I believe three questions are in order when asking for advice on replacing a part. They are:
- What is the probability of failure
- What is the total cost of repair relative to the cost of preventative maintenance
- Can I afford the repair
One can then assign a value to each: 1 point for low or yes answer and 5 points for high and no answer. Then do a little math to gauge the relative risks. If in the green zone (probability is low, the relative cost is low, I can afford it), then wait until the part breaks. On the other hand, if you're in the red zone (probability is high, relative cost is high, I can't afford it), then replace it before the part breaks.
Looking at the part in this way, it much easier to decide what to do. The nice thing about approaching the answer this way is that everyone can assign their own point values to the answers and judge for himself whether to replace or not.
For example, if the probability of breaking a water pump vane is low, the cost of replacing a cracked head is high, and one can't afford ro doesn't want to do a repair costing more than $3500, then he's headed toward the red zone. Replace the water pump before it breaks.
