Originally Posted by jayze
IMO, motors don't need break-in anymore, see it more as a trans break-in, varying speeds and gears help your trans getting smoother
+1, except add the diff to the list.
Engines get broken in at the factory. This really means seating the rings which happens in the first couple of minutes of operation. When you build an engine you put in a high ZDDP break in oil, run it for a minute to make sure it runs, then get it on a dyno or something where you can run some really hard sessions. That seats the rings. Break in is now done.
Ram asks that we don't tow for 500m so the gears in the drivetrain have a chance to work harden before being pushed too hard. Towing a trailer on the flat and going reasonably easy on it should be no big deal, but maybe you don't want to hook up 8000lbs on a 1500 and climb over the Rockies.
I build the engines for my BMW race car. If you replace your rings or camshaft, you need to carefully control the first couple minutes of the engine's life. In our case tho, just jump in the car and drive it home.