The IsaMill (TM) was specifically developed to address the energy efficiency issues associated with grinding to fine and ultrafine sizes. It has been doing this successfully for nearly 25 years in more than 125 installations. Recent efforts by Glencore Technology to further improve the energy efficiency of the IsaMill (TM) have resulted in the development of a new IsaMill (TM) shaft spacer design. The spacer was developed through laboratory, pilot and full scale testing prior to being commercialised. Full-scale results indicate that the new configuration was able to reduce the specific energy consumption by 13-17% in an M1000 gold application grinding to a P80 of 18 mu m. The spacers were also shown to have no adverse impact on the shape of the product particle size distribution. This paper discusses the development process of the new spacers through lab, pilot and full scale trials, operating results and processing implications.