499 and 348cc Manx Nortons, the whys and wherefores...

The Manx Norton, a name synonymous with motorcycle road racing ..in this interview of Joe Craig M.I.Mech.E, M.S.A.E and Norton race team manager, George Wilson from "The MotorCycle" discusses two of the famous Norton production racing engines designed specifically for road racing and published in "Motor Cycle Engines", second series, 1955.....

It is fitting that the words "Manx " and " Norton " should combine to name what are probably the most widely renowned racing machines in the world. In a thousand exploits, Manx Nortons have earned for themselves, and for the entire British motor cycle manu­facturing industry, enviable fame. Based on the design of the famous factory power units, the Manx engines are magnificent, high-performance examples of their type. But high performance is not in itself the sole requirement for racing. It is a truism that a machine cannot win a race unless it finishes. Hence, allied with high perform­ance, there must be absolute reliability.

Therefore, at the beginning of my discus­sion with Joe Craig I said: " To be suc­cessful, any racing machine must have two attributes: reliability and high power out­put. Will you be so kind as to explain in general terms the broad principles by which you achieve such high performance and such a high standard of reliability ?"
Answer : "As you imply, reliability and high power output are entirely inter­dependent and both must be continuously available throughout a race. Reliability is ensured mainly by thorough observations of the engines in use during a racing sea­son. Components which give trouble are subjected to a careful post-mortem examination. Entire winters are spent investigating means of increasing re­liability and performance. Changes are made after test and retest, and then only when it is definitely established that a gain will result. Every care is taken to ensure that the engine will not wreck itself be‑ cause of the high inertia forces. These, as you may know, increase as the square of the speed, and assume tremendous propor­tions in an engine tuned for ultimate efficiency.
"High performance is achieved chiefly by admitting the greatest weight of charge per induction stroke (at high piston speeds) and burning it efficiently. The highest possible weight of charge is ob­tained through having a long induction period and by making maximum use of the kinetic energy of the exhaust gases from the previous stroke. The pressure difference across the exhaust valve en­courages the new charge to flow readily into the cylinder. Engine torque is of prime importance, and although an in­crease in bhp. may be possible between, say, 6,000 and 7,000rpm, the torque at this speed may be decreasing. Briefly, we aim at the highest torque at the highest possible engine speed while ensuring that the torque at low rpm does not suffer unduly in consequence.
Heat Retained
" When the best compromise of inlet and exhaust port shape has been established in conjunction with the length of valve opening period, diameter of valve head, and the displace­ment curve of the valves dictated by the shape of the cams and the required ' overlap,' work begins on improving the com­bustion of the charge. The aim is to provide the optimum amount of ' swirl '—agitation of the mixture—and so improve the evenness of the burn­ing. Swirl is influenced by port shape, diameter and angle, valve diameters, and the con­figurations under the valve head. Governed in this way by compromise, the best set of conditions is established. The com­bustion-chamber shape then receives attention so that the fuel is burned as efficiently as possible. It is of the utmost im­portance that the surface ­area-to-volume ratio is as low as possible, for it is this ratio which decides the proportion of heat that will be retained in the charge and the amount which will be barrel and valves. Heat retained is capable of being converted into useful work, but heat dissipated is responsible for many of the troubles associated with the internal-combustion engine."
Question: "I believe that some amateur racing men are inclined to over-rev their engines, sometimes before the oil is properly on the job. This misuse often brings about inertia failures at the big-end, and valve and valve-spring troubles. Are any steps taken in the Manx engine to prevent bothers of this sort arising ?"
Answer : " No advantage is to be gained by allowing a Manx engine to turn over at speeds in excess of 6,200rpm for the 500cc and 7,200rpm. for the 350cc Engines will usually withstand a sudden increase of, say, 500rpm. momentarily, as for instance when a gear is missed or a chain breaks; but when an engine is over-revved the big-end bearing, gudgeon pin and piston bosses are subject to excessive overloading. It is difficult to take any steps to prevent this occurring, apart from impressing upon the rider how harmful and expensive the practice of over-revving can be."
Question: "Bore and stroke dimensions for the 350 and 500cc engines respec­tively are 71 x 88mm and 79.62 x 100mm. These stroke dimensions are relatively long for modern racing engines in view of the present-day high power outputs. Will you explain, please, the requirements governing the Manx bore and stroke dimensions ? Is it not a fact that the use of a long stroke puts a limit on rpm because of piston speed and inertia load­ing considerations ?"
Power and R.p.m.
Answer : " While there is at present a tendency towards ' squarer' engines, we feel that our 71 x 88mm and 79.62 x 100mm power units have been—and are —quite successful in the hands of the general racing public. Although we agree that the squarer engine may have the ability to rev more for similar piston speed and inertia loading, one should not overlook the important point that higher engine speed is of no merit unless accom­panied by a substantial gain in power."
Question: "One of the most prominent features of the Manx engines is the large, square cylinder head. What is the reason for the use of square-profiled finning ?"
Answer : "Large fins are employed on the cylinder head in an endeavour to get a substantial cooling surface out into the air stream, and not rely entirely on the air that might pass the fork, mudguard and frame tubes. The square shape is em­ployed so that the passing air makes con­tact with the fin surface for the entire length of the head, thus making greater use of the available fin-tip area."
Question: "I believe that the original Manx cylinder head comprised a bronze skull, with the light-alloy fins cast on it. Why did you make the change-over to the present type of all light-alloy head with inserted valve seats? "
Answer : “ In the years before the war when petrol-benzole fuel was used, the composite-type head was most satisfactory. Since the war, however, the available fuel has tended to create higher combustion-chamber surface temperatures. The thermal conductivity of the aluminium-alloy head is superior to that of a com­posite aluminium-bronze unit, and lower combustion-chamber temperatures, there­fore, result."
Question " What is the material used for the cylinder head? "
Answer : "We use a special, low-expan­sion, high-silicon alloy which, among its other virtues, has good thermal conduc­tivity at elevated temperatures."
Question : "What material is employed for the valve guides? "
Answer : " The material for the inlet guide is phosphor bronze and for the ex­haust, chromium bronze. The high thermal conductivity of chromium bronze enables full use to be made of sodium-cooled type exhaust valves. In other words, the hot stem of the exhaust valve is able to lose its heat rapidly through such a guide, thereby lowering the temperature of the exhaust-valve head."
Question : " How are the guides inserted and how is concentricity ensured between their bores and the guide housings? "
Answer : " The guides are arranged to have an interference fit to the cylinder head. Every care is taken to ensure con­centricity of the valve-guide bore to its outside diameter during manufacture, but, to provide absolute concentricity, the valve seats are cut and the valves ground after the guides have been fitted. The actual guide bore is used as a location for these operations."
Combustion-chamber Shape
Question : " Is there anything unusual about the cylinder-head combustion-chamber shape? "
Answer : "No; the combustion chamber is nearly hemispherical. This shape has always been found to be very good, both volumetrically and thermally. The fact that it is also readily machineable ensures good reproduceability.'"
Question : " Are the valve seats pressed, or cast into position? "
Answer : "The valve seats are shrunk into the cylinder head with approximately 0.003” interference. This operation is carried out by heating the cylinder head to a predetermined, controlled temperature, until the degree of expansion enables the seats to be readily inserted."
Question: " What are the valve seat materials and why are they used? "
Answer : " The valve seat inserts are made from an austenitic cast iron—a material with approximately the same co­efficient of expansion as that of the cylinder head, and possessing qualities which render it comparatively free from distortion. Long seat life is thus assured."
Question : " Will you explain, briefly and in as non-technical a manner as pos­sible, how the Manx system of megaphone exhausting affects power output?
Answer : " In an exhaust system for a racing engine, the gases must be dis­charged so that the piston works against the minimum possible gas pressure. The kinetic energy of the gas must be utilized in such a manner as to produce the maxi­mum negative pressure in the cylinder towards and at the end of the exhaust stroke. Use can thus be made of a large valve overlap to get the inlet gas column moving in readiness for the next filling stroke. It has been established that the combination of exhaust pipe length and diameter is important in achieving the best possible results, and that no one com­bination is equally efficient over the whole speed range of the engine. " Usually a long pipe of small diameter is good for power at low rpm, while a short pipe of larger diameter is better for power at high rpm. It is necessary to compromise with a pipe diameter and length that will give the best results at the most used part of the engine-speed range. The length is often less than the regula­tion requirements for racing, and in the early days the megaphone was introduced to bring the length of the exhaust pipe to that required by the regulations. In later years, however, the exhaust pipe has been subjected to intensive investigation, and the angle of taper and length of mega­phone have been found to be important in achieving the greatest possible extractor effect and the consequent beneficial in­fluence on top-end performance."
A Compromise Length
Question : "Is it not true that consider­able induction pipe length and long valve opening periods are necessary in order to make full use of the phenomenon you have described? What are the length and diameter of the 348 and 499cc induc­tion pipes respectively, and how were the dimensions arrived at? "
Answer : "In the 499cc Manx engine, the diameter of the inlet port is 1.219” and the distance from the end of the carburettor trumpet to the inlet-valve head is approxi­mately 10.375”. The 348cc Manx engine has a l.125”diameter port, and the tract measures 10.375” from the end of carburet­tor trumpet to the inlet-valve head. Dis­regarding all the complex problems asso­ciated with the kinetic energy of the inlet charge, it may be said that the indicated mean effective pressure could be raised considerably at any one engine speed by the use of a longer induction tract. In the case of the Manx racing engine, we are forced to use a certain compromise length in order to accommodate the widest that, generally speaking, the greater the length or the smaller the diameter of the tract, the greater is the amplitude of the pressure waves and the later is the arrival of the pressure peak near the time of closing of the inlet valve."
Question : "What is the reason for the centres of the inlet and exhaust ports being offset in plan as they are? If the ports were truly aligned, would there not be greater advantage from the extractor effect of the megaphone? "
Answer : "The offset inlet port helps to impart the necessary swirl (or agita­tion) to the ingoing charge, thus ensuring an acceptable rate of flame travel through it. The offset exhaust port is necessary to allow the exhaust pipe to clear the frame tubes. The offset also helps to direct the hot gases away from the valve stem. The advantage gained in promot­ing turbulence to the ingoing charge is far more important than any possible ad­vantage to be had from straight-through ports and the assumed microscopically greater extractor effect of the megaphone."
Question: " Since the speed of the pressure waves in the induction pipe varies with temperature changes, does not the brake mean effective pressure vary con­siderably with altitude and geographical locations of various race meetings? "
Answer : " Yes, the speed of the pres­sure waves in the induction pipe varies slightly with changes of temperature which, theoretically, have their effect upon the kinetic energy of the inlet charge. In practice, however, this amounts to very little, and the variations of atmospheric pressure due to altitude have a much greater effect than temperature changes. These variations affect the specific weight of the air and consequently the bmep. As I said earlier, it is the weight of the inlet charge that is important for high power output, not necessarily the volume."
Cylinder Charging
Question : " According to the data in the information panel, the inlet valve open­ing period is 307.5°. Is there any direct relationship between the length of the induction pipe, the rpm and the in­let opening period ? "Answer : "In a naturally aspirated en­gine, the ingoing air has imparted to it a velocity sufficiently high at the carburet­tor jet to raise petrol and to continue at an increasing velocity along the port until it enters the cylinder. The force mainly responsible for this velocity is the nega­tive pressure created by the descending piston. This negative pressure occurs only for 180°of crankshaft move­ment and at, say, 6,000 rpm, precious little time is afforded to this cylinder-filling period. However, because the gas column is travelling at such high velocity, cylinder charging continues until the pressure created in the cylinder by the rising piston is equal to the pressure exerted by the gas column. In the case of the Manx engines, this pressure bal­ance takes place some 67.5 degrees after bottom dead centre. This ramming effect occurs as a result of high engine rpm, the shape of the valve-lift curve and the length, diameter and shape of the inlet tract. This ramming effect occurs as a result of high engine rpm, the shape of the valve-lift curve and the length, diameter and shape of the inlet tract. As I have mentioned, the kinetic energy of the exhaust gases helps the inlet charge provided, in the case of the Manx engine, the inlet valve is open some 60 degrees before TDC.

Question: "Obstructions in the inlet pipe, I believe, set up turbulent pockets which result in a falling-off in the size of the pressure waves. In the Manx engines, the inlet valve guide protrudes well into the port. Is this not a disadvantage in a high performance engine?"

Answer : "Even in racing engine design a certain amount of compromise has to be faced. In this instance, the guide does protrude a short distance into the port, but this is necessary to provide sufficient guide length for supporting the valve. The guide is, however, fairly well tapered to offer a minimum restriction to the inlet charge and, furthermore, care is taken to see that there is no reduction in cross-sectional area of the port. As a matter of interest, I might add that it has been found that when an inlet port was very large, a gain in power could be obtained by extending the inlet guide farther into the port."
Question : " What is the reason for double overhead-camshaft operation of the valves? "
Answer : " The reciprocating mass of each valve and its attendant mechanism can be kept to a minimum consistent with reliability. This low inertia of the recipro­cating mass facilitates high-speed valve operation in conjunction with the cam profile necessary for high power output."
Why a Vertical Shaft ?
Question : " Some successful racing en­gines with ohc operation of the valves employ chain or spur gears to transmit the drive from the crankshaft up to the cylinder head. Why do you favour the use of a tubular coupling and two sets of bevels? "
Answer : "The use of a vertical shaft and bevels for the camshaft drive reduces the possibility of cyclic variations in the system. It simplifies compression ratio changes, which, in the case of spur-gear drive with fixed gear centres, have usually to be made by alterations to the piston-crown height and shape."
Question : " How is balancing of the cam lobes effected? "
Answer : " Balancing is by means of a suitable hole drilled through the cam lobe. However, as the cams run at only half engine speed, balance is of secondary importance."
Question : " What is the reason for the Oldham couplings at the top and bottom of the vertical shaft? "
Answer : " These couplings are em­ployed principally to take care of shaft-length variations caused by expansion and contraction. Compression ratio changes, too, are simplified; the couplings can also accommodate slight misalignment be­tween the bevel gears and the vertical shaft."
Question : " Why do you favour the use of separate caps on the tips of the valve stems? Is it not preferable, with racing engines, to harden the tips? "
Answer : " The type of steel used for the manufacture of racing valves is not easy to harden. But as there is no rub­bing motion on the ends of the valves, the caps are not hardened either, but heat treated to ensure toughness. Caps are used, too, for convenience—because the method of adjusting the valve clearance is by the use of shims under the caps. The virtue of this method is that it keeps the reciprocating weight at this critical point to a minimum consistent with the provision of some adjustment."
Question : " Valve timing is, presum­ably, rough set by means of meshing the pinions in the cam box in their appro­priate positions to one another. How is fine adjustment achieved? "
Answer : " Valve timing is rough set in the first instance as you suggest, and finer adjustments are effected at two points : a removable peg locating one of the 12 holes in the cam-box bevel gear to one of the 11 holes in the cam-box bevel shaft, pro­vides for approximately 2¾ degrees angu­lar adjustment, representing approxi­mately 5½ degrees at the engine shaft. In the second case, the same principle is applied to the cam and its pinion : i.e., the cam has 11 equally spaced holes and the pinion 12 holes."
Question : " What material is used for the cam-box pinions and for what reasons is it suitable? "Answer : " The material used is EN.36V. It is used because it gives a very hard case with a strong core, while retaining a high degree of toughness."
Question : " I note that the idler pinions in the cam box have 33 teeth and those of the cam pinions only 21 teeth."
Answer : "This particular train of gears has been arranged to work in with the overall geometrical requirements dictated by the distance across the valve tips. This in turn is controlled by the valve angle and length, and by the relative vertical position of the cam box to the cylinder head."
Question : " What is the reason for sup­porting all the cam-box spindles on these expensive-looking ball and roller bear­ings? "
Answer : "The camshafts are each sup­ported on a roller bearing at one end and a ball bearing at the other. The idler gears run on standard 3/16”-diameter x 3/16”long rollers suitably caged about a fixed spindle. The bevel shaft, as you can see, is mounted on a ball bearing at the bevel gear end and a roller bearing at the other end; the whole provides an arrangement that offers very little fric­tional loss and does not require pressure lubrication."
Question : " Oil-pump drive is by means of spur gears from the timing-side mainshaft. What is the reason for the driven 44-tooth pinion being so much narrower than the 22-tooth driving pinion? I note that the larger is 5/16” wide and the smaller ½” wide."
Answer : " A certain amount of end ad­justment is required for both gears, and the position of the timing pinion on its shaft is dictated by a 30°, included-angle taper. The difference in gear widths ensures full tooth contact."
Question : " I note that the mainshafts are separate from the flywheels and pressed in. Would not the assembly be more rigid if the units were forged integral with one another? "
Answer : " We like to case-harden our flywheel shafts so that a tough core is ob­tained, and also to eliminate the danger of seizure of the bearing inner race to the shaft. To produce hardened shafts and soft flywheels with an integral design would be extremely difficult. In our arrangement, the shafts are of 1.125” diameter where they enter the flywheels, to which they are force-fitted, keyed, and nutted up against a large flange formed on the shaft. A very rigid assembly results."
Question : " The crankpin, I note, is made from the solid. Some racing crank­pins are in two parts, a separate roller track being pressed on the pin proper. What are the advantages of your system? "
Answer : " Although the one-piece crankpin is more costly to produce, we feel that it is worthwhile because of its extra rigidity. In addition, much greater accuracy is always possible where the number of parts demanding concentric accuracy is kept to a minimum."
Question : "The big-end eye of the con­necting rod is 2 5/16” diameter and the small-end bush bore diameter is 0.875”. The rod is no less than 1.250” wide at the small end and 1½” wide where the big-end webs blend to the general rod section. The webs around the big-end eye are on the rod's outer edges. What is the advantage of this design over the more common one of having a single web running round the big-end eye ? "
Answer : " There is less tendency for the outer edges of the big-end eye to be­come bell-mouthed' and there is a web supporting the track under each set of the two rows of rollers in the big end. While this form of construction is costly, we have found it to be worth while."
Question : " Obviously, the Manx con­necting rod must be able to withstand very severe bending loads. What is the material used and what are its properties? "
Answer : "The connecting rod material is KE805 which is a nickel-chrome, molybdenum steel suitably heat-treated to give a high tensile strength, good fatigue qualities and high resistance to shock load­ing."
Question : " Pistons for both 499 and 348cc engines are of slipper design, but the 499 cc piston has a dome top and the 348cc piston a flat crown with a markedly prominent radius between the top land and the crown. Also, with the 348cc piston, there is more land above the gudgeon-pin bosses than there is with the bigger one. Will you explain the reasons for these differences? "
Answer : "As I mentioned earlier, the best thermal conditions in a combustion chamber are obtained when the surface ­area-to-volume ratio is as small as possible. A flat-top piston offers the least surface area of any, and although the 499cc piston has a slight curvature, for all prac­tical purposes it could be considered flat. This slight curvature and any other dis­crepancies between the crowns of the two sizes of piston have been necessary to ob­tain the desired compression ratio in con junction with the combustion-chamber shape."
Question : " What is the piston material? "
Answer : "The pistons are forged from Hiduminium RR59 which, when heat-treated, possesses excellent mechanical properties at elevated temperatures, high thermal conductivity and a low friction co­efficient."
Question: " I note that the big-end bearing has a Duralumin cage while the main bearings have bronze cages. Why is there this difference ? "
Answer: " A main bearing cage, un­like that for the crankpin, does not impart centrifugal loading to the bearing. Hence, bronze can be employed for the main-bearing cage."
Power Outputs
Question: "What is the reason for the very tall crankcase ? "
Answer: "It enables the cylinder bar­rel to be deeply sunk into the crankcase and, by keeping the joint between crank­case and cylinder barrel as high as possible, the whole engine structure is ex­tremely rigid."
Question: "Is a bonded liner used in the composite cylinder ? "
Answer: " No. The close-grained, cast-iron liner, which has a corrugated outer surface for keying purposes, is cast into the light-alloy finned muff."
Question: "What is the recommended rpm range of each of the engines ? "
Answer: " Up to 6,000rpm for the 499cc and 7,000 for the 348cc"
Question: " What are the respective power outputs ? "
Answer: "Approximately 37.5bhp at 6,000rpm for the 499 c.c. engine and 29.5bhp at 7,000rpm for the 348cc. These figures are for 80-octane fuel, and assuming normal barometric pressure and temperature conditions."
Question: " Will you explain briefly at what part of the torque curve a rider should change gear when racing, and why ? "
Answer: " Generally speaking, the torque curve of a racing engine over its useful revolution range is more or less flat, with a pronounced dip at the ends, i.e., at low rpm and at the highest rpm. In order to make full use of the engine torque in travelling from A to B in the shortest possible time, a higher gear should be selected at the engine speed at which the torque (not the hp.), has just begun to “fall off” from peak. By so doing, one not only has the maximum effort the engine can exert available for acceleration, but any tendency for the en­gine rpm to decrease (because of gradient, or headwind) is overcome by the torque increase accompanying this slight falling off in rpm. The engine should be taken up to this predetermined rpm in every gear until top gear is engaged, and in top only should the engine be allowed to reach peak revs. The time taken to reach peak revs after maximum torque has begun to fall off can usefully be employed in accelerating in the next gear. For example, the 499cc Manx engine develops its maximum torque between 4,500 and 5,500rpm, with a slight “fall­ing off” up to 6,200rpm. The time to change gear, therefore, is when the engine revs have reached, say, 5,600rpm."

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Copied from "Motor Cycle Engines", Second Series,published by The Motor Cycle, 1955 with acknowledgement to Mortons Motorcycle Media, holders of the copyright.