From 1983 to 1990 Mercury Marine made the "back bone" of the sterndrive industry.
Improved gear design and bearing arrangements made the Alpha One drive
stronger than its "R" Drive predecessor.
Externally, the housings have a more "squared off" design.
The top cap is flatter with NO loop and the later Alpha top caps
had a camfer down at the front edge to accommodate the higher tilt
angles that resulted by the use of the longer trim cylinders.
Most Alpha One Sterndrives came from the factory with an "A" stamped
on the end of the propshaft for identification.
It is important to note that many Alpha One drives do NOT have an "A" stamped
on the end of the shaft as the replacement shafts are not stamped.
Internally, the upper gears and bearings of the Alpha One
are interchangable with the older "R" Drive.
The lower components differ from the "R" in several ways.
What is most notable is the drive shaft design.
Alpha shaft with NO pre-load pin
The Alpha One shaft (pictured above) has NO pre-load pin on the top.
The pre-load pin which was needed with the "R" units is not needed with the Alpha One
because of the new driveshaft bearing arrangement and gear tooth design.
*Alpha and "R" lower housings are NOT interchangable.
*Alpha and "R" lower gears are NOT interchangable.
*Alpha and "R" driveshafts are NOT interchangable.
*Alpha and "R" water pump bases are NOT interchangable.
*Alpha and "R" lower driveshaft bearings are DIFFERENT!
The Alpha One itself also went through some changes.
The gear forging procedure was changed to increase gear strength.
The shift spool assembly material was changed to be stronger.
The driveshaft eventually lost the o-ring groove at the top.
When Mercruiser updated the "R" system to the Alpha One, they
also updated and redesigned the transom assembly.
The early Alpha One's used an upper steering shaft that turned
out to be a problem in some cases. The older shaft had a fine spline
to match the fine spline of the steering lever.
It was updated to a square spline and used a square splined lever also.
The shift cable was also changed a few times as did it's installation procedure.
The trim limit and trim sender switches were located on either side of the
beefier gimbal ring assembly and were adjustable.
The bellhousing (where the drive bolts on) was modernized to have
a neater square look to it.
The earlier Alpha One's only had a 5/8" water hose and tube leading
from the bellhousing to the gimbal housing.
Mercury soon changed the bellhousing fitting size and water tube sizes
to accommodate a larger 3/4" water hose.
The Alpha One transom assembly also allowed the installation of the
Mercathode System on the bottom of the gimbal housing.
The Mercathode System was an option in most cases and is still
used today on most of the Alpha Gen 2 and Bravo systems.
Another important change for the Alpha design was the exhaust
pipe configuration. Many repowers and "swaps" are complicated by
this exhaust change as the pipe bolt patterns differ as does
the size and shape of the pipe to fit either 3" or 4" elbows.
The Alpha One Drive used what Mercury considered to be a
"high pressure / low volume" water pump assembly which is
incorporated into the drive. The newer Alpha Gen Two uses
a different pump which is "high volume / low pressure" in nature.
The "R" and the Alpha One used a plastic water pump housing.
The Alpha Gen Two has a Stainless pump housing.
An Alpha One can bolt right up to
an older "R" drive bellhousing. As long as the U-Joint splined
driveshaft is the correct length.
Many of our customers will call asking for a rebuilt "R" Drive
when really, it only makes sense to instal the improved Alpha One
in it's place.