break-ins

why some engine manufacturer recommends break-ins of 3000 kms

Depending on materials and application the process of breaking-in can go on for quite a long period of time... Typical production engine usually keeps getting its compression increased up to a few thousand kilometres whilst performance engine can complete the process much much sooner because of different level of engine load.

There are also variations in the tolerances and materials used.

3k kms is rather a long time, though.