It’s New, It’s Pristine, How Long Do You Have to Baby It?
By Kevin Cameron
Originally published in Sportbike 1995 Annual
Break-in is the final finishing operation in manufacturing an engine - and you, the owner of a new bike, perform it. At the factory, the bike builder hones the cylinder walls to a fine finish, grinds cams and tappets to accurate, smooth profiles, and makes connecting rod journals to high standards of roundness and accuracy. But even with all this, metal surfaces remain microscopically rough, consisting of tiny peaks and valleys. When you start a new engine, these surfaces must slide over each other; wherever the peaks stick up higher than the local oil film thickness, metal hits metal, welds momentarily from the intense local pressure, and then tears away. The oil sweeps a residue of particles away, carrying them to sump and filter. Some metal is simply pushed into shape, protected by oil additives, it deforms physically rather than being welded and torn.
Throughout the engine, this process works, quickly at first, then more slowly as break-in proceeds. Once the high spots are knocked or pushed down, the roughness of the surfaces no longer sticks above the oil films. Piston rings have filed themselves into a fine fit to their cylinders. Bearings spin without metal-to-metal contact, on full oil films. Break-in is complete.
This process can have three possible outcomes:
(1) If the break-in begins at high RPM and heavy throttle, the process may generate more heat and metal debris than the system can handle. Then the result is destruction of contact surfaces in some parts of the engine.
(2) If the break-in begins at a lower energy level and builds up gradually to higher revs and throttle, the washing action of the oil will keep up with the generation of wear particles, and the surfaces will bed into each other in such a way that the oil film can carry the load.
(3) The third possibility is that break-in will fail - usually as a result of such light-duty operation that parts are not loaded together forcefully enough to bed-in to one another. Rings glaze and fail to seal. The engine never delivers full power. Fortunately, this is rare where production machines are concerned.
Factory break-in procedures are designed to steer the middle course: not so vigorous as to damage surfaces, not so timid as to have no results at all. Generally, recommended break-in consists of operation at a variety of moderate speeds, alternating with no-load coasting. The idea behind this is that firm part-throttle operation for a period puts a load on bearings and other parts, forcing their surfaces together so they can polish each other to a fine fit. No-throttle coasting removes much of the load, allowing the oil system to flush away the wear particles. Gradually increasing the load (higher rpm and throttle) allows the bedding-in process to build up over time, rather than applying a possibly damaging load right at 1st.
Rob Muzzy of Kawasaki notes: ‘It wont break in until you really run it hard,’ noting that, ‘with today’s thin, low friction rings, you cant get the parts to reach each other without a good load.’ He says his team breaks in its race engines in much the same manner as for the street: 30-60 min of moderate operation on the dyno, just in case there are some really rough areas, followed by several pulls (that is, hard acceleration across the power band). He says that only by the 3rd pull does the engine begin to show its real power.
For a street machine owner, this dyno break-in translates to a period of moderate operation (Muzzy mentions 500 miles), followed by some hard acceleration. Sustained, high-speed operation is not a good idea because it provides no wash time at low load, during which the oil system can flush away any wear particles.
Once the break-in mileage has elapsed, the oil and filter are changed to remove the metal-loaded oil, and the (possibly) metal-loaded filter.
Break-in lore and myths: 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 a poor performance. But the ‘break it in 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 piston ring doesn’t seal. Many engine builders agree that you should not try to break in an engine on synthetic oil. If the oil film is too good, it will support even parts with extensive surface roughness. Only a small amount of local bedding-in may occur on the piston rings, in a poor fit (glazing) that improves only very slowly over time. Manufacturers of synthetic oils are almost unanimous in their insistence that this is not so, and that break-in is normal with their excellent products. But too many engineers and tuners have seen break-in either fail or take too long on synthetics for this to be the entire truth. Muzzy says that his team breaks in engines on mineral oil, and will run the fresh engine the entire first day at the track on the break-in oil, before draining and replacing with racing synthetic.
Synthetic oils are frequently chosen for racing use because low viscosities can be used that will cut friction losses by a small amount. This may be worth the trouble on the racetrack, but for street use, the choice between mineral and synthetic oils is yours. Street engines run well with mineral or synthetic oils of the recommended viscosity.
Your more important decision will be to follow a reasonable break-in procedure. Treat your engine with respect for its first 500 to 1000 miles, and it’ll repay you by delivering its best possible performance.
