Citroen’s engine manager Patrice Davesne explain the 2017 engine regulations.

https://issuu.com/hi-techmedia/docs/...e_october_2016 (pages 44-45)

Regarding the evolutions which will be permitted in the future.

“We can make changes, but some parts of the engine are frozen by the regulations for three years. For example, bore is not allowed to change, the distance between cylinder and the distance between camshafts can’t change. But we have three jokers per year and, for example, we can change in the same year pistons, connecting rods and oil sump. Each year there are three jokers and we can use them as we want – it can be development, increase our power or durability, it’s as we want.”

In the beginning of the article he says that the bore has change from 82 mm, which had been used from the beginning of Citroen’s time in WRC, to 84 mm which is the max bore allowed by FIA.