There's no single answer to what the stock 305 camshaft duration is. The duration varies depending on the year and application of the 305 cubic inch (5.0 liter) Chevrolet small-block engine. Chevrolet used different camshafts in different years and even within the same year, different camshafts could be used depending on the vehicle it was installed in (e.g., passenger car vs. truck).
To find the duration for a *specific* 305 engine, you need to know the year and model of the vehicle it came from. Then you can consult a Chevrolet parts catalog or a reliable engine parts resource for that year and model to find the camshaft specifications, including duration (typically expressed in degrees at 0.050" lift).
