From 1990 to late April 1992 came beefier connecting rods and the use of 6 bolts to secure the flywheel to the crankshaft; May 1992 to 2006 Evolution versions have lighter rods and use 7 bolts to secure the flywheel to the crankshaft. They are referred to as the "six bolt" and "seven bolt" engines, respectively.
http://en.wikipedia.org/wiki/Mitsubishi_Sirius_engine
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