Engine Break In

This is a summation of many articles on the subject found on the internet. Some of the information is from MOTORCYCLIST Feb, 1991, titled GIVE IT A BREAK-IN (How to make your bike run stronger and live longer), and some is from a Textron Lycoming “Key Reprint” article. Some is from my own experience building dozens of engines for racing and performance street/offroad

The first few hundred miles of a new engine’s life have a major impact on how strongly that engine will perform, how much oil it will consume and how long it will last. The main purpose of break-in is to seat the compression rings to the cylinder walls. We are talking about the physical mating of the engine’s piston rings to it’s corresponding cylinder wall. That is, we want to physically wear the new piston rings into the cylinder wall until a compatible seal between the two is achieved.

There are other contact surfaces, such as rod to rod, and rod to crankshaft side surfaces. Crankshaft to engine case end surfaces. Camshaft lobe to valve/bucket surfaces. and gear to gear and gear to shaft mating surfaces. These are all very hard and not ment to wear in. Their clearances are set during assembly and will not change during the break in period set by the manufactor. Also, contrary to some beliefs, the bearings do not need to be broken in. Their clearances are also set at the factory and will not change for many thousands of miles, let alone during the manufactor’s break in period. Again, its all a critical balance between putting enough pressure on the piston rings to seat in the cylinder bores and not so much heat that they will glaze.

Proper engine break-in will produce an engine that achieves maximum power output with the least amount of oil consumption due to the fact that the piston rings have seated properly to the cylinder wall. When the piston rings are broken in or seated, they do not allow combustion gases to escape the combustion chamber past the piston rings into the crankcase section of the engine. This lack of “blow-by” keeps your engine running cleaner and cooler by preventing hot combustion gases and by-products from entering the crankcase section of the engine. Excessive “blow-by” will cause the crankcase section of the engine to become pressurized and contaminated with combustion gases, which in turn will force normal oil vapors out of the engine’s breather, causing the engine to consume excessive amounts of oil.

In addition to sealing combustion gases in the combustion chamber, piston rings must also manage the amount of oil present on the cylinder walls for lubrication. If the rings do not seat properly, they cannot perform this function and will allow excessive amounts of oil to accumulate on the cylinder wall surfaces. This oil is burned each and every time the cylinder fires. The burning of this oil, coupled with “blow-by” induced engine breathing, are reasons that an engine that hasn’t been broken in properly will consume more than its share of oil.

When a cylinder is new or overhauled the surface of it’s walls are honed with abrasive stones to produce a rough surface that will help wear the piston rings in. This roughing up of the surface is known as “cross-hatching”. A cylinder wall that has been properly “cross hatched” has a series of minute peaks and valleys cut into its surface. The face or portion of the piston ring that interfaces with the cross hatched cylinder wall is tapered to allow only a small portion of the ring to contact the honed cylinder wall. When the engine is operated, the tapered portion of the face of the piston ring rubs against the coarse surface of the cylinder wall causing wear on both objects.

Each tiny groove acts as the oil reservoir holding oil up to the top level of the groove where it then spreads over the peak surface. The piston ring must travel up and down over this grooved surface, and must “hydroplane” on the oil film retained by the grooves. Otherwise, the ring would make metal-to-metal contact with the cylinder wall and the cylinder would quickly wear out.

However the ring will only ride on this film of oil if there is sufficient surface area to support the ring on the oil. When the cylinders are freshly honed the peaks are sharp with little surface area. Our goal when seating the rings on new steel cylinders is to flatten out these peaks to give more surface area to support the rings, while leaving the bottom of the groove intact to hold enough oil to keep the surface of the. See illustration:

At the point where the top of the peaks produced by the honing operation become smooth and the tapered portion of the piston ring wears flat break-in has occurred.

When the engine is operating, a force known as Mean Effective Pressure or M.E.P is generated within the combustion chamber. M.E.P. is the resultant force produced from the controlled burning of the fuel air mixture that the engine runs on. The higher the power setting the engine is running at, the higher the M.E.P. is and conversely as the power setting is lowered the M.E.P. becomes less. It’s important to remember that the values produced by the formula are for theoretical analysis only, and do not reflect the actual pressures inside an individual combustion chamber. That said, M.E.P. is still a quite useful way to describe the relationships between the various components, including the rings and cylinders and how engine power effects them.

Pressure is an important part of the break-in process. The spring tension of the rings has no chance of sealing the combustion pressures of an internal combustion engine, When the engine is running, pressure is present in the cylinder behind the piston rings and it’s force pushes the piston ring outward against the honed cylinder wall. Piston rings are designed to take advantage of this pressure. Therefore, as pressure builds during the compression stroke, the rings are pushed harder against the cylinder wall which aids in seating the rings. The more the throttle is opened the more the pressure behind the rings.

Since the higher the pressure, the harder the piston ring is pushed against the wall, the surface temperature at the piston ring face and cylinder wall interface will be greater with high pressure than with low pressure. This is because we are pushing the ring harder against the rough cylinder wall surface causing high amounts of friction and thus heat. The primary deterrent of break-in is this heat. Allowing too much heat to build up at the ring to cylinder wall interface will cause the lubricating oil that is present to break down and glaze the cylinder wall surface. This glaze will prevent any further seating of the piston rings. If glazing is allowed to happen break-in will never occur. Also, if too little pressure (throttle) is used during the break-in period, glazing will also occur.

Many people seem to operate on the philosophy that they can best get their money’s worth from any mechanical device by treating it with great care. This is probably true, but in most cases it is necessary to interpret what great care really means. This is particularly applicable when considering the break-in of a modern, reciprocating engine.

For those who still think that running the engine hard during break-in falls into the category of cruel and unusual punishment, there is one more argument for using high power loading for short periods (to avoid excessive heat) during the break-in. The use of low power settings does not expand the piston rings enough, and a film of oil is left on the cylinder walls. The high temperatures in the combustion chamber will oxidize this oil film so that it creates glazing of the cylinder walls. When this happens, the ring break-in process stops, and excessive oil consumption frequently occurs. The bad news is that extensive glazing can only be corrected by removing the cylinders and re honing the walls. This is expensive, and it is an expense that can be avoided by proper break-in procedures.

We must achieve a happy medium where we are pushing on the ring hard enough to wear it in but not hard enough to generate enough heat to cause glazing. Once again, if glazing should occur, the only remedy is to remove the effected cylinder, re-hone it and replace the piston rings and start the whole process over again.

We asked four top motorcycle engine builders what they do to ensure peak power output and optimum engine life. Here is a capsulation of their responses:

If the wrong type of oil is used initially, or the break-in is too easy, rings and cylinders will glaze and never seal properly. A fresh cylinder wall needs some medium to high engine loading to get the piston rings to seat properly for good compression, but make sure you don’t lug or overheat the engine. Use high quality, low viscosity oil (10W-40 weight), no synthetics, too slippery. If synthetics are used during initial break-in the rings are sure to glaze over.

  • An engine’s initial run should be used to bring oil and coolant (air, oil, and/or water) up to operating temperature only, with little or no load, then shut down and allowed to cool to ambient temperature. Do this in neutral before ever riding it. This is important and quite vital to camshaft longevity. Vary the RPM’s between 2500 and 4000 for 15 minutes, After this run the engine needs to completely cool down to ambient temperature.

  • After a cool down period, start it up again and take the motorcycle for it’s first ride. Get away from other traffic and distractions. Safety is the utmost consideration here. Wear full safety gear of course. Don’t crack up your bike or get a ticket for the sake of a good break-in. Choose your location and time wisely. A closed circuit such as a racetrack is the ideal venue if at all possible.Cool (but not slippery) conditions are desireable if at all possible.

  • This time, give the engine light loads at relatively low rpm and stay out of top gear to warm up the engine completely. Proper warming up of the engine is critical. The clearance between the pistons and the cylinder bore is very tight until they are completely warmed up. The piston expands faster than the cylinder and can scuff (metal to metal contact) inside the bores if the load isn’t kept low until expansion equalizes. Lugging the engine is more detrimental than high rpm. Another key is to constantly vary engine load during the entire break-in period. A constant load is not ideal for breaking in.
  • Now use 4-5 short bursts of acceleration at 1/2 throttle to 50% of redline, followed by deceleration against engine compression (no brakes) to help seat the rings. This deceleration against engine compression is a vital part of the break-in process, as it draws oil up onto the cylinder walls cooling the rings and washing the loose metal particles away. Warning: Make sure no one is behind you when you do this. Use the lower gears. High speeds are not necessary, nor desired. Keep an eye on engine temperature and don’t allow your engine to overheat. Turn it off and allow it to cool down if necessary.

  • Next, providing all is well with the engine and it is warmed up (not overheating), do 4-5 more short bursts of acceleration at 3/4 throttle to 75% of redline following each burst with deceleration against engine compression.

  • Finally, make 4-5 more runs at full throttle all of the way to redline, again followed by deceleration against engine compression.

  • Then head back to the garage, and while the engine is still warm drain the oil, clean any screens, and change the filter.

This gets out the new metal particles that are being worn away. Most of the metal particles will break away within the first 20 miles. To ensure the rings seat well, use the same high quality non-synthetic oil, and don’t be shy about short duration high rpm blasts through the lower gears after the oil has been changed. Don’t worry about hurting your engine. The KTM factory runs every engine to redline under full load on a dynomometer before it leaves the factory, and your dealer mechanic likely hit the rev limiter a couple of times during his “pre-delivery” road test. The key ingredient here is avoid excessive heat. No sustained high speeds, or lugging. They result in too much heat.

After 500 purposeful miles without running at a steady speed while monitoring engine temperature closely, change the oil and filter again. It’s a good idea to check the valves and re-torque the heads at this time also. Wait for at least 1500 miles before switching to synthetic oil. Most of the engine experts warn of the danger of breaking in an engine too easily and ending up with an engine that will always run slow and burn oil. Whether this is from tight tolerances, inadequate ring seal or carbon buildup this seems to be the rule rather than the exception. Engine load is more detrimental than rpm because of the heat created internally, so avoid lugging the engine but rev it freely especially in the lower gears. Basically, be sure not to get it too hot but be sure to seat the rings properly.

As renowned motojournalist Keith Cameron states (my emphasis added):
You often hear something like this: “Break it in fast and it will be fast, break it in slow and it will be slow.” There is some truth here because break-in has to apply enough load to force the parts into mutual machining action. If you timidly try to break it in at very low speed and almost zero throttle, you may never force the piston rings to shave themselves into good contact with the cylinder walls. That will result in a poor seal – and poor performance. But the “break in it fast” part of the saying seems to imply that the faster you push during break-in, the faster your engine will be as a result. Not so. If you push too hard too soon, the parts will score and scuff each other because the heat generated will be enough to destroy the oil film locally. A scuffed ring doesn’t seal.

So that’s it, sure a lot different than keeping under 6500 rpm for 1000km. Maybe bike manufacturers are being super cautious at the expense of your motor’s performance? I think that they take the cautious route that works over time (1000km, or about 15 hours of break-in) versus a faster route that can be more easily screwed up. They must also worry about liability and the wide variance in rider ability. I don’t think they could convey the proper technique for multiple acceleration-deceleration (without brakes) cycles, all the while monitoring engine temperature, in a paragraph in the owner’s manual anyway.