I have some personal experience myself as I participated in the rebuilding of a number of engines and rebuilt 2 engines myself.

The ring has some clearance in the groove and move inside the groove a bit.

At high RPMs under closed throttle there is no compression or combustion pressure to help stablize the piston in the cylinder and and it can **** slightly in the bore. Sure the clearance ain't much but still this is to be avoided if possible.

You may have engaged in breaking in an engine by letting it coast down under closed throttle from relatively high RPMs but that is not the right way to break in an engine.

 

An interesting theory you have-- I'd like to hear more how this is possible on a properly assembled engine.. Are you talking about piston slap/rock? That is more of a design issue in longer stroke engines rather than a break-in. Regardless, this all anecdotal at best and can be traced back to difference of opinion in break in. With modern casting and machining technologies, there is a level of precision that was never seen which would also go against your theory.

My opinion is a properly assembled aren't going to have pistons being cocked slightly during break-in, except possibly during the initial start-up. There are many forces at play... you've got the gas forces from the top and bottom of each ring and the piston itself, axial forces from the ring itself to the bore, friction, inertia, and thermal expansion... you aren't running the engine like this cold, it's going to be operating temp and your ring moving in the groove thing isn't happening at this point. Here is some reading material for you if you want to get into the physics of how things work: http://www.federalmogul.com/en-US/Me...S200976080.pdf

Also, as far as engine braking vs not-- letting an engine brake down creates a vacuum which helps pull the very tiny metal particles from initial run-in out of the cylinder and blown out of the exhaust in the oil that is initially getting by the piston rings... It is the inverse of the typical operation of the 4-stroke engine. The higher RPM you engine brake from, the more pronounced this effect is. Without utilizing this, particles can accumulate in the ring grooves and cause premature wear... More engines run in easy use oil in the long term than not in my (anecdotal) experience.

I can call your way the wrong way just as well as you can call mine. Breaking it in easy isn't going to hurt anything, nor will breaking it in hard-- but max performance will suffer if you don't create enough cylinder pressure to seat the rings very early in the engine's life and odds are higher that the engine is going to eventually use oil or suffer other problems from blow-by gases contaminating the oil due to ring seating issues. How much difference or how long will it take for these effects to be noticeable? Who knows- there is no apples to apples comparison for this or any way to measure this, but my cars and motorcycles have always "ran strong" and used less oil compared to others of same make/model.

 

A properly assembled engine has some clearance between the piston and the cylinder wall.

As I said before under normal operation the piston is supported (so to speak) by pressure during the compression stroke and pessure during the power stroke.

Under closed throttle coasting the piston has essentially no pressure to help keep it centered in the bore and while the clearance isn't much the piston rocks.

There is breaking in easy or breaking in hard and there is breaking in too easy or breaking in too hard. The problem is knowing what the thresholds are. If one shoots for something nearer the middle the less chances there is of being too easy and the rings/cylinders develop a glazed finish which doesn't seal well and worse can suffer from marginal lubrication which can result in bore scoring or too hard which can lead to all sorts of problems like excessive piston/cylinder clearances, often localized, which can lead to ring breakage, excessive blow by which can cause piston/ring failure, comtamination of the incoming charge with oil which can result in detonation even with the proper octane grade of gasoline.

Also, as far as engine braking vs not-- letting an engine brake down creates a vacuum which helps pull the very tiny metal particles from initial run-in out of the cylinder and blown out of the exhaust in the oil that is initially getting by the piston rings...

That is wrong. While there is a low pressure created metal particles are *not* pulled out. The cylinder walls retain a thin layer of oil and this oil remains even in low pressure condition. Any particles that are loosened are loosened by the friction of the rings and piston and the oil that must be and *is* between these and the cylinder wall.

All the coasting does is result in the rings having only their normal ring tension forcing them against the cylinder walls. In cast iron engines this was a ticket to cylinder glazing. Break in is accomplished by the rings being forced to exert pressure against the cylinder walls and this requires the aforementioned compression and power strokes.

It is a testament to just how forgiving these engines are and how well the metalurgy and engine lubrication is that these engines can be "broken in" in so many various ways, some sound, some not so sound, and for the most part apparently manifest no problems.

Still I prefer to try to stay well within the break in guidelines to give the engine every chance to develop the surface finishes at the ring/piston and cylinder wall interface and at the cam lobe/lifter bucket interfaces that will have the engine giving me a long and trouble free service life.