[HO455]

Proper break in is one of those misunderstood subjects. With a non roller cam engine you have to devote the first 20 to 30 minutes to breaking in the camshaft. Depending on the lift and spring pressures you may have to run specific break in springs to prevent lobe destruction. Once you have successfully broken the cam in and replaced the break in springs if used. Checked your valve lash and brought the engine up to temperature. Now the controversial part begins. Every manufacturer and most machine shops tell you to drive the vehicle conservatively, taking it easy. The reason is simply if there are any questionable clearances in the engine they will wear themselves into spec if they are not overly stressed. And let's say a rod bearing is too tight the bearing will wear and gain enough clearance to live. But if your loading the engine and turning the RPM up, the tight spot may get hot and the babbet will start transferring leading to failure. Then the builder has a warranty issue on their hands However to properly seat the rings you must fully load the engine.
When talking about seating rings what do we mean? It is the process to obtain the balance between perfectly smooth cylinder walls that allow complete ring contact and optimal sealing and the need to keep the cylinder, piston and rings lubricated. The need for lubrication requires that the cylinder walls are not perfectly smooth. A properly broken in cylinder wall will have bumps and valleys like sandpaper, but with a microscopic grit. This allows oil to be trapped in the valleys and the bumps to seal against the rings. This ratio of low and high on the cylinder walls doesn't happen by chance. The rings are manufactured with specific materials to wear against the cylinder walls during the break in period. Then a specific honing process is matched to the rings used to obtain a specific finish on the cylinder walls. The idea is that when the honing is finished the cylinder walls are like a new piece of sandpaper with sharp peaks on the bumps. During the break in process those sharp peaks are worn off by the rings. So the broken in cylinder wall peaks are now flat tops that are surrounded by valleys for oil. Too much ground off the tops gets you more power with less oiling and shorter engine life. Too little removed from the peaks means low power and high oil consumption. The window for this grinding to happen is fairly short as the freshly honed area starts soft and gets harder from heat and work hardening and the ring surface coating is worn away. Once this has happened there is very little one can do to correct break in issues short of starting over.
Nowdays manufacturers now days have very sophisticated honing process's and rings developed to seat very quickly under light loads with the selected cylinder wall material, and computer controlled break in systems. But our old engines don't have access to that kind of technology when we rebuild them. So we have to do it the old way. That means the rings have to be forced against the cylinder walls during the break in. The physical ring tension in the ring is not nearly enough pressure to force the ring against the cylinder wall and grind those peaks off. The pressure needed to do that comes from combustion pressure that has made its way along the side of the piston shoved the ring down against the ring land and once behind the ring forces the ring outwards against the cylinder wall. The more combustion pressure the more the ring pushes against the cylinder wall and removes the proper amount of material. (Assuming that the honing was done properly and matches the ring style). The only way to get maximum combustion pressure is to completely fill the combustion chamber with fuel/air mixture. That means wide open throttle.
I know this is opposite of what many people believe to be true but to get the best sealing rings you must have high combustion pressure. When race engines are broken in they are idled for a very short time to get the engine up to temperature, then brought up at controlled rate several times in ever increasing levels until full power pulls are achieved. This level is usually reached with less than 5 minutes of run time on a roller cam motor. They are confident in the machine work and assembly process of the engine so they are not worried about problems coming from improper tolerances. Taking your new engine out and doing second gear pulls on a empty road are how we us folks don't have dynos achieve the same results. Start with a 1/3 throttle pass to letting up about 30% below red line then repeat with a 1/2 throttle to 20% below red line, then 3/4 throttle to 10% below red line. Then 3 full throttle pulls to red line will do the trick. This is the best reason to blueprint your own engine. Knowing everything inside is to the proper tolerances will make this a fun event not a stressful one.
Having been involved in many dozen large Cat diesel post overhaul dyno break in sessions if you know your assembly is right you will have nothing to worry about. When we dyno a 2500 HP 12 cylinder engine it is started up and pressures are checked when everything is in spec the engine is brought up in RPM steps and once it is up to temp and full RPM the engine's full load is applied to the engine and then held for the rated period. For our continuous duty engines that means 4 hours at full load. Standing next to one these engines for 4 hours is not only deafing but boring.
Failure to meet the conditions means you will not get the required amount of material removed from the peaks of the cylinder walls for proper ring sealing.
I hope this isn't too long and boring.