The present disclosure relates to assemblies for engines such as pre-combustion chamber assemblies for spark ignition engines. It is known to provide a spark ignition engine with a pre-combustion chamber but it can be difficult to provide a system with good knock performance at high speed with high loads while still having reliable ignition at low speed with low load. In particular, at low speed with low load, there may be a relatively high percentage of exhaust gas residuals (EGR) in a pre-combustion chamber which can prevent ignition especially when the ignition timing is retarded during catalyst heating. Aspects of the disclosure aim to alleviate at least to a certain extent the problems of the prior art. According to a first aspect there is provided a pre-combustion chamber assembly for a spark ignition engine, the assembly having: an ignition chamber adapted to contain at least one electrode of a spark generator; the ignition chamber being adapted to communicate with a main engine combustion chamber via at least one pre-chamber port; wherein the ignition chamber communicates for removal therefrom of exhaust gas residuals. The exhaust gas residuals are preferably removed to a distinct storage chamber, more preferably via a transfer port.