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I think that the original Formula 4 started with 250cc single seaters imported from Italy in 1966 , e.g. Tecno . In 1967 650cc motorcycle engined cars were allowed and the following season 850cc Imp engined cars joined the fray. The next major change came in 1971 with redundant one litre Formula Three cars being allowed in while in the early eighties 1300 Ford engined cars were admitted with Ford's 1600 CVH and 1800 Zetec being the engines of choice over the next few decades . Surviving cars were mainly absorbed into the Monoposto championship .
The Historic Formula Junior championship has several invitation classes and class G7 allows the use of 250cc and 650cc motorcycle engined Formula 4 cars while I think a motorcycle engined Formula 4 car raced with the 500cc Formula Three series this season just gone . Also Coys sold a c1964 Tecno Formula 4 car in 2017 .
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