Engine-wise, the GN Sigma was offered either with the "Astron" 2.0-litre ( at 5600 rpm and at 2400 rpm) or the "Astron II" 2.6-litre ( at 5200 rpm and at 2400 rpm) units. The "Astron II" had a few changes over the previous models, notably a newly designed head with hydraulic tappets (replacing the manual tappets in previous "Astron"s) and flat-top pistons. The oil pressure switch was now located at the rear of the cylinder block. In December 1985, the 2.0-litre versions were dropped, as Mitsubishi did not believe it warranted the conversion to unleaded fuel. Unleaded was introduced in Australia in January 1986.