Modifications to a 1 1/2-Litre Invicta

This article provides an academic study of some owner-modification to a rather rare car and offers possible solutions to troubles that, the author alleges, beset the users of 1 1/2-litre Invictas. So far the designer of the Blackburn engine found in these cars has not taken issue with the author, but we disclaim any responsibility for the opinions expressed or the alterations advised – Ed.

Having owned a 1 1/2-litre Invicta for some sixteen years, I may justifiably claim to know the car. It is well made, and solidly built in the traditional British style. The front axle is ahead of the radiator and visibility from the front seats is excellent, as both wings stand up nicely in view.

On the road, the car corners just that little better than most, and although, as purchased, the performance was nothing to brag about, the good road-holding plus acceleration of the order to 50 m.p.h. in 21 1/5 sec. enabled it to more than hold its own with cars having larger engines.

It is not a small car, with 9 ft. 10 in. wheelbase, 4ft. 8 in. track and weighing, with two up, about 30 cwt. This for the close coupled saloon. The tourer ran out somewhat lighter. Weight distribution is 17 cwt. rear, 13 cwt. front, and is about right on the road. Steering is exemplary and the “ride” very good if the front “shockers” are watched. The rear “shockers” don’t seem to matter much. The rear axle ratio of 8/48 is low, and the engine is lively, but the weight of the car is all against brisk performance and the engine has rather more to do than it should. The “blown” version had a 9/46 axle and with a reputed 90 b.h.p. from the engine was distinctly lively. I don’t think many were made, however, as I’ve only met two of them to date. The engine in my car, chassis No. L 171, engine No. BA 154, was reputed to deliver 45 b.h.p. at unspecified r.p.m. It has now driven the car over 80,000 miles and I see no reason why this should not be doubled. The engine has six cylinders of 57 mm. bore by 97.9 mm. stroke and alloy is used for sump, crankcase (including bearing caps, etc.) and of course, the unit constructed E.N.V. gearbox, which is a real job of work. The cylinder block is detachable and so is the cylinder head. The combustion chambers are machined all over and are to all intents hemispherical. The valves are set at 90°, with the exhaust on the near side and nearly vertical. The head, in consequence, has a peculiar “tilt” to the off side, and I mention this as it is more important than may at first be appreciated by owners of the breed. I’ve never had an instruction book for the car as at the time I purchased mine the manual had not been published; all my data has been obtained from scratch and may not tally with that published by the makers in every detail.

After some years of ownership I ran into quite a lot of bother with plugs and occasionally exhaust valves bent. Up to this I had been content with what the makers had sold to me, but the plug trouble really set me off. Plugs were normally K.S.5s with the fins removed in order to clear the exhaust manifold, beneath which they were neatly hidden. They were not easy to change and with hot pipes one did a lot of muttering to one’s self whilst endeavouring to avoid being burned. I got fed up with this and decided to attempt to cure the plug trouble and re-design the exhaust manifolds.

Plugs on this engine either burned or oiled; there was no middle course and no amount of changing the type could effect a cure. The plug holes in the cylinder head come just under the exhaust valves and are set at a slight angle upwards out of the combustion chamber, The masking of the plugs was peculiar, the 18-mm. tapped holes finishing off in a hole 1/4 in. diameter eccentrically drilled at the bottom of the tapped portion. With a K.S.5 in position, a pocket was left between the end of the plug and the inside of the 1/4-in, masking hole, which was itself 1/56 in. long. The volume of this pocket and the masking hole was over 2 c.c., and represented some 6 per cent. of the compressed volume of the cylinder. At each charge, therefore, this 6 per cent. consisted of a lop-sided pocket of attenuated gas situated where it was vitally necessary that firing be instantaneous and unquestionable. The lack of these qualities was very apparent in the running of the engine. Periods occurred in the speed range which I had attributed to the carburetter, which on my car was a single 36 rmm. Zenith downdraught. Evidently the makers had thought so too, as I’ve met these engines with one downdraught S.U., two horizontal S.U.s and a single Solex. None of these was any improvement over the Zenith, and to give it its clue, the inlet manifold functioned very well, although naturally charge “waggle” occurred. However, I discounted the carburation theory as the makers’ ignition advance (55°) had always appeared excessive, and I came to the conclusion that this large advance was due to late flame propagation across the piston crown. Measurement of exhaust gas temperatures confirmed this, and indeed the flame did persist throughout the exhaust stroke, burning the plug and the valve quite unnecessarily. The masking was drilled right out and the plug boss on the outside of the head was spot faced so that a T.M.L. 50 plug just came flush inside the combustion chamber. This put the spark in the right place but accentuated the burning and/or oiling tendency.

The cylinder head gasket has water holes up each side, plus a hole of 1 in. diameter between bores 4 and 5. Water is pumped along a channel in the crankcase, up the back of the block, through the holes in the gasket into the head, and finally out through a 1 1/4-in. hole at the front of the head via a thermostat to the radiator. What actually occurred was that the bulk of the water went through the 1-in. hole in the gasket, thus by-passing Nos. 5 and 6 combustion chambers. This caused local boiling to take place in the cylinder head and helped to account for the burned plugs and valves. To offset this, head and block were lapped and a gasket of 25/1000-in. bright annealed electrolytic copper made and fitted, without the 1-in, hole in it. Next, two 7/8-in. holes were drilled between exhaust ports 2 and 3 and 4 and 5 to break into the cylinder head water jacket as high up as possible. Two stud flanges were wangled on the side of the head and pipes run to join up with the 1 1/4-in, hole at the front. This hole was disked to 7/8 in. The thermostat was thrown away and a roller blind shutter fitted aft of the radiator in lieu.

Attention next turned to the exhaust valves. In view of the hard life led by these it was decided to offer them as much cooling as could be effected. The seats were, accordingly, made as wide as possible and a new set of phosphor bronze finned guides fitted. That portion of the guide projecting into the port was counterbored 1/32 in. clearance on the valve stem to prevent it from “grabbing” the latter when really hot on full throttle work. It has proved a sure remedy. Incidentally, to give some idea of the flame velocity, I made it 245 ft./sec. past the valve, and it was, therefore, essential to offer a free passage to the burned gases. The existing two-three branch cast-iron manifolds were, as usual, pretty hefty castings, and on full throttle work ran a glorious cherry red. All this heat was, of course, reflected back to the port and valve head, so a manifold was designed to combat this in the first instance. I made up two cylinders of 1/8-in. M.S. each of length 1/2 in. greater than the span of the three exhaust ports. Each cylinder then had three rectangular section branch pipes about 4 in. long welded to it. The branch pipes were parallel in plan but tapered from 1 in. at the port to 1/4 of the cylinder circumference in elevation. They were inclined upwards from the port face, to give good access to the plugs.

The hotspot arrangements on the engine were merely a pious hope and were abandoned. The two vertical take-off pipes from the new manifolds were each run to a silencer, two of which had previously been fitted in tandem by the makers, and each silencer had its own tail pipe. Silencer resistance was thus halved. As a brainwave, I had a piece of 1 1/16 in. bore M.S. tube welded in the centre of the end plates of the two exhaust manifold cylinders and on assembly a 1-in. O.D. brass tube was inserted through the 1 1/16-in. tubes with a funnel at one end, close to the radiator. The other end was piped inside the car and not only heats the interior but keeps the screen clear. This was done on behalf of an elderly lady, who, in spite of her age, likes to motor quickly and aids and abets my work to this end. So much for the exhaust side of the engine.

The inlet port presented a real conundrum. At the manifold-end it was 1 1/16 in. diameter, reducing to 1 in. some half-inch up the port. It then opened up to 1 13/32 in. diameter across the guide entry point, reducing to 1 1/4 in. at the valve.

The increase from 1 in. to 1 13/32 in. diameter was all on the top side of the port and the changes in diameter boded no good for efficient breathing. To make things more uniform I ran in hard solder all round the guide until the diameter of the port assumed a nice easy taper. The 1 1/16 in. end was ferruled up and then filed out to make a dead 1 in. entry. The manifold ports were then carefully matched up to the head ports, but no polishing was done, as I believe it more important to get good delivery after the valve than before it.

The crankcase next received attention. I had heard of very expensive things happening in this department, but as the engine design was so good as a whole, I found the blame hard to place. However, the shaft was checked over and I considered it to be capable of handling anything that could be delivered from 57 mm. bore, unblown. It has a substantial roller bearing (3 L.R.J. 1 3/8 in.) at the front, two “centre” whitemetal bearings of 1 7/8 in. bore, and a really massive ball and roller bearing at the rear. The last named is 45 mm. bore and generous to a degree. I could not visualise trouble here, except through neglect.

There was evidence, however, that the rollers in the 3 L.R.J. 1 3/8-in. front bearing had been in contact with the retaining circlips in the outer race, to the detriment of both. The designer did slip up badly here, as any wear on the rear-end ball race permitted thrust eventually to come on the front bearing. I wonder how many broken shafts were due to this? This bearing was replaced by an 8 L.R.J. 1 3/8 in., which is the same thing, only with a plain outer race. To locate this outer race the bearing cap was recessed 1/16 in. each side and semi-circular plates rivetted flush to it either side. If the rear race wears, the shaft merely floats through the front race a little and no harm is done. The whitemetalled steel shells of the centre bearings had really comic staggered oil grooves in them. I fancy I know what was in the designer’s mind, i.e. oil spread, but the staggering left portions of whitemetal in very dangerous shear. In fact, these portions were often found adrift after some mileage. These bearings were renewed with simple concentric grooves and an oil pressure of 60 lb./sq. in. “hot” spreads oil where it is wanted. Incidentally, as these centre bearings feed the entire crankshaft, their fitting is a matter of some moment and time spent on this part will pay dividends. All crank oil-ways were cleansed out with strong caustic soda.

The B.H.B. pistons had five rings, i.e., two compression, one stepped scraper, one slotted above the pin, and one slotted below. The block was sleeved and bored to give 2/1000’s clearance (I know, but wait!); the rings were renewed in the following layout: two compression (narrow), one compression (broad), and one slotted above the pin. The bottom ring was omitted. The con.rods were then drilled each side (in the axis of the gudgeon pin) at the big-end with 1/16 in. diameter holes at an included angle of 28°, meeting on the centre of the whitemetal face. At each rev., therefore, oil is thrown out on to the cylinder wall on those faces where the gudgeon pin ends pass. These faces are comparatively lightly loaded and the oil finds its way round to the thrust and inertia faces easily. I cannot understand why cylinder bores are so ill-treated as to have oil scraped from them when they need it more, perhaps, than any other part of the engine, and I do believe the Invicta bores ran dry and so caused rapid wear. For 36,000 miles the bores have remained good, and seem good for as many miles again. I thank the wee holes in the rods for this.

The engine was re-assembled and then it dawned on me as I was replacing the head that, owing to the aforementioned “tilt,” the inlet valve guides were swamped with oil. Drain pipes were fitted in three places on this side of the head to run oil back to the sump and soft felt-packed glands fitted to the guides to seal the valve stems. This appears to be successful, as I’ve never oiled a plug since.

As a matter of further interest, the mean little 3/8 in.-bore crankcase breather was replaced by one of 1 1/2 in. diameter, with an air filter. The oil-filler cover in the rocker box was opened out to 3 in. diameter and another filter fitted here. In consequence, there is no condensation in the engine now and the water of combustion that does pass the bores and guides evaporates rapidly away. Hence sludge formation is nil and oilways remain clear; and so long as this state of affairs endures, bearing failure will be unknown.

When re-assembled the engine was run-in (minus the head at first) at gradually increasing speeds by application of a 350-c.c. motor-cycle engine to the crankshaft front extension.

A brief run was sufficient, and with a song in my heart (pardon the quote), I put the Invicta away until petrol was available. I found I could attain and hold 5,900 r.p.m.-80 m.p.h., and 0 to 50 m.p.h. in 18 2/5 sec. Not bad, but still not good enough.

So I fitted an oil coil and scrounged a 9/42 back-end, ex-Standard, borrowed three Amals, fitted them one per two cylinders and had another go. 90 m.p.h. and 50 in less than 18 sec., and above all pulling propensities in top to rival a diesel. The other three Amals will be added before basic returns, as after all I can always go back to the Zenith when 17 m.p.g. begins to pall.

Was it worth while ? Yes it was! I’ve got a thoroughbred car that can’t be turned over on corners. It seats four comfortably and the bodywork is excellent. It has everything necessary for motoring, and many of the moderns haven’t. To conclude. I have one or two more ideas which will have to stay put for the nonce. One is the fitting of 8 to 1 compressions pistons, but not on “Pool.” They are on hand and have a specially shaped “hump” to “squish ” the gas up to the plug. They also have six compression rings in three sets of two.

The other is more ambitious and entails cutting 1 ft. 9 in. out of the chassis, mid-section, thus doing away with the little bit of prop.-shaft ‘aft the gearbox and bringing the wheelbase to 8 ft. 1 in. The radiator will drop 6 in., tucking in behind the axle and, with 6 in. removed from the alloy scuttle, will restore the status quo of the bonnet line. The floorboards will go on the bottom flanges of the 6-in, chassis members, and, with new steering arms to restore correct geometry, and a light two-seater body, a trials car of some moment should materialise. If anybody with an Invicta fancies the idea, I know where a spare chassis is stored. I can’t, for family reasons, set about mine yet, but I’ll willingly give all the assistance I can.