Dunstan Rotary Valve Grey Holden motor was one of the more unusual experimental engines to emerge in Australia during the 1950s and 1960s. Based on Holden’s original “Grey Motor” – a 2.15-liter inline-six OHV engine introduced in 1948 – it was modified by engineer Rex Dunstan, who developed a rotary valve cylinder head to replace the traditional poppet valve system. The aim was to improve airflow, reduce mechanical complexity, and allow higher engine speeds with less valve-train stress.
The rotary valve system used rotating sleeves or discs to open and close the intake and exhaust ports instead of conventional valves and springs. In theory, this gave the Grey Motor much better breathing, smoother operation at high RPM, and the potential for significant power gains over the stock 60–75 horsepower. Early tests reportedly showed promising results, with the Dunstan-modified Grey running more freely and with greater efficiency. However, like many rotary-valve experiments worldwide, sealing, lubrication, and manufacturing complexity posed serious challenges.
Ultimately, the Dunstan Rotary Valve Holden remained an experimental piece rather than a production reality. Still, it stands as a fascinating part of Australian motoring history, reflecting the ingenuity and ambition of local engineers working to push the boundaries of performance from modest beginnings. Surviving examples or components are extremely rare today and are regarded as unique collector’s items for enthusiasts of Holden’s early engineering legacy.