The process of breaking in, or “running in” a new engine is a subject that has much more controversy surrounding it than it probably should. It is steeped in old rumor, myth, hard fact, and half-truths, with a healthy blending of real science and pure BS. Part of the reason for this jumble of fact and fiction is that the technology at the core of the internal combustion engine has evolved so much in just the past 70 years, and another part is that there is within the engine itself a kind of conflict of interest regarding the needs of various sub-assemblies as they are first put into service.
One school of thought is that the engine needs to be treated gingerly for the first little bit of run time. Another camp insists that if it isn’t subjected to heavy loads very early on, it will sacrifice part of its performance potential.
The fact is that, like a lot of things, there is some truth in most break in philosophies, and the empirical record is full of folks who followed any of several approaches and were successful, ending up with long engine life and extended performance in spite of the advice of the proponents of alternate methods. Why? Let’s examine the issue.
What is “Break In” in the First Place?
In any machine, when there are two freshly machined parts that move against each other, there is a basic problem of preventing them from damaging each other due to too much friction under too much force. Obviously, this is why the machine uses some form of lubrication at such points of contact. In fact, the fundamental goal of lubrication is actually to completely prevent the two surfaces having any direct contact whatsoever.
There two basic states, or modes, of lubrication. These are “hydrodynamic”, wherein the moving parts glide over each other totally separated so that they “plane” on the oil like a skim board, and “boundary”, wherein the two parts have forced their way past the oil film and have come into actual physical contact. “Anti-wear” additives are added to the oil to prevent damage during boundary conditions. More in this in a minute.
Next, there is the fact that even the most perfectly machined surface is never perfectly smooth. When looked at under a microscope, “asperities”, which are craggy looking high and low spots resembling a mountainous landscape, can be seen. If two such surfaces are moved across one another, the high spots of one dip into the low spots of the other, creating friction. This naturally has a tendency for the two parts to knock the high spots off of each other in the process known as wear. The unwelcome byproduct of this process is the debris that results from knocking down all those high spots, which is the major reason break in oil needs to be changed sooner than normal.
With that in mind, the basic goal of running in an engine is to minimize this wear process while promoting a kind of polish between the two moving surfaces, which then reduces the operating friction, makes hydrodynamic lubrication easier to achieve, and extends the useful service life of the assembly. Make sense?
Conflict of interests
As mentioned earlier, different kinds of moving components operate differently, are subject to different kinds of stresses, and have different needs during break in because of that.
“Plain bearing” surfaces like bushings, the bearing inserts on common automotive crankshafts, piston skirts, and the camshafts in typical motorcycle cylinder heads, need to be kept well apart from one another initially until they can develop that high degree of polished compatibility mentioned above. One means of helping this process along are “boundary lubricants”, the anti-wear compounds I spoke of. These are usually metallic compounds of zinc, phosphorus, molybdenum, sulfur, etc., whose purpose is to become embedded in the low asperities of the metal surfaces so as to prevent the neighboring high spots from digging into them. This takes place by running the oil parts together under moderate pressures with a film of oil containing these compounds for a period of time long enough to allow it to take place. Once accomplished, the surface is in effect, “flatter”, which supports the oil film better, improving hydrodynamic lubricity, and the two parts can bear on one another with very high pressures without significant wear taking place, even under “mixed film” conditions where the oil is beginning to fail to separate them. There are two major advantages in this process. First, it reduces the amount of actual wear required to produce a good polish, and that reduces the amount of debris generated during break in. The second is that these boundary lubricants can now permanently protect the moving components against damage at times when lubrication is marginal, such as during startup, or when the stays running while the bike lays on it side after a fall.
Image Courtesy of DIY Moto Fix
Another hazard is “adhesive wear”, which is the transfer of metal from one part to the other. This is seen as a “smearing” of metal from the bearing or piston skirt surface onto the shaft or bore surface it runs against, and from the standpoint of break in, is the result of too much pressure applied to the parts before an adequate amount of the boundary lube additives become embedded in the surface of the parts. It usually always involves the softer of the two metals being transferred to the harder.
Ball and rolling element bearings don’t break in in this way because their components don’t slide over each other as plain surfaces do, but they still depend on hydrodynamic separation, and on the embedding of anti-wear compounds into their contact surfaces. They take considerably less time to receive a viable level of boundary protection, though, and can survive nicely on remarkably little hydrodynamic lubrication after a very short run in period.
Then there are the piston rings. This is a major conflict area, since they do need to be protected from excessive localized wear and adhesive damage, but at the same time, their primary job is to form an effective seal against the walls of the cylinder bore. Therefore, they have to have a particular balance of anti-wear protection together with enough actual wear to produce a nearly complete match to the shape of the bore in which they run.
So, with that understood, what exactly is the answer to the question of how to properly break in a new or freshly built engine?
In the world of engines that was, it was normally the accepted practice to run the engine for a significant distance at not much more than half its potential output. This was true for a number of reasons, one of which was lubrication technology. Highly effective anti-wear compounds such as those currently available didn’t exist in the early fifties and prior, so the process of polishing off the asperities without wreaking havoc on the bearing surfaces had to be approached somewhat more cautiously, and given enough time to take place without the benefit of the filling in process modern phosphorus and moly compounds provide. Now that those additives are available, and generally included in premium motor oils, break-in periods spanning thousands of miles or scores of hours are no longer needed.
Add to that the fact that modern metallurgy and manufacturing methods are now capable of producing much more accurately machined parts that fit together almost perfectly out of the box, and there is much less wearing in necessary in the first place. Twenty years ago, the idea that there would be one size cylinder and one size piston made for an engine, and they would always fit together with the specified clearance range would be considered impossible. Now it’s standard operating procedure for several models including high performance engines.
One of the persistent myths surround the break in process is that synthetic oils can’t be used during the period. This may have been true 50 years ago, but not any more, and perhaps not even back then. The myth is centered on the notion that synthetic oil lubricates so much better than conventional oil that none of the wear required to polish and match things up will take place quickly enough, and that in particular, the piston rings will not wear into a good match to the bore fast enough. If the rings take too long to seal, the story goes, they will build up a glaze from the combustion gasses blowing past the incomplete seal.
One part of this is true; if the rings don’t seat fast enough, they can actually develop a coating of partially burned fuel byproducts, and that will prevent them from ever being close to 100% effective in sealing the force of combustion up in the combustion chamber where it belongs. However, really significant advances in piston ring technology have all but eliminated this problem. More on that in a bit.
The two parts that aren’t true are one, that synthetic oil lubes better, and two, that too much lube during break in is a bad thing. Synthetic oil is almost always the same basic chemical compound that conventional oil is at its base. The difference is just that instead of being dug out of the ground and having a bunch of undesirable stuff removed from it in the refining process, it’s created from scratch in a lab, with none of the bad stuff included. And in fact, while Group IV and Group V synthetics are completely lab created, the so-called “synthetic” Group III oils are conventional oils that have undergone a higher level of refinement than other conventionals, and are allowed to use that term. So there really isn’t a difference in them in terms of their ability to keep two metal parts separate from each other, only in their durability under severe conditions. Frankly, the only sensible reason not to use them during break in is that they tend to be more expensive, and break in oil should be changed after a much shorter interval because the break in process normally produces a lot more debris than will be present after the process is completed.
Even if it were true that they lubed better, that would actually argue in favor of their use. Remember that the wear surfaces of new parts are rougher than we want them to end up being, which creates undesirable friction and more wear than we’re looking for. Good lubrication is more critical during break in than at any other time, so the use of a high quality lubricant is extremely important.
And whether it’s synthetic or not, the use of an oil containing a lot of anti-wear additives is critical during break in because of how important the embedding of the new parts with those additives is to the entire process.
What about the rings?
Ah, yes, the piston rings. Back in the medieval times of the 1950’s, piston rings were almost universally made from simple cast iron. The process of machining both the rings and the cylinder bores was much less accurate than is currently standard, and they required a fairly significant amount of time to wear in to a good fit with the bore. Newly machined bores at the time were considered passably round if their radius varied by less than .0015”, while modern standards are about half that. Rings could not always be expected to be perfectly round once compressed to the bore diameter, either, which produced uneven pressures around their circumference, and uneven sealing to go with it. This was actually made worse by the introduction of chrome faced compression rings, which were brought into common use as a means of extending the wear life of the rings so they didn’t require the undesirably frequent replacement that iron rings did. However, the greater resistance to wear also extended the break in period, the time between installation and the development of a complete seal. Because of that, chrome rings were actually very much subject to becoming glazed over by combustion byproducts, and that was indeed a real problem.
Image Courtesy of DIY Moto Fix
The modern solution was to machine a shallow hollow face into the top ring and fill it with a hard compound of molybdenum. This served two functions; it reduced ring friction, and provided a small amount of sacrificial wear to the ring face that both sped up the “seating” process of the ring, and also protected the bore from wear by depositing the moly compound onto the bore walls, filling in the asperities there with what amounts to an anti-wear coating.
Another benefit of this is found in the fact that since the top, moly-filled ring seats and seals so much faster, almost immediately, in fact, that it protects the second compression ring under it from as much exposure to combustion gasses as it would otherwise get well enough to allow the use of a long-wearing chrome ring without the associated problems of glazing while wearing in.
Combine all that with current machining practices that produce rings and bores that come off the machine almost perfectly round and in matching sizes, and there’s not very much wear even necessary to seat them.
So with all of that having been said, the ideal break in process for a new or completely rebuilt engine is a matter of achieving a kind of balance of causing wear where it’s desirable, and preventing it where it isn’t. Ball and roller bearings don’t need to be dealt with very cautiously, but plain bearings need some respect and gentle treatment. The rings need some force applied.
One popular school of thought is that the engine should be warmed up fairly judiciously to at or near normal operating temperatures, and then placed under heavy loads of at least 85% of the engine’s potential output as soon as practical in order to seat the rings. This method will in fact usually produce a good ring seal that will last a long time, but it carries obvious hazards to any plain bearing surface, including, most importantly, the piston skirt.
Another even more hazardous common practice is “dry building” the top end, wherein the piston and cylinder are not lubricated at all during assembly. The concept is supposed to encourage a more complete seal of the rings by encouraging them to wear quickly, before they have a chance to have any oil glaze onto their faces. On the one hand, this is just a little bit like kidding one’s self, because oil thrown off from the connecting rod bearing in a four stroke will hit the bore walls within 10 seconds of startup at most, in a four stroke, and in a two stroke, the incoming fuel/oil mix will contact the piston and bore below the ring grooves before it ever gets to the top end for the first time.
One thing that is avoided by dry building is an excess of oil behind the rings in the ring grooves that may cook down into a sludgy deposit and interfere with their ability to float freely in the grooves as the piston moves around in the bore slightly, but that can be avoided simply by not slopping the rings up to an excess.
So, then, how to proceed?
The ideal method of breaking in a top end is “dead running” the engine for a short time. The rotating assemblies should be lubed with an appropriate, reasonably generous amount of the same oil that will be used in operation. In a four stroke, the camshafts should be left out of the assembly altogether for this phase when practical. The piston is lubed only at the wrist pin, and the bore and rings are left dry. Then engine is then rotated by any convenient means, including the electric starter, if so equipped, for between 150 to 300 revolutions. On smaller singles, one can put the bike in gear and rotate the rear wheel by hand, or walk the bike around in gear. This will almost completely seat a moly filled top ring and coat the bore in the ring sweep area without placing any undue stress on the piston skirt. A dry moly powder product made for this precise purpose, such as Total Seal’s Quick Seat dry film lube, is a good thing to use in this step. Dust the rings with a little and wipe some on the bore.
After the dead run, remove the cylinder and place one or two drops of oil on each ring, rotate it in its groove to distribute it, and wipe away any oily excess from the ring lands of the piston (the area between and immediately above and below the rings). Wet your fingers with oil and wipe a film onto the bore walls, again wiping away any oil that is more than just a film, and reassemble the top end. In real life, the compression rings of a four-stroke are lubed only by gasoline. The assembly lube should be just enough to protect them during the first 30 seconds of their exposure to live fire. Complete the rest of the assembly, lubricating all rotating and moving parts like camshafts, lifters, etc. with engine oil. Moly “assembly paste” should only be used where specifically called for, and sparingly.
When it comes time to start the engine up live, pay close attention to odd noises, leaks, loose things, and verify oil pressure and delivery to the extent possible. Give it at least 30 seconds to run up normal oil pressure and fill the passages of the lube system. If you don’t have a good sized fan to blow over the radiators, it’s wise to hop on and ride it around fast enough to keep it from heating up too quickly.
Shut it off and let it cool a little while you double check things mechanically. This lets it “soak “ in its own heat a little, and evens out the internal temperatures. Then it’s time for phase two. While it’s still warm, start it up and ride at a level at least 25% of its capability, but not more than 60% for around 5 minutes, then increase that to from 35% to 75% for another 10 minutes. Here, you can take another brief break to recheck your work, then take it out and run it fairly hard, with cycles of acceleration and deceleration at about 90% of it’s full potential for around 10 minutes.
At this point, shut it down, change the oil and service the filter, and call the process done. Break in is over. Go out and ride.
Engines off the showroom floor
If you’re dealing with a brand new bike off the showroom floor instead of an engine you just went through, there’s even less to worry about. That’s because the factories usually do the dead run on the cylinders during the assembly. Methods vary from one brand to another depending on how automated the process is, but almost all of them do it one way or another. Then when the machine reaches the end of the line, it gets started and checked over for any problems. The factory approach to addressing issues that turn up at this point also varies, but any bike that makes it to the dealer has been run long enough to skip the initial steps above and go right to the second phase; run it for about the first 10-15 minutes at up to about 60-75% of its capacity, then step it up for another 10 to 15. You want to avoid thrashing it right at first, but don’t “baby it” during the process, either. Shut it down, look it over, and if all looks well, call it done and have at it.
The truth is that the break in period has been reduced to a less than one hour experience by improvements in metallurgy and machining methods, improved engine oils, and proper assembly practices. The main keys to success are to put it together right, avoid either being too hard or too easy on it at first, use a good oil during the period, and do a complete oil change early.
About the author
Richard Ribley, (aka grayracer513) was a professional motorcycle mechanic and fabricator for 9 years, then moving on to automotive dealerships, where he specialized in engine, transmission, and powertrain overhaul and repair for over over 27 years. He is an ASE and Chevrolet Master Technician certified. During the last 15 years he has maintained his own fleet of motorcycles and built engines and suspensions as a sideline for friends and associates.