How does one “break in” an air-cooled engine after, say, a ring job on both cylinders? Do you switch to mineral oil? Should you be using straight-grade or multi-viscosity oil? Is synthetic really OK to use for this delicate operation? What about heat and rpm on fresh working surfaces? To what degree do cylinder materials (i.e. all iron or iron liners in aluminum) need to be taken into account? Are pistons and more importantly piston rings the key to the friction equation? For that matter, how long should it really take to “seat” a new set of rings? How do you know when an engine has finally broken in?

First things first

Whenever any cylinder is reworked for valves, guides, barrel de-glazing or whatever, the rings (with damn few exceptions) should be brand-new replacements. And whenever rings are replaced, the new ones need to be broken in. In effect, you need to break in your engine all over again, even if only one cylinder has been reworked.

Traditionally, this term “break-in” means nothing more than the wearing away of the highest high points (microscopic ridges in the metal that look like mountains under magnification) on the surfaces of any two metal pieces that rub together. In a typical cylinder, you’re “wearing in” (a term I prefer to breaking in, in deference to the intended outcome) chrome-faced rings against a cast iron barrel or sleeve. (Some few aftermarket cylinders can be exceptions… for instance Alan Sputhe’s patented barrels use a denser, harder material called Lascomite for the liners and those liners are 50-percent thicker than OEM. More of a factor than you might think.)

During break-in/wear-in, the very high pressure on the individual metallic ridges, as they come in contact with piston rings, effectively ruptures the oil film that separates them. The resulting friction causes the highest cylinder ridges to melt and flow (or weld and un-weld) in such a way as to conform to each other; the metal parts “seat” to each other as the piston rings harder surface scuff up. Paradoxically, an oil with anti-scuff agents (or super-high-film-strength lubricants) might actually interfere with break-in by preventing metal-to-metal contact. For this reason, it’s important that molybdenum disulfide, zinc dithiophosphate (ZPD), TCP, and other “extreme pressure” or boundary lubrication additives not be used for break-in. (Many shops unwittingly use “assembly lube” containing this kind of stuff to coat engine parts on an engine build. This is probably a bad practice and may be why as many as three out of 10 air-cooled Harley engines fail to break in properly after overhaul. Although I have a different idea… as you’ll soon see.)

Conventional wisdom says the proper oil to use for break-in is straight, unadulterated mineral oil, containing very few detergents or additives—but times change! These days the advances in piston ring technology, lubricant technology and… wait for it… the widespread adoption of so-called “plateau” honing means far fewer hazards and far better results, regardless of your choice of oil. Issues and obstacles that remain are what we will get into shortly.

What’s tricky is that the mere choice of oil, no matter what that might be, does not guarantee good wear-in; high cylinder mean effective pressures are also needed to promote rupture of the ring/piston oil film. Because from the moment a fresh cylinder (surface) is put into service, the minute valleys in the metal begin to fill up with varnish (cooked oil). When the valleys overflow with varnish, the metal acquires a smooth but useless “glazed” top layer, rendering break-in/wear-in difficult, if not impossible.

If you think about what’s going on, it should be fairly evident that the historic, ritualized process of ring seating is actually a race between the countervailing forces of ridge wear-down, on the one hand, and varnish accumulation, on the other hand. The kinetics of these two processes determine whether you end up with glazed cylinders (and oil consumption), or good ring seating (and dry-running cylinders).

Ring in the new

Air-cooled Harley engines have two compression rings and an oil control ring and everything depends on what happens between them and the barrel. In short, optimum oil control depends on proper break-in of all three rings. But think about what happens when you put a fresh set of rings in service: The top ring, naturally, is exposed to the greatest combustion pressure… therefore heat and friction! The normal pathway for escape of the pressure is for gases to travel behind the rings, then down to the next land or groove, then around the inside of that ring, and down again. The net effect is that the number two compression ring “feels” only about 40 percent as much pressure as the top ring, while the oil-control ring may feel only a tenth the pressure of the top ring. Also, there’s a corresponding temperature gradient: The top ring runs hottest, the second ring a little less so, and so on. What this means, of course, is that the top compression ring is going to break in no matter how you operate the engine (within reason). So, it’s the downstream rings that may never break in/wear in, if they fail to be exposed to proper pressures and temperatures. This is why most mechanics tell owners not to “baby” their engines during break-in… especially low-compression builds, which often produce barely enough pressure below the top ring.

“Recommended” procedures… Ha!

The fact is, if you insist on performing lengthy or repeated “heat soak” start-ups followed by total cool-downs prior to your first post-overhaul ride, you’ll most likely only hasten the formation of cylinder varnish, while creating the conditions that favor the break-in of your top compressions rings to the exclusion of all other rings. So, here we go with what “modern” plateau cylinder honing does to change break-in via:

The “Plateau” method

1) Lightly pre-oil the mating surface before startup, usually best done with the lubricant you intend to use after wearing in. In my honest opinion, it is not only acceptable but preferable to use synthetic oil. Mostly, because of the fact you literally have little flat-topped plateaus on the rubbing surface of the cylinder liner, not those microscopic, pointy “mountains” of yore. Knowing the true nature of modern synthetics and the requirements of those lower rings should put you at ease with this notion. Another hard-sell concept I happen to advocate is that you needn’t frantically change oil and filter over and over. Remember, quality filters that trap down to a few microns (smaller than blood cells) work better with a little “cake” and oil, which checked properly and frequently will tell you when it’s time to change, via appearance, smell and possibly consumption!

2) Start up and wait! The engine needs to be warm before you use it and while that’s happening checks for leaks, noises and smells. If, everything checks good, much as you would with an engine that is broken in, go ahead and ride for about 20 miles, varying the engine speed as constantly as possible. No freeway or steady speed riding! Do not short shift or lug the machine! Ride like you bought it, not like you stole it. When you get home, walk away until the next day, and then go again for 30–40 miles. Use this “ever-increasing circles” strategy for both distance and engine speed until break-in/wearing-in is complete. While you’re at that, try to keep rough track of how much time you have spent with the engine running.

3) Initial ring seating should occur within the first 15 minutes to two hours of engine run time. Final break-in—signified by stabilized oil consumption—should come in approximately 50 hours. You do the math… translating hours to miles based on your own riding routes including stop and go traffic. If oil consumption hasn’t stabilized in 100 hours, it will never happen and you might as well plan on re-ringing and honing all over again. To stress one key point that will keep you from a “do-over”: do not run at reduced engine speeds during break-in! Harley engines are most efficient in the 3,000–5,000 rpm range. Oil, rings and cylinder(s) will respond best to these engine speeds at normal operating temperatures. Remember, rings, oil and rpm finish what the honing process started.

So much for “modern” methodology and plateau-honed cylinders. One word before we continue though; as in discussions of sex, politics, religion and engine building… you never get unanimous agreement about anything! As often as not, that’s because of all the variables of overall engine age and condition, build quality and technique, parts used, fluids choices and not least, heat control… all coupled with human emotions about anecdotal, empirical evidence.

Hone sweet hone!

It seems a little strange to say, but to this day there remain many advocates of conventional honing, as practiced since your great granddaddy’s day. In its purest form it is still a useful technique on solid cast-iron Sportster, Knucklehead, Panhead and Shovelhead cylinders. With the introduction of the Evolution and later Twin Cam “liner-ed” aluminum cylinders came change, controversy, and so-called “torque plates.” Those all-iron cylinders are much more stable and less prone to heat-related distortion, albeit far less able to shed that heat. Sleeved barrels are quite flexible and easily distorted by comparison. By-design stability in these sleeved cylinders relies heavily on the long through-studs reaching from the heads to the crankcase on Evo/TC motors. Torque plates are intended to fool these cylinders into assuming the shape they should have when bolted into an engine. For decades both boring and conventional (stone) finish honing these shape-shifting barrels was the only game in town. In lots of establishments, most capable of turning out beautiful work, it still is! But since the object of the game is to have a perfectly round, straight hole when it’s all said and done there remain issues. Some of them are structural, to do with liners which might be too thin, thus increasing the chances that the hole will go oblong, oval or worse, once in service. Another is that torque plates are used at “room temperature”— not running temps! Let me put it another way. There has been very, very little trouble with liner distortion in 883 engines, because the liner is very thick on 3” bore barrels. Cut them to a 3 1/2” bore and problems pop up, notably the (thinner) liner actually warping until it protrudes up towards the head as manifested by blown head gaskets. Evo 1340 engines don’t have as high an incidence, largely because of the longer (if not thicker) liner and lower (stock) state of tune. (Seriously hot-rodded 80-inchers can be another story!) This saga continues today… ask any 110 owner! The point being, the larger the bore and thinner the liner the more likely a difficult break-in, partly from thin liners and partly from conventional honing. A thin liner with “micro-mountain” honed surfaces and the potential hot spots that accompany the flattening and smoothing process is going to shape-shift and bang rings around much more frequently. Under these circumstances, we should consider another break-in/wear-in procedure, a hell of lot like the one your grandad used and S&S (among others) still requires.

The S&S method

1) Initial start up. Run engine approximately one minute at 1250–1750 rpm. Do not crack throttle or subject to any loads during this period as head gaskets are susceptible to failure at this time. During this time, check to see that oil pressure is normal, that oil is returning to the oil tank, and that no leaks exist.

2) Shut off engine and thoroughly check for any leaks or other problems. Let engine cool to the touch.

3) After engine has cooled, start up again and allow the motor to build some heat. Engine should be run no longer than three to four minutes. When the cylinders become warm or hot to the touch (approximately 150°) shut the motor down and let it cool to room temp. Follow the same cautions as for the initial start-up, and continue to watch for problems.

4) Repeat this procedure three or four times. Each successive time it should take slightly longer to warm up and you can increase the temp slightly each time (+10°). You can be more liberal each time with the rpm, gently vary rpm continuously from idle up to 2500 rpm in the final cycle. Don’t be too concerned with final carb settings at this time because idle speed and mixture cannot be correctly set until the motor reaches full operating temperature. The motor should not reach that temperature during these cycles. Do not allow engine temperature to become excessive. After the motor has cooled to room temperature for the final time you are ready to start the 1,000-mile engine break-in process.

5) The first 50 miles are most critical for new rings and piston break-in. Engine damage is most likely to occur during this period. Keep heat down by not exceeding 2500 rpm. Avoid lugging the motor, riding in hot weather or in traffic. Vary the engine speed. Do not lug the engine. We recommend changing the oil at 50 miles.

6) The next 500 miles should be spent running the engine no faster than 3500 rpm or 60 mph. Avoid continuous steady speeds, and do not lug the engine. Vary engine rpm. We recommend changing the oil again at 500 miles.

Caution: Lugging or running engine prematurely at sustained high rpm may result in damage to pistons and other engine components. S&S voids its guarantee if engine is not broken in properly.

7) For the balance of the first 1,000 miles the motor can be run in a normal but conservative manner. You can be more liberal with the rpm range and the motorcycle can be operated at normal highway speeds. Avoid overheating or putting any hard strain on the engine: no drag racing, dyno runs, excessive speed, trailer towing or sidecar operation.

8) After 1,000 miles, verify carburetor jetting and adjustment. Change the engine oil. Motorcycle can now be operated normally.

9) Have fun!

Yeah, well, we’ll have more fun next time when we continue this discussion… mostly about the characteristics of seasoned cylinders vs. new off-the-shelf ones!


Please enter your comment!
Please enter your name here