many after. A thousand or so accurate reports with good global distribution should suffice.

[rustymotor]

Avulon it's interesting post though, failure of the belt can be due to other components failing, water pump, tensioner, idlers..so unless these are factored in the belt alone figures don't help much.

[Avulon]

Originally Posted by rustymotor

Avulon it's interesting post though, failure of the belt can be due to other components failing, water pump, tensioner, idlers..so unless these are factored in the belt alone figures don't help much.

but would you change the tensioner or water pump without changing the belt and vice versa?

[rustymotor]

Hi, yes I would fit a kit though could any of those parts fail before the belt do you think?

[Avulon]

Originally Posted by rustymotor

Hi, yes I would fit a kit though could any of those parts fail before the belt do you think?

Sure, they could - but the result is the same: a broken belt. Perhaps we should say 'belts, water pump & tensioner' and treat them as a singular item. If you wouldn't change any one of those in isolation then what's the point of specifically blaming one of them? e.g. the tensioner fails, snaps the belt and engine is trashed at 45k miles and 8 years. Should the owner have changed the belts earlier? Yes, because they would have changed the tensioner at the same time...