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Break-In FAQ

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Why Break in?

It is uneconomical to manufacture parts for some precision assemblies to their final fit and surface finish. Instead manufacturers get them close, and get the final fit and finish by rubbing mating parts together to abrade them in a controlled fashion. This is true of many assemblies in an vehicle, particularly piston rings and cylinder walls, final drive gears, and brake pads/disks.

What are we trying to do?

What break in does then, is some final machine work on an engine by running it. It bears a lot of similarities to machining parts with tools on a lathe. The proper lubricant is important. It is important not to bear down too strongly with the tool and to smoothly move the tool across the entire surface many times, rather than keeping the tool at one place and trying to take all the metal there off at once.

What are the potential problems?

One major one is overheating the metal locally. The parts in question are usually sophisticated alloys which may have been subjected to precision heat treating. Ad hoc heat treating by localized overheating due to use too hard too early will weaken the alloy. The other major problem is not loading the parts enough. This can cause the surface finish to become too smooth (glazed) before the fit has been adjusted. The smooth parts then don't fit quite properly, leading to excess friction.

How is this all best accomplished?

Moderate, varying loading of the parts. RPM limits and varying speeds are approximations which work well. There are some ways to move beyond these approximations. It is a good idea to maintain higher rpms under high loads (such as two up on a motorcycle uphill). Low rpms and heavy throttle in these situations is likely to produce high local loads. Downshift a gear, even if it means exceeding the rpm limits, and use light throttle. In addition to varying speeds, getting somewhat on and off the throttle (accelerating and engine braking) will load different areas of the parts, particularly for gears. Sustained full throttle is something to avoid, regardless of rpm.

What about "If you want it to be fast, break it in fast"?

This legend probably arises from the fact that being too gentle during break in will glaze surfaces while maintaining a tight fit, causing friction. It is equally bad, for vehicle reliability, to gouge away metal and overheat parts.

How long does break in take?

This is dependent on the qualities of the metals involved, the machined fit and finish, the desired final fit and finish, and the loading and lubrication of the parts during break in. Race engines are engineered and manufactured for rapid break in, although some (10-100 miles) is still required. Porsche 911s and BMW motorcycles have very hard rings/cylinder walls and take a long time, perhaps as much as 20,000 miles. The only good way of determining the end of break in that I've heard about, other than subjective judgements of performance, is to monitor oil consumption. If it starts out as noticeable, and falls to a constant low level (which may be approximately zero), you're there.

What about synthetic oils during break in?

Pretty much all oil and vehicle manufacturers recommend against of use of synthetics during break in, because of too efficient lubrication. Some anecdotal information I've heard says that this does interfere with break in, while others view it as simply legend. It seems unlikely that waiting to use synthetic could be a bad idea.

What about break in oil?

In the good old days, manufacturers relied a lot on break in, as opposed to manufacture, for final machining. They sent the vehicles out with special (generally light weight) break in oil. These days manufacturing techniques are better, and break in less important, so few do this now.

What about changing oil and filter during break in?

See the above answer. Many manufacturers now do not specifically recommend this, although they used to. As an old fogie (TM) I believe it can't be bad to get the machining residue (theirs and mine) out of there after a few hundred miles, but I may be succumbing to legend here. The minimum standard should be the manufacturers recommendation, and changing more often during break in can't hurt except for (modest) damage to your wallet.

[Thanks for reading. Bob]

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