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http://www.sacoriver.net/~red/breakin.html The purpose of the "break-in" procedure is to GRADUALLY wear down the "high spots" on components such as rings, piston skirts, cylinder walls, bearings and races, etc. after a motor is fitted with new items. ALL machined parts are imperfect to a certain degree and therefore have "high" and "low" areas which must be mated to those that they roll or rub against to achieve a good running fit. Problems can arise however in the process because the mere act of "rubbing down" the high spots creates abnormally high friction. Friction creates heat. Heat creates expansion. Expansion reduces running clearances and increases friction. More friction, more heat, more expansion... Pretty soon you can see that you are rubbing off MORE than high spots on each part resulting in premature part wear (LOW spots). This is what happens when a motor is broken in too aggressively. You end up with a motor that, at the very least, has abnormally LARGE running clearances throughout. Thus you now have an unnecessarily shortened remaining life for your "new" motor accompanied by reduced performance. If the motor is really abused during early "new life" running, the tight initial clearances may get closed up completely due to heat and expansion and the rotating or reciprocating parts will SEIZE. So how to control this "running in and mating" of moving parts becomes the question...
First, before you even start the motor for the first time, do a "cranking pressure" compression test with a good quality, screw-into-the-spark-plug-hole type compression gage. Ignition off, fuel off, throttle held WIDE OPEN. Kick, pull-rope or cycle the electric starter until the gage reaches its' highest reading and stays there. Note the reading and record it. Don't expect a real high number because the rings and cylinder are not mated yet, but you should see at least 100 psi, sometimes much higher depending on the planned compression ratio, port timing (or camshaft profile if it's a four stroke), etc.. Generally speaking, with fuel, air, spark at approximately the correct time, 100 psi gage pressure and exhaust, the motor will run.
I prefer to break-in motors on a petroleum based oil and then switch to a synthetic afterwards (if it's to be done at all). There's lots of opinions on this...... for better or worse, that's mine. My feelings are that "too slippery" an oil will slow down the break-in process too much and I've even seen 600X cross hatched cylinders, chrome and Nikasil bores where the rings never seated and we attributed it to synthetic oils during break-in. If it's a two stroke, you can add a bit of extra pre-mix oil to the fuel, set the oil pump at a slightly higher than normal base setting, or both for the first tank of fuel, but I'd use a petroleum based oil.
OK. Start the motor and allow it to run at approximately 1500 rpm or so. Shut the choke off absolutely ASAP! The excess fuel that the choke supplies can wash the oil film off the cylinder walls and overheat the ring faces quickly, especially in a four stroke. ALWAYS shut the choke off ASAP on ANY motor for this same reason. NEVER let a motor run for long periods with the choke on to warm it up. NEVER ride, drive, fly or place under load any motor driven device with the choke on. It is a quick route to early death for the rings.
Check immediately for oil and compression leaks around the various gasket sealing locations. ANY LEAKS should be fixed immediately, especially head, base or exhaust gasket areas. If there are none, hold your hand against the cylinder and GENTLY vary the engine speed in neutral between approximately 1500 and 2500 rpm. DO NOT OVER REV! There is no "load" on the engine and over revving is very tough on crankshaft, bearings, etc.! When the engine is warm enough to be uncomfortable on your hand, shut it off. Again check for any leaks. Now let the motor cool down to COLD. THEN, carefully re-torque the head(s) at this time.
Now you're ready for your first ride/drive/flight/whatever. Start the motor and warm-up gently exactly as before. When the motor is uncomfortably warm on your hand, stab her in gear and gently accelerate through each gear using about 1/3 to 1/2 throttle as a shift point. DO NOT BOG or LUG the motor. DO NOT "cruise" at a steady rpm. Vary the engine speed up and down at all times. DO NOT OVER REV either! When you reach top gear immediately slow down and ride back to your origin doing the same thing. Limit your initial ride time to 5 to 10 minutes maximum, all the while touching the cylinder frequently with your hand to sense drastic overheating. ANY signs of excessive heating or abnormal engine noises require immediate SHUT DOWN and investigation/cure of the culprit. If in doubt, DO NOT ride/drive/fly back to the garage and then shut it off... TOW it back! When you're done with the initial ride, let it cool down to COLD again.
Continue this procedure gradually extending the running time to 10 minutes, then 15 minutes, etc.. You can also gradually get a bit more aggressive with throttle application (slightly bigger "handfuls/footfuls" of throttle). Speed up, slow down, constantly varying throttle position and going up and down through the gears. Steady cruising at one engine speed or lugging the motor below its' powerband in a higher gear can cause overheating during break-in... AVOID BOTH! Don't worry so much about too high an rpm as VARYING the rpm. Bursts of throttle allow heating and mild expansion which in turn shaves off those high spots while deceleration allows slight cooling and contraction. Stay away from long hills, carrying a passenger or heavy loads during break-in.
After about an hour total riding/driving/flying time has accumulated, recheck cranking compression. As the rings seat, you will see the readings come up and you will also notice improvements in power delivery. Break-in is essentially complete when the readings peak and no longer get higher as more riding time accumulates. For a two stroke, this is typically one to three hours break-in time.
A four stroke has a superior oiling system and therefore breaks in more slowly. Two to five hundred miles is frequently required to completely break-in a four stroke. For a closely toleranced street four stroke it often takes 1000 to 1500 miles or even more! I dump the oil and filter in a four stroke after the first 75 miles, again at 200 miles, 500 miles, 1000 miles and each 1500 miles thereafter on a street engine. Off road and competition four strokes get fresh oil and filter every one hundred to four hundred miles with me, depending on how hard their running life is after break-in. The initial oil and filter change is done into a clean, light colored, plastic shallow pan so I can see any metal particles that drain out with it. Straining the oil through a clean, white paint filter is excellent practice. You can then drag a magnet through the oil to collect the particles that are ferrous for closer inspection of potential problems. Minor break-in particulate or "dust" is normal. I also cut open the oil filter and lay it out on clean white paper towel to see what it has trapped and again look for any signs of trouble. Yes, it's a lot of fiddling and checking but I find it infinitly preferable to engine catastrophies (and a lot less expensive!).
Once it is broken-in, you can optimize ignition timing and jetting, preferably on a dyno. During break-in keep the fuel/air SLIGHTLY rich and the ignition timing essentially stock, NOT advanced.
Even after break-in is done, always warm up the engine thoroughly before riding/driving/flying per the above procedure to avoid cold engine excessive wear or even possible "cold seizure" on liquid cooled motors (most frequently occurs in marine or snowmobile applications).
