[Myetball]

Without new cams and lifters the only real "break-in" is to seat the rings. If you break it in like you're driving Miss Daisy then you will not create enough pressure behind the rings to properly seat them. It's critical to put a load on the engine within the first 20 miles. If not, the rough surface of the cylinders, after honing, will be smoothed and your rings will not seat properly. End result, poor rings seal, less than optimal compression, blow-by, poor performance, poor fuel economy. Nothing like spending $3,000 to rebuild an engine that runs like crap.

Bearings do not get broken in. If they are shedding material after initial startup they are either starved for oil or not within tolerance. Either too tight or too loose. Plastigauge is not a reliable way to measure tolerance.

To be exact when checking bearing clearance you need inside and outside micrometers that measure to 10thousandths of an inch. Install the bearings, either in mains or rods. Install and properly torque caps (block halves when measuring SVX mains), without crank installed. Use an inside micrometer to measure the bearing diameter. Now measure the corresponding crank journal. Subtract crank measurement from bearing measurement and you have your clearance. Clearance is pretty tight 2-15 10thousandths on the mains and 8-18 10thousandths on the rods.

Changing the oil after a rebuild is essential. Also, do not use synthetic oil until break-in is complete. Synthetic oil will interfere with the friction necessary to seat the rings. After it's all broken-in and there's no more material from the rings showing up in the oil, use the best oil you can afford.

As for new cars not requiring oil changes right away, the engines are already broken in and all the ring and cylinder material is already cleaned out. This wasn't always the case. Back in the day they built up the engines, started them up to see if they worked and left it up to the owner to break them in....the wrong way.