In the early post-war years, the model was reintroduced in virtually its 1939 form, but was dropped after 1948; such were the manufacturing difficulties at the time that it was decided to concentrate production resources on the LE and MAC models. In 1951, the MAC engine appeared in a considerably redesigned form, with light-alloy cylinder and cylinder head.
As a result of the easing of manufacturing problems during the last two or three years, and the demand from Velocette adherents for a larger-capacity machine, it was decided to work on a new MSS power unit. In the interests of economic production, the MSS engine would be housed in the newly developed MAC rear-sprung frame, and would embody the proven features of its ancestors together with the lessons learned from the MAC.
Although it bore a similarity to the earlier version, the new MSS engine differed considerably there from in its internal details. Possibly the most obvious change concerned the bore and stroke which, from being 81mm x 96mm, became “square” at 86mm x 86mm. The engine had the familiar high-camshaft operation of the overhead valves, and emerged as a functional and efficient-looking unit
Charles Udall, who was responsible for the design, has been with Veloce, Ltd., since 1927 and, in pre-war years, was on the racing side under the late Harold Willis; on Willis’ death in 1939, Mr. Udall took over the racing department and, after the war, became development engineer. In this capacity he is concerned not only with laying out a design on paper but also with ensuring that it comes up to expectations after its translation into metal- an ideal combination to a technician in search of information.
To after the bore and stroke of a successful engine is a major step, not to be undertaken lightly by any designer. Hence my first query:
Question: “Does the 86mm bore and stroke mean that you are now in favour of comparatively short strokes and big bores, or is some other consideration involved?”
Answer: “The reason for the changed dimensions is very simple. We decided to use the current spring frame to house the new MSS engine, and the size of this frame is such that the old “long-stroke” engine was too tall to go in- so we shortened its stroke until it would fit. Where very high rpm. are necessary, as on a racing engine, a short stroke is essential, but the average roadster engine is in a different category and there is, in my opinion, no intrinsic advantage, in a bore/stroke ratio approaching or exceeding unity. A very tall engine will tend to be heavier than a short one and, if the stroke is over-long, difficulty will be encountered in getting adequate sizes of valves. With in certain limits, however, I consider that I could get almost identical characteristics and performance from any bore/stroke ratio.”
I had expected a technical lecture and had received a straightforward admission of expediency! We then considered the crankshaft assembly, and Mr. Udall pointed out that the flywheels, though comparatively narrow, are of large diameter with rims of fairly deep section, thus providing maximum flywheel effect with minimum weight.
Question: “I notice that there are no nuts for securing the mainshaft or the crankpin in the flywheels. Presumably you employ interference fits, but the absence of nuts is unusual in the case of the crankpin, and represents a difference from the earlier MSS engine. Why have you adopted this particular method of construction?”
Answer: “The mating parts have a taper of 0.008" per inch. This taper enables each shaft to be entered in its hole without difficulty during assembly and gives an interference fir of 0.003" to 0.0035" when fully home-ample for complete security. The elimination of the crankpin nuts has meant that we no longer have to counter-bore the flywheels to accommodate the nuts, and thus have almost twice the length of crankpin shank held in each wheel than we had before. In addition, we have increased the diameter of the shank; the combined effect of the two alterations, plus the reduced throw of the crank, is a very much stiffer flywheel assembly. Incidentally, we have employed the shallow-taper method of construction since 1925.”
It is of the utmost importance that the materials used for the various components of the crankshaft assembly should be up to the loading imposed on them. For this reason, the flywheels are stampings in a 0.3%, carbon steel, the crankpin is a 3 %, nickel-chromium, case-hardening steel, and the drive-side and timing-side shaft a direct-hardening steel is employed.
For the connecting rod En18, a 1% chromium steel, is employed and the forging is heat-treated to 60 tons/sq in tensile strength. The heat treatment is carried out before machining, to avoid distortion, and the usual hardened sleeve is pressed in to form the outer race of the big-end bearing.
Question: “On the ‘iron’ engine you balanced 70% of the reciprocating weight, whereas on the new engine only 55% is balanced. Does this alteration result from the reduced weight of the engine?”
Answer: “While the reduced weight might have had an effect, one cannot say that there is an optimum balance factor for an engine on its own: there is only an optimum factor for a giver engine-and-frame combination, which includes the method of mounting the engine in the frame. Here we have an engine of altered dimensions and weight from the earlier model, housed in a completely different frame. It would have been most surprising had the best balance factor proved to be the same for both machines. There is no known method of forecasting the best balance factor for any particular combination so it has to be ascertained by experiment.”
Interesting comment, as the balance factor was changed back to 70% around the introduction of the Venom in 1956. DQ.
High Load Capacity
The MSS engine is probably unique in that it has taper-roller bearings to support the mainshafts, in place of the more usual ball or parallel-roller pattern. These taper-roller bearings were first introduced on the post-war “long-stroke” MSS engine and proved so satisfactory that they were retained in the new design.
Bearings of this type have a high load-carrying capacity for their size, and are less affected than are other varieties by out-of-line forces caused by shaft deflection, which cannot be completely avoided in any engine. Also, the taper-roller bearing is intended to withstand axial as well as radial loading and so is admirably suited to dealing with the end-thrust imposed by the helical timing gears.
With quiet operation in mind, a degree of pre-loading is applied to the main bearings; this “nip”, as it is called, is not in any way harmful to the bearings. It ensures absence of play between rollers and races, so that “grumbling” during running is avoided.
The inner races are pressed on to the mainshafts. The outer races are pressed into the crankcase halves, where they are shimmed to provide a nip of 0.004" when the engine is cold. At normal running temperatures this nip comes down to between ½ and ¾ thou. It is recommended by the makers that after 10 to 15,000 miles, by which time the bearings will have thoroughly bedded down, the crankcase should be dismantled and the outer races re-shimmed to restore the pre-loading to its original figure. Thereafter no further attention should be necessary for the rest of this life of the bearing.
Question: “In this new engine you retain the traditional, narrow crankcase, with only one bearing for each mainshaft. Since so many other manufactures employ two bearings, at least on the drive side, can you tell me the reasons for your layout?”
Second Bearing Unnecessary
Answer: “Many years ago we decided to keep the primary chain line as close as possible to the engine centre line. This necessitated our putting the primary drive inside the final drive. Although this feature resulted in a more complicated clutch-operating mechanism, the absence of overhang and the consequently short, stiff mainshaft enabled us to dispense with a second drive-side main bearing which would undoubtedly be necessary if we had a conventional drive layout. On the timing side, the mainshaft pinion is located right up against the outside of the bearing, so that here, too, the shaft is short and stiff enough to require no out board support.”
Question : “For the big-end bearing you employ a single row of 3/16" x 9/16" rollers in a Duralumin cage, in place of the more common two rows (or even three) of shorter rollers. What benefit do you consider to accrue from the use of these long rollers?”
Answer: “All bearing rollers have radiused- or chamfered-ends, to avoid flaking of the case at these points. These end radii reduce appreciably the effective length of the roller, so that three rows of 3/16" x 3/16" rollers would have a lower bearing capacity than has our single row. You will note that the ends of the cage which runs on the crankpin, are located in shallow recesses in the flywheels, so that the rollers virtually fill the gap between the wheels. This avoidance of wasted width assists in achieving a narrow rigid crankshaft assembly.”
Engine lubrication is by gear-type pump driven by a bronze worm on the end of the timing-side mainshaft; to ensure through scavenging of the crankcase, the scavenge-pump capacity has been increased and is nearly two and a half times that of the pressure side.
On leaving the pump, the oil is forced to a gallery in the timing cover, whence it passes to four separate feeds: one leads to a big-end bearing via the mainshaft, another lubricates the cam-spindle bearing, the third directs oil to the cam faces, and the last takes oil to the rocker gear. Oil is supplied in the desired quantity and at a suitable pressure for each duty by means of jets and metering holes.
Several features requiring comment emerged from a study of the lubrication system, so I put the following questions to Mr. Udall.
Question: “A disadvantage of the gear-type oil pump is its tendency to let oil seep past it into the crankcase, so that over-oiling can result on starting after the engine has not been running for some time. Have you made any provision to deal with this difficulty?”
Answer : “We encountered the trouble on earlier engines, and have taken two steps to eliminate it on the MSS unit. In non-return ball valve, held on its seat by a light spring when the engine is not running. Pump suction is sufficient to take the ball off its seat so that oil can flow from the tank to engine. Though unusual, this method has proved entirely satisfactory, provided only that the feed pipe is fully primed with oil before it is coupled to the tank. On the scavenge side, the oil is returned through a fabric filter in the tank and, to prevent the oil in the filter from draining back, the return pipe is extended above the top of the filter.”
Question: “Big-end lubrication on the earlier engines was by the usual drill ways in mainshaft, flywheel and crankpin, but I note that the crankpin is no longer drilled. Instead, the drill way in the flywheel is inclined and emerges at the inner face slightly nearer to the centre than the crankpin. What is the advantage of this over the former method?”
Answer: “Where the crankpin is drilled axially, centrifugal force results in the oil tending to go only to the outer part of the bearing, so that the rollers nearer the axis of the flywheel may be under-lubricated. By supplying the oil to the point of the bearing nearest to the centrifuged through the whole of the bearing, so that more through lubrication is obtained.”
Question: “The rocker gear is fed from the pressure side of the oil pump, and not from the scavenge side as on many other engines popular today. Also, the pipe from the timing chest is no less than 5/16in in outside diameter. Why do you not lubricate from the scavenge pump, and why is the pipe so large?”
Answer : “Lubrication from the scavenge pump may result in some restriction in efficiency and hence inadequate scavenging. Also the pressure is so low that, in conjunction with a fair length of small-bore piping, little real lubrication can result, particularly when the oil is cold.
“We prefer to use the pressure pump to make sure the oil gets to where it is wanted, to give it passages of adequate size through which to flow, and to restrict the quantity by the use of metering holes at the component in concerned-in this case the rocker bearings, where the oil emerges through 0.046" holes.
Another feature to be carried on unchanged from the earlier-series engines is the timing gear. All push-rod Velocette engines have had small-pitch, helical-cut teeth in the timing train; despite their rather higher manufacturing cost, such gears are considered to be well worth while since they operate more quietly than do straight-toothed gears. The reason for this quieter running is that more than one tooth is always in full engagement over part if its length; thus, the driving load is not transferred suddenly from one tooth to the next but the changeover is relatively gradual and smooth.
Adjustment of Backlash
A diametral pitch (number of teeth divided by pitch circle diameter) of 32 is employed, and the intermediate idler gear between the crankshaft pinion and the cam wheel has a hunting tooth to distribute wear. This idler also has an adjustable mounting whereby backlash can be taken up-a further point making for quiet running. The fixed spindle on which it revolves has a circular back plate with three tapped holes; there are three similarly spaced holes in the crankcase wall through which pass the set-screws securing the back plate. Adequate clearance is allowed in these second three holes to give the necessary meshing adjustment which is held when the screws are tightened. On erection, the adjustment is set so that all backlash is just taken up with the engine cold.
As mentioned earlier by Mr. Udall, the mainshaft pinion is carried close up to the main bearing. It follows that the intermediate and cam gears also have little overhang from the crankcase wall; both are bronze bushed, and the cam spindle is pressed into the case. An outboard steady plate supports the outer ends of the two gear spindles and that carrying the cam followers. The steady plate is tied to the crankcase at two points, and thus not only maintains the spindles in correct relation with each other but also with the crankshaft.
The taper-interference fit mentioned in connection with the crankpin and mainshafts is also found between the cam wheel and the sleeve on which the cams are formed. In this case the taper that the parts are self-gauging: if the fit is correct, the sleeve will enter exactly half-way through the wheel when inserted by hand. If it went less far, the interference fit on pressing it home would be too heavy, while entry beyond the halfway point would result in too light an interference.
Question: Compared with that of many other engines, the valve timing of the MSS engine is very ‘moderate’ and provides only 38 degrees of overlap. This reduction from the 60 degrees of earlier MSS engines was presumably made with a view to economy and good torque lower down the speed range. Has any serious loss in top-end performance resulted and, if not, how have you avoided the loss?”
Answer: “ You are right in your assumption that we were guided primarily by the need for economy and better low speed pulling. However, there is no serious loss at the top, as is indicated by the peak power output of 23b.h.p. at 5,000rpm, with air cleaner and standard silencer. We have obtained this peak performance by an alteration to the bottom rockers (cam followers). These rockers have a radius of 1" instead of 3/8" as with the former engine. This larger radius results in quicker acceleration of the valve off its seat, and a longer deceleration period towards full lift.
“With a fairly high valve lift, the deceleration period must be as long as possible if the valve mechanism is going to follow the cam motion at high rpm. which it must do if float is to be avoided. Softer timing means that the total period available for opening (or closing) the valve is lessened, so that only by speeding up the acceleration stage was it possible to maintain an adequate valve lift with the new timing.”
While the fuel consumption under road conditions must await an independent road test, Charles Udall is confident that the MSS will prove economical. The effect of the modified valve timing on the torque curve has been most marked. Although the actual torque peak occurs as before at about 4,000 rpm the torque is very nearly constant between 3000 and 4,500rpm. after which it falls off rather sharply. Since the torque figure is still good at as low as 2,500 rpm. it should obviously be possible to drive the MSS largely as a top-gear machine if desired, and it should have excellent characteristics for sidecar work.
The cam followers referred to by Mr. Udall are of 3% nickel, case hardening steel and have a hard-facing alloy in the rubbing radius. Push rods are of Duralumin tubing with hardened steel ball-ends pressed into the lower ends.
Checking the Clearance
An unusual arrangement is employed at the rocker adjusters. The rocker carries a threaded, ball-end adjuster which seats in a cup formed in a hardened-steel mushroom. The stem of the mushroom is a sliding fit in the bore of the push rod, and the underside of its head forms a flat shoulder. On to the end of the push rod is pressed a shouldered sleeve; the shoulder of the mushroom seats on the flat top face of this sleeve.
It will be evident that the clearance between shoulder and sleeve face will be a measure of the valve clearance, which is checked by sliding a feeler into the gap. The adjacent corners of sleeve and mushroom have a small radius to assist the insertion of the feeler.
Apart from improved accessibility, this adjustment scheme has an advantage over a threaded adjuster at the valve end of the rocker. The thrust button which bears on the valve can be radiused in one plane only instead of being part-spherical. The resulting line contact causes less wear of the valve stem than did the almost point contact of the part-spherical button employed for the earlier engine.
When redesigning the MAC engine, Y-alloy was adopted for the cylinder head on account of its mechanical strength and its good heat conductivity. The latter feature permits the use of a higher compression ratio than does cast iron, thereby improving both performance and fuel consumption. Since it was clearly desirable that the MSS should not lag behind in these two respects, Y-alloy forms the head material on this engine also.
Shrunk into the head are valve seats of austenitic iron. This material was chosen because of its resistance to impact loading at elevated temperatures, and because of the security it provides through having a coefficient of expansion approaching that of Y-alloy. Four long holding-down studs pass through the head and the upper fins of the Al-fin cylinder barrel, and these studs gave rise to my next point
Question: “The four cylinder and head holding-down studs screw into steel sleeves which in turn screw into the crankcase. Surely such sleeves are unnecessary with semi-permanent items such as the studs?”
Answer: “They would be unnecessary if the studs were semi-permanent, but the height restriction of the frame is such that the studs have to be removed before the head can be lifted. Removal of the head will normally be infrequently required, but we thought it as well to guard against the inveterate “dismantler” by ensuring that no crankcase threads would strip.”
An unusual detail is that the head holding-down nuts have nylon inserts which, in addition to being self-locking, seal the threads against oil leakage from the head and down the studs.
On top of the head is bolted the rocker box, a die-casting in DTD424 aluminium alloy. Two half-bearings are machined in the box to take the journal portion of the rockers; the detachable bearing caps, also of aluminium alloy, are each secured by four screws. This uncommon construction gives very rigid support to the rockers but was dictated primarily by the difficulty of evolving any other satisfactory method of rocker mounting with hairpin valve springs.
As mentioned earlier, the rocker gear is fed from the pressure oil pump; from the union at the rear of the box, the oil enters a longitudinal gallery from which passages lead to the metering holes in the half-bearings.
Oil oozing from the end of the bearings is centrifuged along the rocker arms to the valve-stem ends and push-rod ends.The rockers are 3% nickel-steel forgings, case-hardened all over. In order to provide ample bearing area without excessive weight, the large-diameter journal portion is hollow.
Question: “Although hairpin valve springs have been employed for many years on the racing KTT engines, the new MSS unit is the first Velocette touring engine to be so equipped. What is the reason for this change?”
Answer: “It was found during early experimental work that coil springs would not give us what was required. To get the necessary characteristics with coil springs would have involved overstressing the material. With hairpin springs, the loading can be such as to give a considerably higher stress than is permissible with coil springs. A further advantage in this particular application is that the natural frequency of vibration of the hairpin spring is higher than that of the coil spring which it replaced, so that the possibility of spring surge is eliminated.”
Question: “The springs fit in an upper holder which is separate from the collet collar and is a sliding fit thereon. This practice is unusual in that most other hair-pin spring engines have the collets seating directly in the spring holder. What is the object of your rather more complicated layout?”
Answer: “Our layout is, of course, less simple, and involves a slightly higher reciprocating weight than the alternative you mention, but it is identical with that employed on the KTT engines. The advantage is that the valves can rotate. Freedom to rotate is particularly beneficial to the exhaust valve which is always unevenly heated and so tends to distort. If the valve is free to rotate we have found that it will certainly do so, although we cannot say why; thus it will not always be distorting in the same direction, and permanent deformation is less likely than with a valve which is held.”
Silchrome is used for the inlet valve and an austenitic steel for the exhaust valve. Guides are of aluminium bronze, a good bearing material which has a coefficient of expansion similar to that of the head alloy and so will not tend to loosen when hot. Also the absence of flanges simplifies machining and requires less weight (and therefore cost) of material.
Question: “The piston crown is almost flat, having a radius of 5", and has no cutaways to provide valve clearance. Also the included angle between the valves is 70°, as on earlier engines, so that the combustion chamber is rather shallower than a true hemisphere. What factors affected your decision to adopt this particular combination?”
Answer: “The absence of valve recesses results, of course, from the moderate valve timing employed, although the compression ratio, at 6.8 to 1, is probably above average for this type of engine. We retained the valve included angle at 70° in order to get a fairly open chamber, with adequate depth at the sparking plug; such depth assists propagation of the ignition flame and so ensures good combustion characteristics and freedom from detonation. Having decided on the bore and the size and angle of the valves, the shape of the piston crown followed automatically.”
In the interest of consistency between one engine and another, the inlet port is almost fully machined, so as to leave the minimum amount of hand work and, therefore, possibility of variation in shape. The port has a straight taper from the carburettor flange to the valve guide; this results in a small-radius bend at the bottom of the port which would appear to mask this portion of the valve opening area.
However, Mr. Udall considers that in any engine the area of valve opening at the inside of the port curvature can almost be ignored as regards cylinder filling; in his opinion the aim should be to make the best possible use of the remainder of the area. This is achieved on the MSS by having a layout such that the axis of the straight section of the port, when produced, passes through the area of opening at full lift. In other words, if the eye looks exactly along this axis, it can see straight into the cylinder head, so that the passage for the inlet gases is obviously unobstructed.
On the electrical-equipment side, there is one respect in which Veloce have long ploughed a lone furrow in the motor cycle world, although their method is used on the majority of cars.
Question: “Belt drive to the dynamo has for many years been a feature of Velocette machines and you retain it on the engine under discussion. What do you consider to be the advantages of the system over gear or chain drive?”
Answer: “The belt drive is cheaper than any other form would be, save direct drive as featured with A.C. generators; it is extremely quiet and requires no lubrication. Further, it is more resilient than any positive drive could be without the inter-position of a flexible coupling, and it can slip if overloaded, thereby avoiding damage to the dynamo.”
Search for Quietness
The legislation recently introduced on noise emission in Germany and Switzerland has underlined the rather unsatisfactory position of the motor cycle vis-à-vis the car. Whereas motor cycle exhaust noise is probably more noticeable to the man in the street than is mechanical noise, the latter often predominates as far as the rider is concerned. This thought gave rise to my final question, to round off a most interesting and instructive interview.
Question: “Velocette engines have acquired a name for unusual mechanical quietness, a reputation which, I gather, is maintained by the latest engines. To what features do you attribute this quietness?”
Answer: “As would be expected, the ‘alloy’ engines proved more of a problem than the “iron” engines which had better noise-damping qualities. In consequence we have had to pay more attention to the sources of noise, rather than to damp it out after it had been started. The adoption of taper-roller bearings, as has already been stated, has gained us a little, and the rigidity of the flywheel assembly and close-up support of the timing gears assist towards silence. Ample lubrication, the helical teeth of the timing gears, the belt dynamo drive and careful attention to the profile of the engine-shaft shock-absorber lobes are all contributing factors. In common with other manufacturers, we employ quietening ramps on the cams; but it may not be generally realised that we have been doing so since the ‘twenties.”