TECHNICAL TALKS. By F. T. BERSEY, M.I.A.E.
No. 1 —ON ENGINE OVERHAULING.
N attempting to describe some of the more important -2operations connected with the overhaul of sporting engines, I realise that many of my readers may be regarded as experts, especially in respect to what is needed in securing the extra ” pep” so necessary for success in racing and competition work. But it is one thing to know just what is wanted and another matter altogether to convert theory into practice. For example, there are hundreds of amateur motorists, who, given adequate facilities, could do much of the work carried out at Ewer Street, but the necessary
equipment is so costly and much of it has to be specially designed for particular operations. That, therefore, is just where our place in the motor industry has become recognised. To become a specialist in this class of work is not a matter of a few years concentration, and after twenty-four years of continuous effort, we find our works still undergoing improvement and expansion, for fresh demands for increased speed call for improved workshop methods and, like London, the Laystall Works” will be a fine place when it is finished.”
Why Special Machinery is Necessary.
Let us picture to ourselves the position of the private motorist, who having some race or competition in view, decides to overhaul his engine. Now, as you know, it is possible to do quite a lot to improve the efficiency of an engine by the aid of the simple equipment possessed by the keen amateur mechanic, and, from my own experience, I must agree that a lot of pleasure is derived from the work of getting an engine up to its best performance by one’s own efforts, but the more one indulges in practi
cal repair work of this description, the more obvious becomes the need for proper equipment. We all know how disappointing it is to work on a set of con-rod bearings, if the crankshaft journals happen to be oval to the extent of a few thousandths of an inch ; to say nothing of the anxiety concerning the ultimate fate of a bearing possessing blow holes or cracks. One could enumerate many other similar instances, but such is unnecessary when we realise the fact that success in any kind of competition demands absolute accuracy
in every detail of the engine overhaul. Such accuracy can only be ensured by modern workshop methods and the highest degree of mechanical skill.
The Danger of Super “Hotting.”
Not only are the two latter factors of the greatest importance in engine overhauling, but one must also be conversant with the materials of construction employed for different parts of the power unit. It must be remembered that there are certain risks incurred in attempting to obtain abnormal performance from an engine designed for ordinary touring purposes.
We are sometimes called upon to make alterations to engines of certain types and if we were to follow the instructions to the letter, the results would be absolutely disastrous. There is always a limit in ” hotting up ” any engine, and what that limit is in individual cases can only be determined by a wide amount of practical experience. Nobody has the monopoly of the information available on this point, but our experience proves the desirability of working to calculations, rather than waiting for a wrecked engine to proclaim that the factors of safety have been passed.
Of course, there are many engines running to-day, which according to all theories have been fined down beyond the limits of safety ; but the average sporting driver prefers to keep a little in hand rather than run unnecessary risks from mechanical breakdowns.
Many and serious are the responsibilities shouldered by the repair specialist, more particularly by those who undertake work for the racing driver, and it is the sense of this responsibility, as much as anything else, that has dictated the methods adopted in the works which I control.
Well, so much for my introductory remarks, and, if you are interested further, let us have a look round the shops to see what is going on.
Preparing the Engine for Overhaul.
Before any engine is overhauled, it is submitted to a very exhaustive examination and, in some cases, may be mounted on a Froude Dynamometer to obtain an accurate power output before the work is commenced. This is very useful as it provides a means for determining the exact improvements resulting from the overhaul, or from any modifications that may have been made during the progress of the work.
It stands to reason that if an owner intends to spend a fair amount of money on his engine, he is entitled to know exactly to what extent he benefits from the outlay. Incidentally, the data thus obtained is of considerable value for future reference, for, so far as is humanly possible, experimental work has to be kept to its proper sphere.
The next stage in the operation is interesting as it throws some light upon the cost of really first-class overhaul work. Experience has proved that the work of dismantling an engine is at least as important as the assembly operations and, therefore, is entrusted to highly skilled mechanics, instead of to men of the semiskilled category. This means that any obscure defect in any part of the engine is detected at the right time and any failure discovered is examined in its relation to other parts which may be affected.
Suppose, for example, we have received instructions to carry out certain work to a given engine and, on examining it internally, a serious flaw or other defect is located. The owner is at once communicated with and advised as to the true state of affairs, so that he can come to the Works to examine the parts himself. He will be advised as to the best course of procedure and can then give his authority for the extra work to be done. When any special work is in progress, it is quite usual for an owner to make several visits to the works and every
part of the overhaul is open for his inspection at any time.
Viewing the Dismantled Engine.
The mechanic in charge of the engine dismantling bench is also made responsible for viewing the different component parts, the dimensions of the wearing parts being carefully checked by means of precision instruments. With very few exceptions, the engines submitted for overhaul show a certain amount of wear on the crankshaft journals, which is corrected on one of the machines installed for the purposes. (See Fig. 2.) In connection with this operation, the mere fact of eliminating oval wear is not sufficient, for the dimensions of the reground journals must be regulated so that they are
all of uniform diameter when finished, not only with the object of securing a correct balance for the crankshaft, but also to maintain uniformity in machining the connecting rod bearings. Thus, if a set of crankshaft journals is machined to a definite size, with a tolerance of 0.0005 in., the machinist who rebores the newly metalled connecting rods can work to plug gauges with a similar limit, thereby reducing the amount of hand work in fitting the bearings and ensuring accuracy and economy.
How Accurate Bearing Alignment is Assured.
With the enormous stresses imposed upon the crankshafts of racing engines, great attention has to be paid to the question of bearing alignment and in the case of six and eight-cylinder engines with a high power to
weight ratio, the aluminium crankcases are apt to suffer a certain amount of distortion. It is easy to imagine the difficulty of bedding a set of crankshaft bearings under such conditions and in certain cases it is found desirable to restore the alignment of the crankshaft bearing housings by taking a slight skim from the latter on the special boring machine shown in Fig. I. By this method the job is started with a good foundation and the expense of setting up the machine is more than counteracted by the subsequent saving in hand fitting, quite apart from producing a first-class job when the engine is finished. This detail of engine overhaul work throws interesting light upon the need for employing costly machinery in securing the highest possible results and shows how
the old-fashioned methods of lining up bearings by the aid of brass-foil packing have been entirely eliminated in favour of really sound engineering practice.
Skilled Fitting Essential.
Notwithstanding the employment of elaborate machinery on engine bearing work, the need for skilled fitting remains paramount, for, though many production built cars of to-day are turned out without ever having seen a scraper, no method of producing a good working fit for bearings has been discovered to take the place of hand scraping. The life of a high efficiency engine depends very largely upon the degree of accuracy with which the bearings are scraped to fit the crankshaft, but it is
surprising to find how few mechanics are really expert in this class of work. Even after the careful machine methods described above, nothing is left to chance to make quite sure that all the bearings of an overhauled engine fit with the greatest accuracy and, after they have been fitted and examined, the engine is run for many hours under electric power in order to ” bed ” the bearings. At the end of the given time, the whole engine is stripped for a close examination of the bearings, which serves as a final check and affords an opportunity for making further adjustments if necessary.
By fitting the bearings slightly on the tight side and easing them off by electric power, a hard skin is formed on the surface of the bearing metal, which ensures long life as well as the fining down of any high spots which may have been left on the surface of the metal during the hand scraping progress. 0,