Although the basic engine design has remained constant for the XE engines there were a few minor changes made leading to the distinction between 'early type' engines (those up to engine number 14142838) and 'late type' engines (those from engine number 14142839 onwards).

The two types of head bolts are quite easy to tell apart at a glance, simply by looking at the bolt heads. Both bolt designs are a six splined TRX fitting:

• The early type head bolts are a conventional bolt design which need a socket in order to fit them. These bolts are fitted with seperate head bolt washers.
• The later type head bolts are a 'cap head' design with an internal six spline TRX. These bolts need a TRX key in order to fit them. The later bolts are also supplied with a captive head bolt washer.