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Engines and Gearboxes

1933 Silver Eagle TC 19.82 engineAll Alvis engines are strong and capable of high mileages without overhaul. The first thing to check is the oil pressure with the engine really hot. It can take 30 miles of driving before the oil is at full temperature. Handbooks state that pressure should be 25 psi on the 12/50 and 40 psi on other pre-war models at 2000 rpm and above. It seldom is, about 10 psi is said to be the safe minimum. From the 12/70 onwards, larger capacity pumps mean that 50 psi is normal. Whilst 40psi is to be regarded as the safe lower limit of oil pressure the early Three Litres (TA21) have been known to run for years on an oil pressure as low as 25psi when hot. Much higher but totally meaningless pressures will be shown when the engine is cold. Alvis engines tend to have slightly noisy valve gear, but any obviously loud knock usually spells expensive trouble. Speed 25 SC engine - from the Instruction Book. Note that it has coil ignition only.Once heard, the tap of a run big end is never mistaken. It is usually worst under neutral load conditions and may disappear altogether under acceleration or over-run. The Three Litre series has shell bearing big ends and mains, these are expensive to replace and 40 psi is the safe minimum pressure. The combination of a seven bearing crankshaft and sludged oilways can allow an engine with every big end knocked out to show 50 psi. Some 4.3 engines also have shell bearing big ends. All other pre-war engines, plus the TA 14, have cast in white metal bearings which can be replaced by any reasonable engineering firm working on pre-war cars. There are now reclamation techniques for crankshafts already reground to their limits.

1938 12/70 engine about to go back in. Working conditions less than ideal! Most Alvis engines are of pushrod overhead valve design. The pre-war ones, excepting the 12/70 and Silver Crest, have cast aluminium crankcases bolted to cast iron cylinder blocks with cast iron heads. Sumps are also of aluminium. The threads used are (mostly) BSF; these are barely adequate when tapped into aluminium and so stripped threads are quite common if the studs have been over tightened. Conversely they are prone to fall out if not tightened enough. There are standard engineering techniques for dealing with stripped threads, but to do a proper job the components need to be dismantled and on the bench. Post-war engines follow the 12/70 and Silver Crest with integral cast iron block and crankcase. A design feature of all engines up to the Three Litre is the oil return via the pushrod gallery. The exhaust ports pass directly through this so the oil gets thoroughly cooked. Invariably large carbon deposits are found on the ports, so frequent oil changes are essential. Synthetic oils are said to be able to withstand higher temperatures (they cope with jet engines) so their use could be advantageous. On many models up to the TD 21, oil filtration is by no more than a piece of coarse brass gauze, adequate to catch only the grossest debris, another incentive for frequent oil changes. Piston and bore condition can be assessed with a compression check, turning the engine over with the starting handle gives a good indication. It should not be too easy, but that depends on your strength! Open the oil filler at tick-over on a hot engine. A lot of huffing and puffing and clouds of blue smoke indicate worn bores, as does a dirty and dripping breather. 1932 Silver Eagle 20 hp engine - from the Instruction BookLeave the engine ticking over for a few minutes and then snap the throttles open up to about 3,500 rpm. A large cloud of blue smoke from the tailpipe(s) is a sure sign of worn bores. The textbooks say that it can also be caused by worn valve guides. Pre-war engines used a separate transfer port at the back of the engine to avoid water circulating through the head gasket, and on many engines the "land" where the gasket seats between adjacent bores is very narrow. This means the gasket can blow here with no loss of oil or water, or overheating, merely a mysterious loss of power or a misfire. Usually a compression check will show such a fault. All engines are subject to rampant electrolytic corrosion in the alloy components of the cooling system. Fortunately these components are nearly all available. Modern anti-freeze used all year round (take care to use a type suitable for classic vehicles), retards the rate of decay.

Early 12/70 engine - from the Instruction Book Overheating is common these days and seems to be exacerbated by modern petrol. Blocks, heads and radiators sludge up if tap water is used in ‘hard’ water areas .The effects of this can be mitigated to some extent by adding a Kenlowe radiator fan but the radical clean out is the only lasting remedy. The bronze water pump on the early six cylinder cars is commonly held to be inefficient, but if everything is in order then only the most severe conditions should cause anxiety. The models which had honeycomb radiators when new should have replacements of this type, expensive though they are; fitting a modern film core will not work as the water simply cannot flow through fast enough for thermo-syphon or the early water pump to cope. Deformation of the grille slats on the Park Ward Three Litres or removal of the baffle plates below the front bumper and behind the number plate can deflect the airflow away from the radiator core, resulting in overheating.

Nearly all the models commonly encountered have robust and easily overhauled SU carburettors, proprietary petrol pumps, and straightforward Joe Lucas, Rotax or BTH electrics. If a car has been standing unused the wiring may have been damaged by squirrels, rats or mice.

The "Silent Third" gearbox from the TD 16.95 Silver Eagle - Instruction Book illustrationGearboxes were Alvis built pre-war, and post-war up to the TD 21. Without exception these boxes are beautifully and strongly made. Serious trouble is almost unknown, and parts are available. The vintage "crash" box gears eventually wear out, but new gear sets are made from time to time. Later all-synchromesh boxes are virtually trouble free. From the TD 21 onwards, it is not such a happy story. The BMC 4-speed box, acceptable in Austin-Healeys, is generally regarded as not up to the torque of the Alvis engine and can break. It also has weak synchromesh on second and peculiar ratio spacings. First is high, second is hardly any higher, yet there is a big gap to third, which is very close to top. Since, however, it is used in the Austin-Healey 3000, parts can be had easily.

Internals of the All-Synchro box. Note the double helical constant mesh gears and the centre bearings. Straight-cut gears are first and reverse. From the Speed 25 instruction book.This is not always the case with the ZF 5 speed unit used in later 3 litres (Series II TD and TE/TF models) which is pleasant to use but can be fragile. It has a slick change and excellent ratios, but it is sometimes prone to whine in the indirects when even slightly worn. Beware that the propshaft used with the ZF does not have a sliding joint, it is in the gearbox. Members have fitted other gearboxes and overdrives and a conversion using a new proprietary 5 speed unit is available. The original automatic gearbox for the later Three Litres is the ubiquitous Borg Warner automatic: robust, easy to fix, parts are plentiful. Conversion to a modern 4 speed automatic is a popular upgrade.

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