Thornett suggested, or rapidly adopted the suggestion, that the new post-war engine be a wet-linered light alloy 6 cylinder unit.  It would be perceived as being state of the art.  In any case, lighter weight and higher output were highly desirable for the 16hp cars.  To reduce weight yet further and lower the mean piston speed, Thornett chose almost square dimensions.  It was considered that anyone who might purchase an Armstrong Siddeley was not likely to worry about the small (in comparison to purchase price) additional annual taxation that this would cause.  The RAC horsepower taxation regime that penalised short strokes still existed in 1946.  In any case, such taxation was mostly unknown abroad and the government of the day had decreed that car manufacturers must export much of their output.

The known leading particulars of the 2.6 ohv aluminium engines as built were:

        6 cylinders 80mm bore x 86mm stroke;  7 main bearings

        2593.7 cc capacity;  Tax rating: 23.8 hp RAC

        Compression Ratio: 7.0 : 1 [standard combustion chamber]
                                       6.8 : 1 [average of the "squish" combustion chambers]

        Inlet diameter: 1.475" in 1.375" port     } Final "optimal" dimensions
        Exhaust diameter: 1.5"                          } of early 1948

        Valve lift: 0.350"

        Inlet valve opens: 10° BTC, closes 54° ABC         } Initial design, as per 16hp;
        Exhaust valve opens: 48° BBC, closes 16° ATC    } timing frequently changed

        Spark plugs: Champion NA-8 long reach

        Carburettor: 1 Stromberg 42mm downdraught

Unfortunately, weight remains unknown.  No drawing has been located at the time of writing [or since].

From the outset, the 2.6 proved to be exceedingly troublesome.  Only a partial list of the defects will be given here.  The unit leaked liberal quantities of water into the sump as well as having numerous oil leaks.  The water leaks occurred even when the engine was not running!  Fuel consumption was considered to be too high.   Breathing, as might be expected given the general design of the porting, was poor.   However, given the lower mean piston speed, mechanical efficiency at 82-83% for most of the running speed was higher than the 16hp’s very low top end figure of 72.5%.  But as a seven main bearing design had been chosen, the result of the stresses caused by this layout could not only be felt but also, so it is said [by John Densham], seen!  This obviously did not help with the oil leakage problems and mechanical efficiency.  To this day [1994], John Densham (who protested at the time) does not know why Thornett chose this arrangement rather than a 4 main bearings one which if designed properly causes less trouble, especially in a light alloy car engine with a less than substantial crankcase.  As a partial remedy to some of the problems, a "squish" combustion chamber was also developed in parallel with the standard head from about March 1947 with certain benefits.  The breathing difficulties included very poor fuel distribution around 1250-2500rpm, leading to a very high specific fuel consumption at this important engine speed range.  Experimental wanted completely external porting - as suggested by Allen for the 16hp - but this does not seem to have been adopted.  Volumetric efficiency was not helped by the 2.6 having a tendency to overheat.  There were difficulties with valve timing, clearances and springs.  Hydraulic, special hydraulic and ordinary tappets were tried.  There was also quite severe problems of cylinder wall wear after times as short as 28 hours running.   Roughened bores were one of the measures adopted in an attempt to reduce such wear.

Another major problem was that the engine was very sensitive to small variation in ignition timings at lower speeds.  Maximum power, about 90bhp with the standard head, could only be obtained by tuning and timing that caused heavy detonation up to 3000rpm.   The later squish head gave about 92bhp with less detonation.  It was specially adapted to give up to 100bhp in March 1948 on better fuel.

Experimental and developmental work carried on in earnest until about the end of February 1948 and some work continued until at least late August 1948.  Valve timing and clearance problems - there was a loss of lift - were still not sorted out by September 1947.  This was one of the causes of the continuing poor volumetric efficiency.  Overheating remained.  A saloon, FDU 113, was fitted with a 2.6 engine with both two and four blade fans, and with a standard bonnet and a bonnet with special louvres on the bonnet side.  It was found that the radiator temperature did not vary by any appreciable amount.  Finally, the valance was cut away, and a slight improvement was obtained.   However, whilst there was an increase in power output up to 3000rpm, there was a very small decrease thereafter, no doubt due to the fan absorbing rather more power at higher speeds.

During the 2.6 project, John Densham carried out a large number of tests and modifications, both on the test bed and on the road.  His view does seem to have been that if there were actually any way of making a success of the 2.6, he intended to find it.  However, it is fair to say that he, and others, had grave misgivings about the engine’s design from its inception.  He has not [1994] changed his mind after 48 years.

It is not known how much the Lagonda engine would have cost ASM, but by now ASM wanted an engine that could produce at least 110-120 bhp.  ASM viewed the design of Lagonda unit as a good basis from which to make a quick start.  Further, Bastow and Bentley were or were about to become legally "free" to offer their services to anyone prepared to pay their fees.  Either Chapman or Sopwith, or most likely both, considered that given the problems with the 2.6 and the comparative refinement of the Lagonda, it would be sensible to hire Bastow, Bentley and their small team.  A very tentative suggestion that Bentley act as a consultant had already been made in 1939 by his former V12 chief designer at Lagonda.  There would of course be the very important commercial advantage that Bentley’s name could, with some legal care, be attached to the car!   This last fact was probably the most important reason for hiring them to design what was immediately and proudly referred to in internal memos as the "A.S. Bentley".