tech32

Breaking-in a New Motor
by Lynn Bennett

After rebuilding your classic motorcycle four stroke engine how should
the engine be treated?

A complete rebuild normally includes several plain bushing bearing
surfaces, several roller or ball bearings, new valve guides (and/or new
valves), and new pistons and rings.

Plain bushings, like the big end of the rod, the small end of the rod,
transmission gear and shaft bearings, and maybe a crankshaft main
bearing, are not perfect just as is true of their mating surfaces.
There are high and low points, maximum and minimum clearances, for
each. Initially the high spots need to be worn down enough so that a
little excessive engine heat does not reduce the clearances to
interferences, with the resultant seizure between any of those bearings
against their mating surfaces. Normal operation of the engine will wear
those high spots down pretty quickly. But in the meantime the engine
load must be keep to a minimum to keep the heat level below the point
of excess expansion of the mating bearing surfaces. Such expansion can
lead to the surfaces being microscopically welded together, ripping the
bearing's surfaces, and ruining the bearing and its mating surface. So
for plain bushes keep the engine speed low, with minimal loading, and
change the oil often to remove the materials removed during the initial
stages of engine operation.

For roller/ball bearings the hardened bearing surfaces are precision
ground for optimal wear and clearances. But the bearings must be
intermittently and lightly loaded for the bearings to correctly seat
on their shafts and into their bores. Any slight misalignment can cause
binding which generates heat causing the bearing either to seize
internally or wear excessively. During this wear-in process the
lubricating oil must remain clean to minimize a grinding compound
action from the scrubbed off debris floating in the oil from other
bearing's break-in process. So for these kinds of bearings keep the
engine speed down, only under light loads, and change the oil
frequently.

Valves have stems that run in guides made of cast iron or bronze or
aluminum alloys. Again, nothing is perfect, so the valve to stem
clearances in the newly rebuilt motor must be worn-in. The excessive
use of high loads or high RPMs can make the valves stick to the guides
through that local welding mentioned earlier. Such sticking can result
in the piston bumping the partially open valve, perhaps bending its'
stem or punching a hole in the piston's top. Avoid the high loads and
high RPMs and get those micro particles generated during the wear-in
process out of the oil through frequent initial changes.

Piston break-in involves several things. Pistons also are not perfect
and the cylinder wall to piston clearances are not always uniform and
correct over the entire piston surface. Again using light loads and
reduced engine RPMs keeps the heat factor low, minimizing the
possibility of the aluminum piston expanding too much to create an
interference fit to the cast iron cylinder bore (commonly called a
piston seizure). The rings move in the lands on the piston (in and out)
following the irregularities of the cylinder wall. Such motion creates
heat. The initial imperfect fit of those rings to the piston lands can
cause the heat to get excessive if the engine is operated at high loads
or high RPMs. The high heat will cause that local welding action making
the rings stick in the lands eliminating their ability to meet their
number one requirement of controlling compression gases or controlling
lubrication oil in the bottom end. The rings also must wear into the
profile of the cylinder walls. The "cross hatched" surface that we put
in the cylinders walls with a hone during rebuild wears the rings outer
cylindrical surface to conform to the cylinder walls. All of this
initial wearing-in of parts generates particles of removed metals, just
waiting to contaminate other bearings and bearing surface throughout
the engine. So, change the oil often, initially.

The constant here is to change the oil regularly! I like to change the
oil after the first ride, usually about 100 or so miles. Then again
I'll change the oil out at about 500 mile, then at the normal regular
intervals of 1000 miles. Yes, I know the manual recommends much longer
normal change intervals, but these British bikes normally don't have a
really good oil filter. New high detergency oils keep those abrasive
particle suspended, allowing the oil to carry those particle to all
bearing surfaces, acting like grinding paste. Oil is cheap so change it
often, even after the break-in process. Since our bikes are generally
used only minimally perhaps after the first two oil changes it would be
prudent to change oil based on the calendar not the mileage. A good
plan is to change the oil just before storing the bike away for the
winter and then again mid summer time. That change before winter
assures that all the acids generated normally during combustion go out
the drain plug before storage.

Years ago cars used to come with factory break-in oils installed. Such
a practice is not used today. Regular oils are used today but are
recommended to be changed very early. What oil is best for your new
motor? I like to use Valvoline or any other manufactures "racing" oil.
This is not a vegetable oil like castor based oils but rather a regular
oil that has extra friction reducing additives like STP blended in. I
use the 20W/50 or mono grades 40 or 50 weight here in Southern
California, The 20 weight for the 20W/50 at start up temperatures
assures easy kick over while the 50 weight after the oil temperature
climbs to normal operating temperatures assures an adequate oil film
used for lubrication. But it is becoming apparent based on more recent
information that mono grade oils are really the ones to use. Do not use
synthetic oils during break-in as they are too slippery and will slow
down the wear-in process.

For transmissions I like to use the excessive pressure (EP)
transmission oils. The standard 85 weight is adequate. For break-in I
like to change out the transmission oil along with the engine oil at
100 and 500 miles, then at the regular 1000 mile engine oil change
intervals.

For the primary any 20 or 20w/30 engine oil works fine and only needs
to be changed out at regular engine oil change intervals of 1000 miles.
For motors (later Triumphs and later BSA singles) that use the engine
oil for the primary lubrication be sure to change that oil at the
regular intervals. Such motors require a "priming" quantity be present
in the primary chain case at start-up. Use the same oil here as for the
engine. See your manual.

To reiterate, break-in your engine for at least 1000 miles, changing
the oil at 100, 500, and the normal 1000 mile intervals. There after
change the oil every 1000 miles (or use the time interval schedule).
Change the trans oil out at the same intervals as the engine oil.
Change the primary oil at the 1000 change intervals. Or if your primary
needs the "prime" then change it out with the engine oil. Be sure to
clean any and all screens used as filters in the oil system. During
break-in running keep engine revs to the first half of the red line,
slowly and progressively increasing the allowed RPM until you are at
full red line at 1000 miles. Lengthening the break-in process to 2000
miles can only make the engine last longer . So if you can bear it,
stretch the slow, light load running up to 2000 miles, taking it
occasionally in the latter half of the break-in to higher revs.

Well, I hope this helps you assure that your newly rebuilt motor will
last a lifetime: yours.