The design concept for this new range of pistons from Mini Spares raised a few eye-brows at their introduction almost a year ago now

However, the design was arrived at after consultation with a couple of very learned and experienced A-series engine specialists and the piston manufacturers concerned – themselves a highly rated company that produces extremely hi-tech pistons for all manner of piston-driven engines including the mad, seriously engine-punishing world of 'drifting' cars tuned to run at extreme rpm producing extreme levels of BHP for protracted periods. Certain features of the piston and ring pack from pistons designed for these 'drifting' car engines were employed, receiving most attention of those used to more familiar, old school spec designs. Most completely over-looked the quality and material/heat treatment spec of the piston itself. A very durable, very high silicone content material heat treated to a specific requirement ensures tough, reliable and long-living pistons.

Many wondered why the ring pack was chosen as 'current' designs use thinner rings – the Evos using 1.5mm width rings, others such as Omega use 1.2mm wide rings. Surely thinner rings are better for power as they cause less drag. A fair point. The answer to that is simple – there were more alternative ring pack options available at 73mm. There is not an off-the-shelf ring pack available at 1.2mm width. Having them made especially would have hiked the piston/ring set price considerably. And since they are an off-the-shelf ring pack, world-wide customers shouldn't have issues getting replacements. And as for the drag/performance side of things – it doesn't matter how wide or thin the rings are if they are sealing properly. But that's all I'll say on that subject for now.

Since their introduction I have used something like a dozen sets in engine builds from road-sports/sports-tourer engines up to full race engines – they have performed and are still performing superbly. One engine was the 1360cc project engine I did for Mini Magazine March 2006 and consequently made it in to the 'Mini Expert' Special Edition issue 1. The main aim of the feature was to create a decent power unit for maximum road-orientated fun but at a sensible budget. A bonus being with the 73.0mm Evo pistons meant once bores and rings were worn, there was still life left in the block as it could be bored/honed out to 73.5mm and the bigger brother to the 73s installed. Basically the block was just a stripped standard A+ engine bored/honed out to take the pistons, an RE13OT cam fitted, but little else in the way of lightening or balancing, topped off by a Super-sports head like an MSE3. The result was stupefying – really. On a local rolling road it gave 12% more BHP than one of the more usual-suspect type fast road 1380/286 cammed engine and a way more impressive torque curve that made driving the car seriously habit-forming. The engine was sold on to a Mini nut in France, who has driven the wheels off his car zooming round France and impressing other Mini owners since. The engine is still banging out blistering performance – so no piston/ring reliability worries there.

At the other end of the scale I built 3 full race engines at the beginning of this race season (2007) with the 73mm pistons in too. All three have completed a full seasons racing with no problems whatsoever. On two of the engines I wanted to use different piston rings – and this is where their slightly wider, more common sized width came in. Had they been 1.2mm wide I couldn't have done this – at least not without investing a monumental amount of money. I was able to try the most current range of gap-less top rings from Total Seal. The arrangement means that the top ring is actually thinner where the ring meets the bore, so a friction reduction is gained there. Although determining any specific performance gains on the dyno was not really on, they must be doing something to help as the leak-down on them over the year has remained at barely measurable. This must positively affect performance since you are getting a better induction 'suck' and loosing no compression on the firing stroke.

As for the other engine that ran with the rings as supplied with the piston set in the first place, the pistons are perfect and the rings acquitted themselves admirably. They do need replacing after a full seasons racing, but then that is very far from abnormal in most instances.

The crux of this review then is that, despite some initial reservations from folk because these Evo pistons are not 'the norm', they at the very least every bit as good as any other high quality cast A-series piston out there, are more adaptable in terms of piston ring options and cheaper in to the bargain!

Useful Part Numbers:

C-STR313 - 73.0mm piston with rings (sold individually)

C-STN98 - Replacement ring set for 73.0mm pistons

C-STR311 - 73.5mm piston with rings (sold individually)

C-STN97 - Replacement ring set for 73.5mm pistons

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