If unsure of size required email Lee Turnham for advice on lee.turnham@talktalk.net

Note on later guides BMW counter bored the guide  8.1mm Ø and 5mm deep at the combustion end which may have been done to stop valves sticking due to carbon build up. 

The guide does not come with this undercut therefore it will need machining if you think its still required with modern clean burning fuels