How can I tell if my car has a 7-bolt engine?
You would already know this (hint, hint) if you had used the FAQ Locator; specifically, this section. The two engines have different oil pans, among other changes.
Before the oil pan method was identified, the only way to 'tell' if you had a 7-bolt engine was the production date of the car. According to the staff of Conicelli Mitsubishi, cars produced between March, 1989 and the third week of April, 1992 have 6-bolt motors; May, 1992 to the first week of May, 1994 have 7-bolt motors.
Another way might be to check the front brakes. 7-bolt engines are associated with big brakes; cars that have big brakes often have 7-bolt engines. Unfortunately, nobody seems to know if this method is reliable or not.
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How can I tell if my car has a 4-bolt rear end?
According to the staff of Conicelli Mitsubishi, all 1G cars produced before the second week of June, 1991, have 3-bolt rears. This essentially covers all 1990 and 1991 DSMs. All 1992-1994 cars have 4-bolt rears.
Many DSM owners are under the mistaken impression that the changeover between the original 3-bolt rear end and the stronger 4-bolt version occurred halfway through the 1992 model year. This is not so.
Those curious to know the production date for their car can check the driver's side doorjamb sticker. The acronym MDH stands for month, day and hour of production.
excerpts from
http://members.shaw.ca/costall/1000Q/