Timing chain's tend to have a much longer life span than timiing belts. I had an X-type which never suggested changing it at all. Check BMW recommendations in your service manual. Also look out for chain tensioner - the bit that helps to keep the chain on the cogs. The engine in My friend's 320d is shagged due to a faulty tensioner. It is 6 wks out of warranty and BMW are curremtly saying 'nowt to do wif me Gov'