Tuning the 12/50 and 12/60 Alvis
[The following informative article is contributed by an enthusiastic reader, J. A. Cooper of Leicester, who was until recently with Alvis Ltd., of Coventry. It follows on our series of tuning articles, which have dealt with Frazer-Nash, M.G., Talbot, Bentley, Riley, Austin, Delage and Hotchkiss cars. —Ed.].
The Alvis 12/50 has been referred to in my hearing as a "grossly over-rated motor car," and, at that, by someone who should have known better. For a firm to manufacture successfully a relatively expensive sports-car, with out altering a single major feature of the design, for eight whole years, is surely no mean achievement. I think nobody who has owned one of these cars in decent condition can deny their charm, specially on a long run; a satisfactory and all too rare feeling, similar, though in a lesser degree, to that engendered by the old type of Bentley.
I do not propose in this survey, to go back further than 1925; prior to that date, of course, came the well known side-valve 12/40 Alvis, and the first o.h.v. 12/50, with the dynamo belt-driven as on the s.v. model.
Therefore, the models directly referred to in this article are the TE and TF 12/50s, of 1925-6-7; the TG and TH 12/50s, of 1923-9; the sports SD 12/50 of all the above years, and the TJ 12/50 and TK-L 12/60s of 1931-2. Towards the end of 1929 the 12/50 went out of production, to be reintroduced early in 1931 in slightly redesigned form.
These cars have all the same general engine layout, the same camshaft (and valve timing), the same push-rod valve gear, and the same wheelbase, track and chassis arrangement.
Engines—The engines of all the models mentioned above, except those of the TE TH and SD 12/50 models, are of 69 x 110 mm. bore and stroke, giving a cubic capacity of 1,645 c.c. These engines are stamped with "L 200" on the timing case, following the engine number, except those of the TJ-TK-TL series, from which it was omitted. The engines of the TE, TH and SD cars are of 68 x 103 mm. bore and stroke (1,496 c.c.), and are similarly stamped 'L 201'.
All 12/50 engines have the parallel H-section steel connecting-rod, with the exception of L 201 engines numbered 6875 and over, that is manufactured after February 1928. These engines have taper H-secsion con-rods: both types of rod have integral white-metal big-end bearings.
The steel connecing rods have a 1/4 inch B.S.F. bolt, together with nut and split pin, locating the gudgeon pin, and all but those in a few early engines have also a sawcut down from the gudgeon pin boss to enable this bolt to clamp the gudgeon pin tightly, as well as to locate it endways. The duralumin rods can be substituted for the steel rods in any 12/50 engine, but end pads or circlips must be provided to locate the gudgeon pins, as there are no pinch bolts in these rods.
The breakage of these pinch bolts in the steel rods is one of the few troubles associated with these engines; mild steel bolts must never be used, the Alvis bolts having 5 per cent nickel content.
The crankcase, sump, oil pump and drive, etc., are identical on all 12/50s. The timing gears vary in material, but all are interchangeable; the best combination is probably the original steelbronze-steel, in spite of the claims to silence of fabroil.
The L 200 engines have the "long-throw" crankshaft, the L 201 engines the "short-throw" crankshaft; apart from 3 1/2 mm. difference in the throw these are identical. The crankshaft in the TJ, TK, TL engines is similar to that in the L 200 engines, but is machined at the front end to accommodate a vibration damper. This combination can with advantage be fitted to the earlier engines, the only other part that is different being the timing gear cover, which must also be changed.
The cylinder blocks of all those engines with 110 mm. stroke are interchangeable, likewise the blocks of those engines with 103 mm. stroke, but not the one with the other, unless the crankshafts and pistons are also changed.
All the early engines, and those of the TJ series, have cylinder heads in which the combustion chambers are 19.5 mm. deep; 2 or 2.5 mm. can be removed from the face of these heads with impunity, this raising the compression ratio from 5.35 to around 6 to 1. A better plan, however, is to use a "big-port" cylinder head from a later SD or TH 12/50; this has combustion chambers 17 mm. deep, together with considerably enlarged ports. The TL 12/60 engines have a "small-port" head with combustion chambers 17.5 mm. deep; not much can be removed from these heads as the wall is rather thin, but their initial compression ratio is 5.8, and some can always be removed from the face of this block if it is desired to raise this figure. The TE, and early SD cars, of 1925-6-7, have the touring cylinder head, and rely on h.c. pistons to make up for their shorter stroke. The later SD, and the TH, 12/50s have the "big-port" cylinder head, together with h.c. pistons and the duralumin con-rods; their compression ratio is initially 6.3 to 1, and need not be raised much for touring work; common sense must, as usual, be the limiting factor. These "big-port" heads have masked sparking-plug ports, providing an "explosion pocket"; these can with advantage be drilled and tapped straight through and long reach plugs used, as in the touring engines. The best plugs are probably K.L.G. L583, but these motors are not at all plug-conscious, and will run on any decent variety. All these cylinder heads repay port-polishing; the TJ-K-L heads, together with an indeterminate but small number of the earlier types, have machined combustion chambers, but the others will repay polishing and corner-removing in this department. It is permissible, also, to remove the boss and portion of valve guide which together break the symmetry of the port design, but it is doubtful whether the benefit accruing from this operation outweighs the added possibility of a bent valve.
Valves, guides, valve springs and cups, rocker shaft brackets and water pipes are identical on all 12/50s. Taking these in order:
Valves are adequate for all ordinary purposes. The valve guides should be treated with great care, and tapped into position with a drift or box spanner over the thin portion, as this will inevitably crack if hit. They should not be fitted too tight in the cylinder head, as hitting the drift too hard will neatly remove the apparently substantial shoulder from the middle of the guide.
Lightening the rockers is not worth the added risk of breakage. The standard valve springs are strong enough for anything up to around 4,800 revs., and more revs. are inadvisable in any case with standard assemblies.
If it is desired to fit a water thermometer, the best place for it is in the water transfer port at the rear of the engine; the TL engines have a port suitably adapted for the purpose. The TJ-K-L engines have a rocker shaft different from, but interchangeable with, that fitted to the earlier engines, with springs between the rockers instead of distance pieces; this helps to cure rattle but is unimportant. The metering pin in the rocker shaft opposite the oil feed pipe must be treated with respect; cutting the oil feed down may give valuable pounds of oil pressure, but if this is overdone an air lock is apt to develop, and by the time "mice" are audible to the driver much wear may have taken place. Starvation here also affects the centre and rear camshafts bearing, which are fed by the overflow from the valve gear.
The push rods, which are hollow, are supposedly the same length in all 12/50 engines, but some, at least, of the L 201 engines seem to have push rods about 1/4 inch shorter than standard. The tappets are all the same, but whereas on the L 201 engines they run direct in the block, on the other engines they rim in separate tappet guides, which are held in the crankcase by pressed steel bridge pieces. These last should be inspected occasionally, as they can work loose; split pins and castellated nuts do not effect a cure, because the studs can still work out of the crankcase.
The aluminium plate bearing the name "Alvis," which conceals all this mystery, drums badly, and a bolt and nut through it, arranged to press on the side of the cylinder block, cuts out a surprising amount of mechanical noise.
The TE, TF and early TG engines have the induction and exhaust manifold cast together; the induction part of this manifold almost invariably cracks, but insulation tape and copper wire are perfectly satisfactory for preventing air leaks (if welded it usually cracks again). For this reason the later TG and the TJ engines have the manifold (otherwise similar) cast in two pieces and bolted together; this is more satisfactory. These engines have a single 30 mm. type MV Solex vertical carburetter.
It is much better, however, to obtain the manifold off a TK or TL 12/60 engine, with its attendant twin horizontal S.U. carbs., type HV3HC. This will fit without alteration, except to petrol pipes and controls, but will probably necessitate an autovac or petrol pump even with a front petrol tank to maintain a head of essence; the correct needle for these carburetters is AC. The SD and TH engines, with the " big-port" head, utilise a correspondingly enlarged manifold with a 40 mm. type MV Solex vertical carburetter, the best normal setting for which is: Choke 27, Pilot 55, Main 120/51. For sheer power this carb. is unbeatable, if you can put up with the Solex flat-spot, which seems inescapable. No other manifold is available, unless home-made; twin S.U.s seem to provide more even acceleration but no more speed. The original specifications provide the SD model with an outside (and outsize) exhaust pipe and the TH model with the inside version; however, many of the former model (including my own) were turned out with the inside pipe, which was standardised for 1929, and the difference in performance is very slight. The TJ-K-L engines have coil ignition (except for special orders) while the rest have magnetos; the latter is perhaps preferable, especially for enthusiasts with dud batteries and no money, but it is doubtful whether the performance gain is appreciable, especially if a high voltage coil is used.
The valve timing is: Inlet opens TDC, closes 50° after BDC; Exhaust opens 55° before BDC, closes 10° after TDC.
Ignition timing 28° before TDC, fully advanced; incidentally, each degree represents 2.7 mm, measured round the circumference of the clutch-body.
The oil-pressure should be 25:30 lbs./square inch at 3,000 r.p.m. when hot, but many engines have run for many miles with much less. However, this is not advised, and attention to the bearings and to side play in the oil pump gears will work wonders. Adjustment of the relief valve is by shims, and in the usual way.
Coming to the back of the engine, the flywheel cannot be lightened much. The clutch body is of cast iron, and while a certain amount can be removed from it, this procedure is fraught with danger, as these have been known to disintegrate at high revs. The makers once cast a small number of these clutch bodies in aluminium, for racing purposes; this is definitely worth while, if you can find one or afford to have one cast. These clutches all have nine springs, the remaining three spaces only being utilised on the six-cylinder models; nine are normally ample. The only alteration in design to this clutch from its introduction in mid-1924 to its demise with the 12/50 in 1932 was the fitment of a slightly different toggle arm and operating sleeve, at engine number 6903.
Rev. counters were only fitted when racing, and occasionally to order, until 1932, when the TL 12/60 was fitted as standard with this invaluable instrument. These are all driven by the somewhat barbarous method of a belt, the drive being taken from a split pulley clamped to the shaft running between engine and gearbox. Some time ago, however, the Speedometer Supply people made a very compact right-angle drive to the order of an Alvis owner, to enable the rev. counter to be driven from the rear end of the camshaft, and they would doubtless undertake repeat orders.
The clutch stop should not come into action until the clutch shaft is entirely free; the spring behind the arm, however, can be compressed as much as is compatible with the free movement of the arm, so that the full force of the left leg can, when necessary, be exerted on the clutch stop. A valuable fitment is a segment of a ring of inch flat metal, lined with Ferodo, and riveted in turn to the standard clutch stop arm, thus increasing the friction surface. Incidentally, if the undershield under this portion of the car has been removed, wet weather will render the clutch stop inoperative, as the next upward gear change will effectively demonstrate.
Up to 1927 the engine and gearbox were mounted in a subframe, while after that date this was replaced by the well known chassis frame with the extended lower flange. The engine and gearbox units were not affected by this alteration. Those cars produced in the earlier batch (i.e. up to late 1929) were fitted with gearboxes with 3-point mounting, and also with the large propeller shaft equipped with Timken taper roller bearings and nine grease nipples. Those made in the revival period, 1931-2, had 4-point mounted gearboxes and Hardy Spicer prop. shafts; the latter are well worth procuring, as the Timken prop. shaft, unless in perfect condition and adjustment, is definitely illegitimate. In this connection it is worth while noting that to change from the one to the other involves taking out the gearbox, not only because the front end of the prop. shaft is hidden inside a cross-member, but also because the speedometer drive pinion is carried on that portion of the prop. shaft driving flange which lies inside the gearbox, and to change this flange it is therefore necessary to remove the gear box rear cover. Trouble will probably be experienced in removing the other prop. shaft flange from the bevel pinion, on which it is mounted on a long and shallow taper, and some form of puller will be found very useful here.
Axle ratios – Concerning crown wheels and pinions; all the cars of TE, TF, TG. TH, TJ, TK, TL types have an axle ratio of 4.77 (9x43) except those fitted with high-pressure tyres; before chassis number 5126 "stub-tooth," and, after that number, "deep-tooth." 'The latter gears can be fitted to the earlier cars if the six holes in the diff. box to which the crown wheel is bolted are reamed out from 5/16th in. to 3/8 in. dia. The SD cars of 1925-6-7 are fitted with 4.33 (9x39) ratio, replaced after the above chassis number by the "deep-tooth" versions. Towards the end of 1927 these were supplanted by 4.55 (9x41) gears which were fitted till this model went out of production. The mesh adjustments, laterally for the crown wheel and fore and aft for the pinion, are quite simply carried out by shims, while as the pinion is carried in taper roller bearings any end play can be taken up.
Gear Ratios — All T series cars except the later TK and the TL models, have the following gear ratios: 4.77, 7.32, 11.04 and 18.54 to 1, giving at 4,000 r.p.m. the following speeds: 76.8, 50.1, 33.3 and 19.8 m.p.h.; let me say here that the standard 12/50 will not produce 4,000 r.p.m. in top gear except in very favourable circumstances. These speeds are calculated on the dimensions of new Dunlop racing covers; I am well aware that they do not agree with those given in the 12/50 instruction book, but you can check them if you like. These ratios are much too wide for comfort, and the S.D. cars were fitted with higher 1st and 2nd gears, which produced the following ratios: 4.33, 6.64, 9.22 and 14.05 (84.7, 55.2, 39.8 and 26.1 m.p.h. at 4,000 r.p.m.), later, of course, changed to 4.55, 6.98, 9.69 and 14.77 (80.6, 52.6, 37.9 and 24.9 m.p.h.). This is better, but third is still a long way from top, so nearly all the TK 12/60s, and all the TL 12/60s, were fitted with not only the high 1st and 2nd gears but also a higher third; this results in ratios for these models as follows: 4.77, 6.73, 10.17 and 15.49 (76.8, 54.5, 36.1 and 23.7 m.p.h.). These gears which are the closest fitted as standard to 12/50s, were first used in the 1930 sports "Silver Eagle" models, and can be fitted to any 12/50. Note, however, that the whole set must be obtained, for reverse gear is also different (meshing as it does with 1st and 3rd) and so is the upper constant mesh pinion, which is cut away in accordance with the requirements of the altered third gear wheel.
A better, but more expensive, plan is to obtain (or keep) the 1st, 2nd, 3rd and reverse gears of an SD type box, and to have made special constant mesh gears with 19 and 30 teeth, respectively, in place of the standard Alvis constants, which have 17 and 32 teeth. This results (supposing, say, a 4.55 axle) in ratios of: 4.55, 5.86, 8.13 and 12.39 (80.6, 62.7, 45.2 and 29.6 m.p.h.). Remember that these speeds are all calculated at 4,000 r.p.m., and are therefore not (necessarily) maxima. Special constants of this type were used by the Alvis Co. when they raced the four-cylinder FWD cars in the 1928 T.T. (the gears from which, however, do not fit 12/50s), and since then, inter alia, by Michael May, noted Alvis death-dicer, etc., to whom I am indebted for the idea.
This gear ratio problem was made considerably more complicated by the fact that the tooth form was changed, and not on all the gears at once, but in two steps. Thus, all TE, TF and SD motors made up to mid-1927 have shallow "stud-tooth" gears throughout (first fitted case-hardened, incidentally, at chassis number 4427). All TF and SD motors made from then till near the end of 1927 have "stub-tooth" 1st and 2nd gears, but "deep-tooth" 3rd and constants, while the last few TF, all the TG and TH, those SD made after that date, and all TJ, TK and TL motors have "deep-tooth" gears throughout. Also, numbers of the earlier ears have been converted! Note, by the way, that each of these alterations, whether to tooth contour or ratio, affects the sliding reverse gear, as this meshes with 1st and 3rd, so that there are no fewer than seven different 12/50 reverse pinions in existence.
All those boxes with either high 1st and 2nd or high 1st, 2nd and 3rd gears, i.e. those out of SD, TK, TT, and the sports "Silver Eagle" models, have an "S" stamped on the lid in addition to the box number. Of course, the gears may have been changed...
Chassis — Essential points of the brake gear remained unchanged during 12/50 production, though the linkage was altered slightly on two occasions: 1926. and 1931. The only noticeable difference here is that in the revival batch (1931-2) the handbrake operates on all four wheels, as against the previous system of operation on the rear wheels only. The hubs were unchanged until 1932, when the TL 12/60 was introduced with the Dunlop knock-off hubs, using corrugated driving plates. A variety of wheels will be found, all those used with low-pressure tyres, however, being of 20 inch dia., except those wire wheels fitted to the early SD models, which were of 21 inch dia. and possessed a peculiar and dastardly form of split rim. Low-pressure tyre sizes were at first 4.50" x 21", then 4.75" or 4.95" x 20", and later 5.00" x 20", which latter size is convenient. Oversize tyres may interfere with standard wings.
Those cars built roughly before April 1928 were fitted with a steering box of Alvis manufacture: those after, with Marks units; the latter are lighter in operation but also lower geared (1 1/2 turns lock-to-lock against 1 1/8). Any wheel wobble can usually be cured by attending to the drag link ball joints; this means packing up or replacing the small but very strong spring used in these joints, and possibly replacing the lignum vitae inserts. If the inner insert at either end is damaged it can easily be drilled out, and a new one lightly tapped in. The track rod ball joints rarely give trouble; if wheel wobble still persists and the wheel alignment is correct, wedges between the axle and the springs if not already fitted, usually effect a cure.
The front springs are all interchangeable, though improved in 1926 by the extension of the second leaf to roll round the spring eye. The rear springs are also interchangeable, but vary in initial curvature according to the year and the type of body fitted; however, all the sports two-seaters have identical rear springs.
All 12/50s have double Hartford shock-absorbers front and rear, the latter mounted transversely. The 12/50. radiator was deepened somewhat in 1928; in the opinion of many this spoilt the frontal aspect, but in any case it was only a minor alteration. The 1931-2 cars have an entirely different radiator block with a separate, chromium plated, shell.
The touring motors have had a startling range of bodies which I am not going to attempt to review; the sports two-seaters, until the end of 1926, were the well known boat-shaped duck's-well, "duck's backed" body, usually of polished aluminium. The 1927 version was beetle-backed, still aluminium panelled, but usually painted; in 1928 the same body, but panelled in sheet metal, while in 1929 the hood was concealed beneath a miniature tonneau cover. In 1931-2, running-boards were dispensed with, bucket seats were used, and the disappearing hood was neatly concealed by panelling; this body was rather deeper, and also heavier, than the earlier versions. All these two-seater bodies were built by Carbodies Ltd., whose factory is practically opposite that of Alvis Ltd. I hope these remarks will be of interest to some of those who read them; I have intentionally made no mention of radical alterations such as lowering or shortening chassis frames, or hotting-up engines to the point where they are no longer suitable for road work, because such doughty deeds must of necessity be to a great extent individual, and any attempt to reduce them to a formula is doomed to failure.
[If anyone would like further details, or part numbers, I shall be pleased to helpthem to the best of my ability.—Ed.].