Breaking-in
Breaking in any type of machinery consists of wearing the surfaces together to obtain fits and finishes which are beyond the capabilities of the equipment used to manufacture the components and also mating the surfaces under actual operating conditions of stress and deflection, which, being largely unknown, cannot be allowed for in manufacture. One cannot, for instance, easily measure the slight ovality of a cylinder bore in an engine turning at 10,000 rpm. We know ovality will be present because all materials give to some degree under the load and, in the case of engines, the load on most components varies from a positive to a negative value with every stroke, and varies also in magnitude, according to the temperature and frequency of each operation. We therefore crudely manufacture as precisely as practicable and leave it to the machine itself to complete the process.
Break in is therefore a period of rapid initial wear and its rapidity and severity have a very definite bearing on the eventual performance and useful life of the machine. Engines undergo this process in most unfavorable conditions, as the beat which is their motivating force introduces an additional set of stresses and distortions which have to be accommodated. Additional heat produced by the rubbing of new microscopically rough surfaces makes temperatures shoot up beyond the designed values for the engine. Lubrication breaks down, more heat is generated by friction, parts expand and distort beyond limits, running clearances disappear and more and more heat is produced until the engine either seizes or chews its working surfaces into an easier fit.
At the end of this brutal business the engine will run freely and without overheating, and it will also run fairly well, but not for long. Because the wearing-in has taken place at temperatures well in excess of those arising under normal conditions, distortion beyond the normal has been accommodated, and the engine has been partially worn out. The most important thing to observe, therefore, when breaking in a new engine, is that temperatures do not exceed those anticipated at full power after break-in. All other considerations are offshoots of this main maxim. The thing that most people will want to know is what procedure and circumstances will meet this condition.
Since friction between the working surfaces is the source of excessive heat, the first objective must be to control the friction within certain limits. However, the more friction and wear we can tolerate, the faster will be the break-in process. Therefore, let us establish a rough and ready standard for gauging the maximum allowable temperature during break-in. Seventy degrees Centigrade is about right and, as most people have no means of measuring accurately, the simplest yardstick that approaches that temperature is the hottest you can grip with your fingers without severe pain. This does not mean a quick reluctant squeeze, but a good hearty grasp of the cylinder and head fins lasting several seconds.
The degree of friction depends upon lubrication, rpm and load. A fuel with an oil content of one-third is ample for lubrication, but in model engines a high viscosity oil should be used, of between 50 and 70 grade, or castor. Many modern fuels contain synthetic oils which are excellent. However, some of the cheaper fuels contain a grade of synthetic which tends to gum the cylinder walls. Therefore, use one of the recognized superior brands of low nitro content for break-in if you cannot mix your own. It is a fundamental fact that the load capacity of a plain bearing increases with rpm up to a critical surface speed which owing to the small size, is seldom reached in model engines. If the bearings are revolving fast under a given load, there will be less friction than if they were moving slowly with the same load. During break-in the engine should therefore be allowed to run at medium high revs, or about two thirds of maximum.
That leaves us with the load factor to take, care of. Many people keep the revs down by fitting a heavy coarse pitch prop. This is a mistake, as it actually increases the bearing loads in the engine. The condition to aim at is fast free turning with the minimum possible cylinder pressure, which necessitates the use of a light fine pitch prop of medium diameter. Rich mixture should be used to keep speeds down on a glow engine and lowest possible compression setting on a Diesel. As the engine loosens, these settings may be adjusted progressively, subject to temperature conditions, until prolonged flat-out running is achieved, when the engine can be considered broken-in.
Since friction determines the temperature and friction depends upon fits and finishes, the actual running time required by an engine before becoming ready for maximum output hinges upon the quality and accuracy of manufacture and the soundness of its thermal design. Every engine is different and wheras a top quality, carefully checked racing engine may be ready after an hour of short, high-speed runs, a job like the home-built MAN .19 may take several hours, depending on the workmanship.
The purpose of this article is to suggest a break-in program for the MAN .19, but the foregoing explains why this cannot be done in specific terms, since the surface finish in your particular example are known only to yourself. However a 10x4 wood Top Flight or equivalent is a good prop for the purpse and, if your piston and cylinder are free enough to bounce off compression after five minutes running, another two or three hours should be quite sufficient. Do not be unduly worried if the piston bubbles slightly at first as this will disappear with running if it is not obviously excessive. That is a virtue of the cast iron piston: its pores absorb carbon.
