Dear Tom and Ray:
I recently purchased a new Mini Cooper S and was surprised to read in the owner's manual that the break-in period for the engine is 1,200 miles. Until I reach 1,200 miles, I am supposed to vary my speed frequently, keep the tachometer under 4,000 rpm and keep the speed under 95 mph. I am not looking for a reason to circumvent the break-in period, but I am really curious as to why the break-in period is so long. What's going on in the engine during this time? -- David
TOM: This is when the gerbils are in training, David. As I'm sure you know, they run around on their wheel, and that's what makes the car go. But until they can build up their leg muscles, they can only do about 4,000 rpm. So give 'em a break, will ya?
RAY: I think the gerbil wheel is what powers my brother's brain. What's happening during break-in is that the piston rings are "seating" to the cylinder walls.
TOM: What does that mean? Well, at the heart of the engine are your pistons. They look like soup cans, and they go up and down inside the cylinders. It's crucial that there is a perfect, tight fit between the outside of the pistons and the inside of the cylinder walls.
RAY: So, the pistons are surrounded by spring-loaded rings, which push out against the walls and keep the seal tight. Otherwise, oil will get past the rings and you'll "burn oil."
TOM: And the theory of "break-in" is this: If the rings and the cylinder walls don't come out of the factory matching up perfectly, the break-in period gives them a chance to conform to each other during relatively "light duty" service (which involves going slowly and varying the speed).
RAY: Why is it 1,200 miles on a Mini, and 600 on a Honda? Because it's not an exact science. I think each manufacturer is making its best guess as to how much time the rings will need. It's probably based on how many people have whined to them in the past about their cars burning oil. They look at their warranty claims and say, "OK, guys, let's jack it up another 200 miles and see if that helps."
TOM: It's interesting to note that some car makers are so confident in their precision manufacturing that they require no break-in period at all. Porsche, for instance, says you can get in a new Porsche and drive it out of the showroom however you want.
RAY: Or, maybe certain companies require no break-in period because they've deemed it hopeless. They've decided, "No matter what we say in the book, this jerk is gonna go 100 mph as soon as he hits the freeway entrance, so why waste the ink?"