Read the model and serial number on the drive's own plate, not just on the engine.
Find the engine number and serial number
Compare the features below and check against the manufacturer's chart.

Our own simplified schematic sketches, not dimensioned drawings. Parts may have been replaced. Cavitation plate, skeg and paint colour are clues, never conclusive proof of model or gear ratio on their own.

Official identification charts

Volvo Penta SternDrive Models – publication 7738236-4 (12-2006) (fybmarine.shop)
fybmarine.shop · PDF · 2006
The manufacturer's identification plate at Volvo Penta dealer FYB Marine.
Sterndrives.com: MerCruiser Sterndrive Chart (years, serial numbers, gear ratios) (sterndrives.com)
sterndrives.com · HTML · 2004
R/MR, Alpha One, Gen II and Bravo I/II/III by serial number range. Always check both the upper and lower gear sets.
Volvo Penta Sterndrive Identification Chart (reproduced by an authorized dealer, 69 drives AQ–DPI) (external link)
www.sawleysautoandmarine.com.au · HTML
Table of features, serial number ranges, oil capacity and gear ratios for each drive. Drawings are by Volvo Penta.
Volvo Penta: Find your engine and drive serial numbers (external link)
www.volvopenta.com · HTML
Where the ID plate is located; the drive has its own plate, separate from the engine's.
Volvo Penta propeller guide (drive types and propeller series) (external link)
www.volvopenta.com · PDF
The propeller type (A/B/C/J, D/F/I, E, G/H) tells you which drive family it is.
Volvo Penta workshop manual saildrive 130S/150S (7742984) (external link)
www.bavariayacht.org · PDF
Describes the saildrive's identification plate and version.
Mariner's Warehouse: How to identify a MerCruiser outdrive (Alpha One, Gen II, Bravo) (marinerswarehouse.com)
marinerswarehouse.com · HTML
Visual differences between generations and where the serial plate is located.
MercruiserParts: Where is my serial number (www.mercruiserparts.com)
www.mercruiserparts.com · HTML
Serial number locations on engine, transom plate and drive.
GLM Marine: 30 Years of the OMC Sterndrive (external link)
glmmarine.com · HTML
History: 400/800 Stringer to Cobra and King Cobra.
sterndrive.info: OMC 800 (1978–1985) and Cobra (1986–1993) (external link)
www.sterndrive.info · HTML
Photo-based walkthrough of model years.
Mariner's Warehouse: How to identify an OMC Cobra lower unit (marinerswarehouse.com)
marinerswarehouse.com · HTML
How to tell Cobra gear housings apart.

Frequently asked questions

How do I tell AQ280 and AQ290 / 290A apart?

AQ280: New lower gear housing; Single propeller. AQ290 / 290A: New transom shield; Single propeller.

How do I tell Alpha One (Gen I) and Alpha One Gen II apart?

Alpha One (Gen I): Single propeller; External trim cylinders; Older upper gear housing. Alpha One Gen II: Single propeller; Rounder upper gear housing; External trim cylinders.

How do I tell Bravo One and Bravo Three apart?

Bravo One: Heavier-duty drive for larger V8s; Single propeller; Shift mechanism in the transom plate. Bravo Three: Two counter-rotating propellers; Heavy-duty lower gearcase; External trim cylinders.

Where is the serial number on a Volvo Penta drive?

The drive has its own plate, separate from the engine's: on the upper gear housing or the transom shield. Saildrive: plate on the gear housing; read the model, product number and serial number.

Where is the serial number on a MerCruiser drive?

Decal on the port or starboard side of the upper drive shaft housing. On newer Bravo drives the number may be stamped behind the rear cover. The transom assembly has its own number on the inner or outer plate. Confirm the gear ratio from the markings or a serial-number-specific parts catalog.

Where is the serial number on a OMC drive?

Check the model and serial numbers, not just the engine's output. Tell the Stringer (beam mounting and ball-type gears) apart from the Cobra (transom mounting, U-joint and trim cylinders). 400/800 denote small and large gear housings respectively.