The siamese bore
As an engineer and a power unit engineer at that, many might say that I have little need of a dictionary. And in truth, since the vocabulary of engineering terms tends to be so exact and rarely found in all but the most comprehensive of lexicological texts, for very many years I have simply done without. But it was while thumbing the latest version of the OED (Oxford English Dictionary) - the concise version I hasten to add, and a present given to me only recently, when I came across the word 'Siamese.' Defined as 'a native of Siam (now Thailand) or an old fashioned word for Thai, the reference expanded further onto the subject of Siamese twins and that most famous 19th century pair, Chang and Eng Bunker who were physically joined by the hip at birth.
At the outset of any engine design, in the search for efficient use of engineering materials, the designer has to package all the components parts inside the smallest of dimensions. In the case of the crankcase this means getting the largest size piston diameter inside the shortest of cylinder blocks and therefore keeping the bore centres (the distance between adjoining bore centre lines) to the absolute minimum. At first this might sound relatively easy and straightforward and in the early stages of a design when these critical decisions are made, such is the case. However later on, it is an undeniable fact of life that customers will eventually want a larger capacity or a reduction in weight and in either case, one of the options available to us is that of Siamese, or perhaps more politically acceptable these days, conjoined bores.
In the ideal world of engine philosophy, the surface of the cylinder bore should be surrounded by a uniform thickness of supporting material, which in itself will be fully surrounding by the cooling water jacket. Having a uniform thickness should ensure that, with the heat flowing out radially, the strains as a result of temperature variations will be minimised and the metal temperature especially that towards the top and between the cylinders, will be kept within reasonable limits. However, with unchanged bore centres increasing the bore diameter and yet retaining sufficient material around them for strength, a time comes when these two surfaces effectively fuse together forming a figure of eight shape where no cooling water can penetrate them. At this condition the bores are said to be 'siamesed' or conjoined. Whatever the term used, these bores are highly likely to suffer from severe distortion and high temperatures around the upper 'bridge' area which will limit the reliability of the gasket in this zone. In cast iron cylinder blocks where this design route has been taken, the lack of critical cooling in this area has caused temperatures to be as high as 360 deg C in this area at times. It is therefore little wonder that engineers try to avoid such design approaches whenever possible.
But to any rule, there would appear always to be exceptions. In this case it would appear to be the introduction of an alloy block Cosworth BDG in 1972 to the Works Ford rally Escorts. With the cylinder block in this engine based on the Ford 1600 711M component, the maximum safe bore size even based on the most carefully produced cast iron blocks, was 85.6 mm. This gave an overall capacity of 1790cc. Machining the blocks and inserting vacuum brazed iron liners would enable 90 mm bores and bring this up to 2.0 litres but even then distortion was an issue. A change to an aluminium alloy cylinder block brought about a saving of 40 lbs (18 kg) but handicapped with the same bore centres as before and the reduced strength of the cast aluminium, came the necessity to Siamese the bores as described above. However despite the potential issues outlined, the engines were surprisingly reliable.
Quite what the poor Bunker brothers would have thought we can never know. But as showmen to have such a technology named in their honour, I am sure they would have approved.
Fig. 1 - Siamese bores.
Written by John Coxon