Q I have a 1991 318i with a 2.5 engine, and the original 4.1 diff’ is a bit too low – it’s quite revvy and noisy on the motorway although it goes well! I want to fit a 325i diff’ – I’ve bought a 3.73 open diff’ but I’m told it won’t fit straight in. Can you confirm this and if so, which diff’ will fit?

Phil Jones, Exeter.

A The 325i uses the bigger ‘188’ type diff’, as do the 323i and M3 cars. The others use a 168 small diff’. The bigger diff’ will fit. Although it’s wider, the actual distance between the CV joints is the same. The only thing is, the diff’ is longer so the prop won’t bolt straight on. But no problem – just slacken the collar nut just behind the centre bearing and tap the end of the prop inwards with a mallet until the splined sliding section goes inwards enough for the prop to fit. Fit and tighten the prop flange to diff nuts, retighten the collar nut and it’s job done.