Lots of interesting thoughts on the science of "break in" in the thread.

I'll simply add my two cents.

A whole lot has changed since the days of my father's last "new car" purchase in 1965. Think about buying a new car (with a brand new engine, transmission, rearend (or more likely, transaxle) today. They are usually started for the first time on the assembly line. When they show up at the dealership, they rarely have more than a mile or two on them. Almost everything shows up with synthetic oil in them from the very beginning. And the average public doesn't have a clue about "break in" -- including the people at the dealerships selling the cars. I've bought a few new ones over the last 25 years - and I can recall one or two that had a line in the owner's manual (which no one bothers with except to figure out how to set the sound system up) about not exceeding 5000 rpm during the first 1000 miles or some such thing. Nothing about constant stops/starts. Nothing about varying engine speed. Nothing about retorquing fasteners. Nothing about oil/filter changes after the first hundred miles. Nothing. About. And the vast majority of these cars go 100k-200k miles (often, many more) usually with less than stellar maintenance by the average owner.

If you read up a bit more about all of this -- with the ability to carefully control the manufacturing and machining processes as well as vastly superior material (metals) control -- those new engines seat their piston rings within a few seconds of start up. Do things get cozier over the next couple of hundred miles? Yup. But "break in" as it was necessary back in the day is a thing of the past with most of today's new engines. In 2016 I installed and started up a brand new crate LS3 V8 in my Volvo. Its crankcase was filled with Mobil 1 synthetic -- from the factory for the FIRST start up. It fired on the first try, settled immediately into a 700 rpm idle and ran like a champ from the very beginning. Has never smoked or burned a drop of oil or leaked. There was zero "break-in" -- I just drove the car. I did prime the oil system -- more on that later.

All of that to say -- have a long talk with the folks that did the machine work on the motor and the folks that supplied your new bits - especially rings, bearings. See what THEY say about break in and go from there. As mentioned before, if you replaced the cam and shim pads - there's sometimes a procedure for having those get to know each other. With old school flat tappet followers it's typically to be sure you get any required lash set correctly - and then holding a steady 2000-2500 rpm from 20 minutes or so at first start up. Having said that - I've changed (or helped change) those follower shims on Volvos, VW's, Jaguar's, etc. and after the valve lash has been set with new shims (usually on just a few valves) you fire the car up and drive it. If you have block/head machine work done, then you'll have to recheck/reset lash anyway and perhaps all the shims will change either because you needed some new ones of a different thickness or because you had to move one to another location.

Of course - hopefully you used the proper assembly lube when putting things together - lots of special stuff often used for cam lobes/followers, thread sealers in certain places, loctite, etc. ALWAYS prime the oil system so all passages are full of oil all the way to the bearings and top end of the engine (including filling up hyd lifters) - and the filter is full. Ideally, have someone with a socket on the crank pulley slowly rotate the motor while you're priming so oil gets everywhere. Then fire it up. Check for oil pressure. Be sure it's controlling temperatures as it should. Then follow the advice of the folks that supplied the parts and did the machining. With quality parts and proper machining - it's likely going to be check for leaks and improper noises, then drive the car. Me -- whether called for or not, I like to do an oil/filter change after the first 100-200 miles. Just to get any assembly debris out of the motor. And I cut the filter open just to be sure there are no surprises there.

Clearly, I had too much time on my hands this morning... Good luck.