the importance of the new engine “break-in” period


I bought a new motorcycle today (brand new, odometer read 0) and about 3 hours into a VERY cold ride home, my numb extremities failed me. I missed a shift and redlined it pretty hard.

While everything seems fine, it got me wondering: apart from the obvious risk of instant catastrophic failure, what are the “later in life” repercussions of such abuse during the break in period, and how do they differ from abusing an engine later in its life?

In: 44

During a break-in period, you treat the vehicle gently and try to maintain steady speeds to avoid straining the engine and transmission.

The reason for this is that all the parts need to shift and even sometimes change shape a bit to fit together well. If you do this slowly, everything just settles down. If you do this quickly, then parts may be strained by being forced to adjust too fast. They probably will not break, but they do come out of the break-in as well and this can cause repairs or even failures down the road.

Think of it as the difference between gently taping something with a hammer repeatedly until it fits where it belongs and whacking it hard once to get the same result.

So when a car is brand new everything fits tightly and parrts haven’t had a chance to be worked to be thoroughly lubricated and to have parts settle into place.

Breaking in a car is driving the car gently and delicately until those parts have been properly lubricated and settled into place. Being gentle with the car during this stage helps prevents parts from violently settling into place or causing unnecessary stress on parts not properly lubricated.

It also gives people a chance to find out if there are any manufacturer defects. Usually there is a brief window of time where if a part is defective, it will break in those first few miles. After the break in period it’s generally assumed that the part will last it’s effective service life.

Modern engines are assembled to very high tolerances, necessary to have high compression ratios and long service intervals (eg low blow past piston rings to minimise oil contamination). The oil from the factory may have additives to help the engine bed in during the first hundreds of miles.

Decades ago, engines weren’t nearly as well made, and you’d have to run them in, to smooth out imperfections and loosen up tighter parts. Then at a certain mileage you’d flush the oil and gearbox oil, so that any metal particulates were flushed. You also changed the oil more regularly because it wasn’t as good and would get contaminated faster with exhaust gases, traces of fuel, and traces of metal from engine wear.

(This might be a bit too advanced for ELI5 but I’ll give it a go)

On one hand, break in periods are basically null and void with modern engines, as the manufacturer has already properly broken them in before delivering the vehicle. Apart from an early 1000km oil change or so, the end user rarely has to treat the car any differently from usual early in its life.

However, in the case of rebuilt engines where the pistons/rings/cylinder sleeves have been changed, the general consensus is that after it’s been warmed up you should hit the engine quite hard. The newly-honed and slightly abrasive cylinder walls wear the fresh piston rings down, creating a good seal that stops blow-by and excessive oil usage. While not much actual research has been done, a lack of engine load during this break in period does appear to contribute to both a less-than-ideal piston ring seal and allows glazing of the cylinder walls, both of which decrease performance and increase oil burning for the life of the vehicle.

Modern engines are made very very well to extremely tight tolerances. Once it’s up to heat for the first time, if something is going to break, it will break regardless of how hard you push the engine. (Flat tappet camshafts are an exception though).

All the parts in your engine are getting to know each other.

Think about a new friend or new roommate, when you first meet, everyone is tentative and courteous. After a while you get to know each other and settle into a groove. You don’t really want to apply high stress early in this relationship or it might come apart. If you do apply high stress, you’ll find out quickly a person’s true build quality.

Your engine is fine and is high build quality. Enjoy a lifelong relationship, maybe buy it some rose flavored oil and say sorry. It’ll go a long way.