For smaller drives, it MIGHT be an Alpha 1, Alpha Gen 2 or one of many older #1 or "R" style drive.
Look for serial numbers and ratio numbers.
Note engine/ drive year if possible.
Observe what other characteristics the unit has.

For example, for an outdrive... what does the top cap look like?
Does it have a loop, is it squared or does it camfer down in the front? ( a loop MAY indicate an "R" drive)
How many inches long is the u-joint assembly splined shaft? (older in-line drives had a shorter shaft)
How many o-rings does the splined driveshaft have?(3 o-rings generally indicates Alpha Gen 2)
Is the cavitation plate rounded on the aft end or is it squared? (squared is characteristic of an Alpha 1)
Does the end of the prop shaft have an "A" stamped on it? (indicates that the lower assembly MAY be an Alpha)
Does it turn clockwise (righthand) rotation in forward or counterclockwise? (determines RH or LH rotation unit)
Does the drive have an oil passage where the drive mates to the bellhousing? (indicatesAlpha Gen 2 style)

Sometimes the best or only way to tell exactly what drive unit you have is to do a partial disassembly of the unit.
If you seperate the upper from the lower then you can tell if the drive is an Alpha or a "R" drive.
The verticle driveshaft of a pre-Alpha, "R" or number one drive has a pin at the top.
If it has a pin then it is NOT an Alpha style lower-unit.
The Alpha lowers vary in verticle drive shaft styles. The latest Gen 2 and some of the regular Alpha drives have no o-ring groove at the top of the driveshaft.
Many older Alpha's that have had their driveshafts replaced may have the newer non-o-ring style drive shaft.
Confused yet?

Ratio determination is VERY important.
The size and horsepower of the engine, to a large extent, determines what ratio the drive should be.
The drive ratio is determined in the upper unit by the number of teeth on each gear.
In general, the smaller the engine, the higher the ratio.
Sometimes the only sure way to tell what ratio a drive is, is to access the upper gears and carefully count the teeth.
This SHOULD be done by a certified mechanic.
Small block V-8's usually have ratios of 1.50:1 and 1.47:1
V-6 engines and older in-line 6 cyl engines use 1.65:1 ratios.
In-line 4 cylinder engines will use a 1.98:1 or a 1.84:1 ratio drive.

Before the Bravo drive was introduced, Mercury mated the big block engines to an Alpha with a heavy-duty 1.50:1 drive (labeled HD 1.50:1) which really didn't hold up too well.
They changed the ratio to a 1.36:1 which held up better.

Alpha Generation 2
The Generation #2 Alpha has caused some confusion in the drive area.
As of this date, Gen #2 drives are the latest smaller drive design.
Both the transom assemblies and drives are NOT interchangable with the older units.
The Gen #2 drives have a different mounting hole stud pattern. It LOOKS the same but it's different so
an older Alpha or "R" drive will NOT bolt to a Gen#2 transom and vice versa.
So... if you want a newer style Gen 2 Alpha then you will also need to replace the transom assembly to match it.
One easy way to tell if you have a Gen#2 Alpha is to look at the way the upper is bolted to the lower. A Gen#2 uses through bolts to mate the upper to the lower.
An older Alpha and "R" drives have nuts on studs.

Design Changes
Through the years, Mercury has updated their drive designs for improved performance and servicablity. Really old #1 drives used to bolt the top bearing to the upper driveshaft.
That was soon changed to a pressed-on type bearing.
Some older "R" type drives did not have the plastic removable water intake gills. Newer drives do.
Perhaps one of the biggest changes was the gear cut in the Alpha Drives.
Changing the direction of the lower gear teeth allowed for a more dependable drive shaft bearing arrangement.
It made the gear clearance more predictable hence stronger.
Changes in this gear and bearing design meant that some of the older parts could no longer be used in the newer Alpha Drives. Parts such as the lower drive shaft, gears, some bearings and even the housing itself was changed.
This is important to know when servicing an "R" or Alpha Drive unit.
Many of the parts used in the Alpha One Drive are still used today in the new Gen #2 Alpha Drives.
Some bearing changes in the Alpha Drive occured "mid-model". The double bearing set in the upper unit must pressed on to the newer replacement drive gear.
Also, If a replacement lower Alpha housing is needed, it will require the use of a special bearing to fit the original forward gear into the newer, smaller housing race area.

It's stuff like this that can drive a mechanic crazy!

Bravo Drives
The Bravo drive saved the day for the big block puppies.
It replaced the need of using a transmission as the TRS drives needed and took the place of the weaker Alpha Drives that couldn't handle the torque.
The Bravo uses a shifting cone mechanism much like the proven Volvo shifting system.
Big beefy gears and thick shafts make the Bravo tuff.
It can be identified by the removable rear plate on the back of the upper driveshaft housing. The plate is held to the drive with three twelve-point bolts.
Bravo's come in three basic types and a few different ratios. Bravo One is the most common. Bravo Two has a larger lower housing and prop-shaft so it can throw a larger diameter prop and the Bravo Three is a dual prop configuration which is great for medium size cruisers for better handling.

TRS / TR Drives
Lots of TRS drives are still operating out there!
They are very large and very tough. The main draw-back of the TRS drive was that it needed a transmission as no shifting occured in the drive itself.
Newer and faster boats needed less weight and more room so the TRS has been discontinued by Mercury Marine and it's market replacement is the Bravo series.
The TRS initially was mated to Mercury Marines TRS transmission. This was Mercury's own design which used some Ford inner parts. The tranny had an internal oil cooler and an aluminum housing.
Mercury then changed to the Borg Warner Transmission which required that the TRS Drive input shaft be a splined male shaft instead of the older, hollow female splined shaft.
When ordering TRS stuff, make sure you specify what tranny set-up you have.
The TR drive used the same upper driveshaft housing as the TRS but it's lower housing and propshaft were much larger for larger props.

Many other drives such as the 215E and the 215 as well as the "A" and "C" drives still pop-up for service now and then but parts are hard to find and perhaps they are best displayed in a museum!

Black Hawk
The Black Hawk drive looked promising for the high end sport performance boat but it looks as though Mercury may have put the brakes on the project.
Many boats were rigged with the dual-prop surface piercing with great success. It's stout, short housing reduced shaft bowing and it's sleek design mean't drastically reduced drag coefficients.
It's main draw-back was that it could only be used on boats that had a certain step in it's hull design.

Know what drive you have before ordering parts, replacements or service estimates. Transom models and types have also changed along the years as well as trim rams and trim pump styles too.

At Sterndrives.com, we have been working with drives for over 25 years and have replaced or serviced every model that Mercury has made.
Please feel free to call us if you need help in determining what unit you have or need!