I’ve written before about timing belts vs. timing chains. I strongly prefer the latter, because they don’t have to be replaced every 60,000 to 100,000 miles at a cost of 300 to 600 dollars (nearly all of it labor).

But TrueDelta’s repair rate analysis has disadvantaged engines with timing chains. Because most of the cost of replacing a timing belt is labor, when replacing them repair shops usually also replace other fairly inexpensive parts in the same hard-to-access area, most notably the water pump and timing belt tensioner.

As a result of this “preventative maintenance,” car engines with timing belts won’t have these parts fail nearly as often as those with timing chains.

To keep things “apples to apples” between the two engine designs, water pumps and timing belt tensioners won’t be included in the analysis retroactive to the November results. Well, unless someone else has a better solution.

Update: one change already made, another coming.

First, water pump and belt/chain tensioner failures have been reclassified as regular repairs as long as the odometer is under 70,000, since in these situations they wouldn’t have been replaced yet as part of preventative maintenance anyway.

Second, starting next month we’re probably going to start tracking timing belt replacements and “preventative maintenance.”