Why do practically all engine-specification sheets include bore and stroke? Hardly anyone I know has any idea what these numbers mean, apart from their literal meaning. Why is an 85 mm stroke better than an 80 mm stroke, or vice versa? What do the terms square, undersquare and over-square mean?
V WOLMARANS – Kranskop
The answer to this question deserves a book but I’ll try to be brief. The bore and stroke determine the engine’s cubic capacity, which emphasises the fact that, for a particular capacity, the bore and stroke are interdependent, i.e. choosing one fi xes the other one.
For example, in the 1,6-litre capacity class, a bore and stroke of 79,85 mm will give a capacity of 1599,5 cm3. Such an engine with equal bore and stroke is called square and, according to some enthusiasts, has mystical qualities, but there is no technical reason why that should be the case.
If the bore is increased to 90 mm, the stroke will have to be 62,85 mm to stay just below 1,6 litres. The big bore will make the engine block longer and wider, but it will also allow bigger valves to be used. The heavier pistons will put greater stress on the big-end and main bearings at high revs, but the shorter stroke will tend to reduce these loads.
Reducing the bore to 70 mm will require a stroke length of 103,9 mm to remain just below 1,6 litres. The small-bore pistons will ensure a shorter and narrower cylinder block and smaller valves, and also decrease highspeed bearing loads (but the long stroke will increase these loads).
This demonstrates that engineering is about fi nding the best compromise. Careful thought goes into choosing these dimensions, which is why a knowledgeable enthusiast likes to know what they are.
