The Owner's Workshop




The Owner’s Workshop

HAVING got the crankshaft properly fitted in its bearings, the shaft, rods, and pistons balanced, and the whole ready to refit, attention must then be paid to the other details of the engine. The operations described in the last article of this series are essential as a basis for the more subtle tuning operations which follow, and it is quite useless spending time and trouble on the details of a ” top overhaul” if the essentials of the engine are not in perfect condition.

Before coming to the questions of valve and piston treatment there is work to be done in the way of seeing that other portions of the mechanism are perfectly free to function correctly. The crankshaft, having just being refitted, will naturally be stiff until fully run in, but before finally assembling it, the camshaft and all auxiliary drives should be examined. The opening and closing of an engine’s va.ves requires quite enough power without ally additional load due to auxiliary drive losses. Therefore when the valves and drive are disconnected it is possible to check the camshaft for freedom and lack of play.

In camshaft bearings there is no great uniformity of design, but whatever type is fitted (ball, roller, bronze, or whitemetal), they must be perfect. The removal of the camshaft is harder on some designs than others, but if there is ever to be a good chance of removing it easily, it will be when the whole engine is out of the frame and dismantled. It will be easy enough to see if the bearings are free by spinning the shaft, and it may be possible to see if there is any play in the end bearings. As the camshaft is likely, if of any reasonable length, to be carried on 3 or more bearings it is often impossible to see whether all bearings are O.K. without removing the shaft. The loads on a camshaft are of course radial, are very high (a point which is often lost sight of) and of an uneven and hammering nature. It is therefore quite possible for one bearing, say, of a 3-bear Success in tuning an engine for increased power and speed entails care and attention to every detail. In this article some of the more obscure points are

dealt with.

ing camshaft to develop excessive play, due to failure of metal or breaking up of a ball race, and yet to appear rigidly supported when tested by hand in position, especially if a centre bearing has gone.

Under load the unsupported portion is bound to flex and the immediate result will be a loss of valve lift and consequently of power, while further results may be failure of the shaft itself through fatigue, and if a bearing has gone at the driven end, the drive itself may suffer.

Naturally in a good engine all these occurrences are rare, and in fact the camshaft bearings and drive are usually taken for granted by amateur tuners, but they should not be. Nothing in tuning an engine should ever be taken for granted, and if success is desired in racing and competitions a great deal of this sort of work will have to be done. It does not require a great deal of experience of tuning engines for racing to turn one into a mechanical pessimist, but this is a very essential state of mind as ” hoping for the best” is precious little value as a means of securing reliability. Many people would regard it as wasted labour if, for instance, they spent some time in removing and checking up a camshaft, its bearings and oilways, only to

find that they were in perfect order.’ This is completely wrong, as they now know for certain that every part of that assembly has settled down correctly and is in perfect order, and this knowledge is worth much work and time. The more logical way to look at such work is:this. If nothing is required after the parts have been dismantled, one is actuallyrsaving the extra time that would be needed to put right any faults.

Having tested the camshaft bearings, and replaced any necessary parts, the same procedure must be applied to any other drives such as magneto, water pump, oil pump, etc. If any bronze bearings have to be replaced there must be carefully fitted by scraping or by use of reamer, according to the size of the bearing. Expanding reamers are very useful, but somewhat costly, and if a slightly increased quantity of metal is required to be shifted, a strip of brass foil laid alongside the reamer will have the desired effect. Bronze bushes must be fitted free, however, as the extra hardness of the metal will not allow it to bed down like white metal, and there is a danger of seizure if it is not perfectly free. There must not be any play, however.

The next thing to do will be to decide if any alterations are to be made in the piston department, and this will depend chiefly on the type of engine being dealt with. If it is a racing engine to start with, and the standard pistons are satisfactory, they should be left alone, apart from examination for possible flaws. If there are any signs of tightness on the bearing surface these spots can be eased off carefully with a smooth file. The clearance of the piston in the cylinder should be carefully measured ti see that they are correct. The makers will supply information as to the correct clearance, but if the engine is to be used for speed work this may be increased slightly with advantage. Increased clearance may mean a slightly noisy engine, and will also mean that special care must be taken t:) see that the rings are in perfect condition. There are many common fallacies with regard to piston rings, so that an explan

ation of some of the chief requirements will not be out of place. One of the commonest troubles in a highly tuned and not too new engine is oiling up, due to excessive oil getting past the rings, and many people imagine that the oil gets past the gap in the piston ring. Actually the oil gets past the back of the ring, and this can only be prevented by the rings being a perfect fit in their grooves. Otherwise the rings move up and down relative to the piston and act as a pump in transferring the oil to the upper side of the ring. If the ring grooves are really badly worn the pistons will probably be generally worn and new ones are indicated. If an attempt is being made to increase the performance of the engine considerably above standard, special racing pistons will be required, either from the makers of the engine if they supply them, or from someone who specialised in the production of racing types in special alloys.

The whole business of piston design and manufacture is sufficiently complicated and important to warrant special treatment on its own, and this is shown by the number of firms who make pistons alone. Among the smaller firms who specialise in racing pistons to the exclusion of commercial types may be mentioned the Brooklands Engineering Co., of Weybridge who make the Martlet piston. These have been well known to racing men for a long time and are fitted, incidentally, to George Eyston’s record breaking M.G. Midget. If special pistons are being obtained, the opportunity may be taken, in cases where there is room or necessity, to raise the compression ratio. This is, of course, one of the commonest methods of increasing the power and maximum revs, of any engine, but it must never be done in a haphazard manner. A very high compression ratio is apt to introduce many snags, as the loads on all working parts will be increased, the engine will be made rougher at low speeds

increased temperatures may lead to special requirements in the way of plugs, and therefore the purposes for which the engine is required should be carefully considered. Whatever is being done, one of the first things to ascertain is the compression ratio of the engine as it was before alteration. The makers usually state the compression ratio in their catalogue, but this is not always correct, sometimes very far from it, and the only way is to measure it. This is simplest when the engine is complete, but if it is not it will have to be built up temporarily so that at any rate one combustion chamber and piston are functioning. If the connecting rods are all out (as they will be if new pistons are being considered) one rod and piston should be fitted, the cylinder head bolted down with the valves in that particular cylinder fitted, and that cylinder can then be measured. The compression ratio is the ratio of the volume of the combustion space at bottom dead centre to that at top dead centre.

It is reasonable to suppose that the bore and stroke of the engine are what the makers state them to be, so that all that need be measured is the volume at T.D.C. This is achieved most simply by running in suitable fluid—thin oil, or oil and paraffin mixed—from a burette, into the sparking plug hole, and reading off the volume required to fill the head. This operation may require a certain amount of tipping the engine about to get the plug at the highest point. A burette, incidentally, is a long glass tube accurately graduated in C.C.s., and having a tap at the bottom. The actual trial of volume should be repeated to make sure there is no error, and from this the compression ratio can be worked out as follows :—The swept volume of each cylinder being known, the ratio is the sum of this and the cylinder head volume measured, divided by the latter volume. Let us take for example a 1500 c.c. 4-cylinder in which the T.D.C. volume is found to be 75 c.c.s. As the swept

volume of each cylinder is 375 c.c. the compression ratio will be 375 + 75, i.e., 6 to 1. 75

If the compression ratio is to be raised the increased height of the piston above the gudgeon can be worked out, or if the ratio is to be altered by milling a certain amount off the head or block, this amount can be calculated. Let us suppose that the same engine mentioned above, has a bore and stroke of 69 min.. x 100 mm., and that it is required to increase the compression ratio to 7 to 1. The T.D.C. volume of the combustion space must be reduced, and the new volume V such that- 375 Vt or 6/ =375 . • . V1

=62.5 c.c.s.

The change of volume (/—/ 1) 12.5 c.c.s. Therefore, the depth to be removed from the block can J:?e found. The stroke is 100 mm. giving a volume of 3.75 c.c.s. per mm. of stroke. 12.5 c.c.s are equivalent to 12.5 or 3.4 3.75

mm. of cylinder barrel.

If this amount is removed from the block (or added to the height of the new pistons, if any) this will produce the desired effect.

The above figures have, of course, been taken merely as an example, so that other alterations may be worked out according to requirements.

The actual fitting of the rings should be completed by lapping them into the barrel with jewellers’ rouge, using an old piston and a made-up wooden handle for the purpose.

The engine is now finished in its essentials and requires only the grinding in and refitting of the valves, and final details of assembly and tuning, which we will deal with in a subsequent issue .—W .S.B.