The engines were lightweight aluminum ohc and were advanced for their day. I think they were used extensively in the 750 formula class at one time.
That engine was originally a Coventry Climax fire pump alloy engine that I've used in that original guise in a fire pump trailer that was towed behind light trucks and Land Rovers.
When the Rootes Group designed the Hillman Imp as a twin engined air-cooled car intended as a Citroen 2cv competitor, they discovered the Mini was to be four cylinder water cooled at around the same price and they panicked.
Casting around for a suitable four cylinder, which had to be very light due to the overhang rear position, they came across the Coventry Climax alloy unit and got a licence to use it in the Hillman Imp, boring it out to 850cc. It was still rear heavy, hence the Imp's tyre pressures of 30 lbs rear, 15 lbs front to keep the steering in contact with the road!
Bond adopted the same engine for the Bond Bug, and when Reliant took over Bond they gained it too. The original Reliant three wheeler engine was the venerable Austin 7 one, a 1920s design!
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