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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
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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