[Corner carver]

Originally Posted by Donner

Hello, I posted the following statement in another thread about "Who's doing a hard break in?" That one, like this was addressing a similar issue. There are plenty of folks that have their own minds made up and for you, have at it. To anyone that actually is concerned about whether the manufacture is correct, here's my take.

Hello,

If I might offer some insight into breaking in the motor. There is the school of thrash it and trash it and I subscribe to that belief. The issue with rushing the breaking in of the motor components is that no seat of the pants measurement will be able to show one what was lost by not doing right by the engine and the advice of the engineering professionals that designed the motor.

To spare the readers a long discourse, consider just the piston and rings. The cylinder that the pistons ride inside of are nearly circular cylinders but they are made by tools that leave a slightly abrasive uneven surface and cutting marks through a process known as honing.

The rings sliding up and down within the cylinder’s walls will eventually polish these minor ridges away and create an optimal interface if properly broken in, or something less than that if the process is rushed or the motor is abused prematurely. The pistons themselves contain ringlands (grooves that are machined or cast to create circular trenches) but they are not circular. That is because the piston isn’t exactly a circle either. Its proper shape, when cold, is elliptical when viewed from above (looking down) and conical when viewed from the side. The top of the pistons are truncated cones replete with whatever shaping that the valve relief was designed to create.

The reason that the pistons are not full circles when cold is because of expansion. Metals grow in proportion to temperature in an “inches per inch” relation. There is more metal (solid in fact) at the top of the piston (the dome) and a complete hole at the bottom extending upwards. Since the top is solid, it grows more in relative diameter than does the bottom of the skirt but once up to temperature, it will become a circular cylinder as well.

Now consider that the wrist pin has protrusions on the internal walls of the piston’s insides. The piston will be narrower by the holes of the wrist pin and wider along the transverse axis. Because of the extra metal needed to support the wrist pins, the piston will expand further in that direction and again, become circular once it is heated up. Why this matters is that the rings are not fully supported in the ringland until the engine is at operational temperature.

As the pistons cycle up and down, the flexibility of the rings cause them to wave in relation. Rings do not remain flat because until the motor is broken in they are being jolted about by the honing marks. Since they do wave, the tips at the upper and bottom edges of the rings can get chipped or gouged if the engine is pushed too hard before the full sealing and polishing has occurred.

What this will mean in the long run, is that the rings will not hold the most pressure possible thus the engine loses compression (admittedly it’s a very slight difference at first and will degrade more rapidly than a properly broken in motor would over time). Those chips create little wedges that can allow more pressure to bypass the interface so maximum compression suffers.

Why altering the range of RPM and the load is important is because those wavering rings get the best chance to polish the surface rather than wear in ruts. It’s better for the rings and the cylinder walls to limit the power early on.

Bottom line: If a better service life and greater performance matters to you, follow the break in advice given by the engineers.

Kindest,

Donner

Great write up, thank you!