Re: pre aplpha drive newer lower?
ok sns, time for a bit of Merc drive history
from 1967 to 1982 the drive was known as MC-1.
In 1983 Mercruiser made some small changes and it's known as an 'R' (Revision) drive. These 2 drives are very similar. Both have a 'pre-load' pin at the top of the drive shaft. This is to load the bearing in the lower box. The one under the water pump. It's a tapered roller and is installed so that the shaft needs to be pushed down to make it run right, because as the pinion and forward and reverse gears come under load they try to force the driveshaft up and the bearing out of contact with its race. Both top and bottom boxes are completely interchangeable. They run the same water pump too. When you pull your boxes apart, make sure the pin is working. I usually just tap them with a hammer. You should feel the spring bounce the hammer back.
In 1985 (?) Mercruiser made a lot of changes to the drive, mostly the lower box. That's called an 'MR' (Major Revision) drive. Changes include turning the driveshaft roller bearing over and removing the pre-load pin. The top box got a facelift with a flat top, as opposed to the earlier ones with the lifting lug on the top, and the back edges of the housings (top and bottom boxes, where they join) were squared off.
In about 1987 Mercruiser introduced the Alpha One. Very similar to the MR. Again MR and Alpha one bits are very interchangeable.
The top and bottom boxes of the 4 different units are interchangeable, but most of the internals are not.
The plastic water pickup inserts you speak of started in the lower unit on drive # 3899470. I suspect that's about 1974.
With all 4 iterations the water pump impeller, housing and gaskets are all the same. What's different is the pump base.
1991 saw the introduction of the Alpha One Generation II. Nothing on this drive is changeable with anything on any of the 4 previous drives.
I hope all this helps you. If not, well, I'm not going to remove it
Chris...........
