Differences in Mercruiser Outdrives...
What are the differences in the Mercruiser 1/R/MR/Alpha and Gen 2 Drives?
We are going to start in 1974 and go to 1982 which is called the Mercruiser 1/R Drive
This drive has a preload pin on the upper driveshaft.
Note: The 1/4" diameter pin is about 5/8" long sticking from the drive shaft.
This pin takes the thrust of the two lower unit gears trying to separate from each other while turning. Also there is no nut holding a bearing in place under the water pump housing and the bearing cone is pointing down. On these models the Lower unit housing, lower water pump housing, drive shaft, lower gear set and the upper and lower drive shaft bearings are not interchangeable with any other Mercruiser design units.
Next from 1983-1990 came the Mercruiser MR/ Alpha 1 Drive:
This drive did away with the preload pin and used a bearing under the water pump housing which is held down with a nut to take the thrust of the two lower unit gears trying to separate. This was a much better design. The parts that changed were the lower unit housing, gearset, drive shaft, upper and lower drive shaft bearings and the lower water pump housing and the addition of a nut under the water pump housing pictured below :
The upper units on all of these models are interchangeable. All differences are cosmetic in the upper units for this range of drives. On the upper unit the inline 4 and 6 engines mostly used an 8 inch Yoke whereas the V6 and V8 engines used a 9.5 Inch Yoke.
Next From 1991-1997 came the Alpha 1 Generation 2:
With this unit, the upper and lower units Changed and are not interchangeable with the earlier drives. The Gen 2 Drive now has a drive oil reservoir as standard equipment and a speedometer pickup. The top and bottom are held together with Bolts and nuts along the side of the drive unit, the water pump has been completely redesigned and now has a stainless steel impeller housing with a much larger volume impeller. The gear set in the upper unit is the same but takes some different bearings and different seals. The lower unit gear set changed as well which made the final drive ratio change about .03 like a 1.50 became 1.47 a 1.84 became 1.81 and so on. This was a change in the lower unit gear set not the upper.
We hope this helps you identify or understand what Mercruiser drive you have.