Gear Ratio

Are all MerCruiser Alpha Gen II lower units the same?

No. They're made in standard rotation, counter-rotation and high-altitude (HA) versions. The HA lower is only used with a 4-cylinder engine in the 2.0 and 2.4 ratios and is a standard-rotation-only drive. A single-engine boat at 1.62 would be running the standard lower. Give us your drive serial number and we'll tell you which of the three you have.

Can I change to a different gear ratio when I replace my drive?

We strongly recommend staying with the ratio your boat was manufactured with. It keeps the boat performing the way the builder intended and avoids a propping problem you may not be able to solve - fitting a high-altitude gear set in the lower, for example, takes a 1.94 setup out to 2.40. Anything custom along these lines isn't backed by us and carries no warranty.

Can I confirm my Alpha ratio by counting gear teeth?

Yes, and it's the best fallback when the serial is unreadable. Drain the unit, remove the top cap on the upper and count the teeth on the two gears inside. A 22/20 count is the 1.47 ratio and 17/19 is the 1.81. Send us your counts and we'll confirm which unit you need before you order.

Can I convert my drive from a 1.47 to a 1.81 ratio by changing the gears?

It takes more than the gear set - the roller bearing is different between the 1.47 and the 1.81, so you'd need that too. Beyond the parts, it has to be shimmed properly and the rolling torque set correctly, which is precision work. In most cases buying the drive in the ratio you need is the more sensible route, and a custom build wouldn't carry a warranty.

Can I order a custom gear ratio or a modified setup?

We don't offer custom configurations. Anything custom isn't something we can back, and it wouldn't carry a warranty. We match new units to old units like for like so that no disassembly or modification is required on your end.

Does the upper or the lower unit determine my gear ratio?

Both halves have gears, so the ratio comes from the pair working together - which is why you can't read it off one half. The short answer: don't try to work it out yourself. Send us the drive serial number and we'll tell you the ratio outright. If you want the mechanism: the number on the tag is the combined ratio, and some upper gear sets are one-to-one, so a tag reading something like 1.62/1.65 isn't a contradiction - it's the upper and the combined figure side by side.

How do I do a spin test to work out my gear ratio?

With the unit in forward gear, mark both shafts and count how many turns of the driveshaft it takes to get one full turn of the prop shaft. That number is the ratio: a 1.87 is 1.87 turns of the driveshaft to one turn of the prop shaft, and a 2.00 is two turns to one. Take the impeller out first so the shaft turns freely. It only works if the unit still rotates in gear - if it's locked up you'll need a tooth count instead.

How do I find my ratio if the driveshaft is seized?

If the unit is locked up a spin test won't work, so the remaining option is a tooth count. Disassemble the prop shaft side and count the teeth on the pinion gear and the drive gear - that gives us an exact ratio to match against. Send us the counts and we'll confirm the correct unit for your engine.

How do I find the gear ratio for my MerCruiser Alpha drive?

The most reliable way is the drive serial number - send it to us and we'll look it up and tell you exactly what ratio it was built with. You'll find it on the engine ID tag on top of the engine, or on the port side of the drive on the MerCruiser sticker. Don't go by engine size or what usually came on that boat; the serial is what we ship against.

How do I find the gear ratio for my drive or outboard?

The most reliable way is your serial number - the drive (or engine) serial decodes to the exact ratio it was built with. Don't go by engine size or "what usually came on the boat." If you can't read the serial, count the teeth on the upper and lower gears, or call us and we'll help confirm it at 813-925-7127.

I was told my ratio is 1.65 but your site only shows 1.62 - will that work?

Yes. SEI's 1.62 replaces the older 1.65 - they're the same application. The same is true for other close pairs (for example a 1.94 replaces a 1.98). When in doubt, confirm with your serial number at 813-925-7127.

My drive is a 1.50 or a 1.98 ratio but you only list 1.47 and 2.0 - will those work?

On a complete drive, yes - our 1.47 replaces the older 1.50 and our 2.0 replaces the 1.98. Be careful if you're buying only one half, though: the 1.98 and 2.0 half units are completely different from one another, so that substitution only holds when the whole drive is being replaced. Tell us whether you need a complete drive, an upper or a lower, send your drive serial number, and we'll confirm the exact match at 813-925-7127.

My outboard's gear ratio isn't marked anywhere - how do I find it?

Outboards usually don't have the ratio marked on the unit, so it comes down to either a spin test or a tooth count inside the gearcase. Start by sending us the engine serial number - in many cases we can look up what it was built with. If the serial won't resolve, do the spin test with the impeller removed and count driveshaft turns per prop shaft turn.

What does the 90-116-01B upper suit - will it work on a 5.0 or 5.7 V8?

No, not unless you're operating above about 6,000 feet of elevation. That's the high-altitude gear set. Likewise the 90-116-02H lower only fits the 2.00 and 2.4 ratio units, whereas the standard 90-116-02 covers the rest. Tell us your engine and where you boat and we'll point you to the right one.

What gear ratio does a MerCruiser 3.0L use?

Every MerCruiser 3.0L from the 1970s through 1997 is a 1.94 ratio - the older 1.98 designation was replaced by our 1.94. From 1998 the 3.0L moved to the 2.00 lower paired with the 1.65 upper. Send us the drive serial number and we'll confirm which side of that change your drive falls on.

What gear ratios does the SE216 come in?

Just two: 1.75 and 1.62. The 1.62 is very rarely requested - we sell only a couple a year - so in almost every case the 1.75 is the one you want. Send us your engine serial number and we'll confirm which applies to your motor.

What happens if I fit a lower unit with a different gear ratio than my original?

Two things change, and both cost money. You'll need to re-prop the boat, because the old propeller will no longer be right, and depending on the unit you may need different prop hardware with it. Beyond that, a gearcase that isn't the one your engine was built with is generally sold without our warranty. Unless you have a specific reason to change, stay with the ratio your boat was built with - and if you're unsure what that is, send us the serial number at 813-925-7127.

What's the stock gear ratio for a 4.3L OMC?

The stock ratio for the 4.3L is 1.84. We'd still want to confirm against your drive before shipping, and we'll also need to know whether it's a single or twin engine boat so we get the rotation right.

Generation

Are Alpha Gen I and Gen II propellers interchangeable?

Yes. Props from the Gen I and Gen II use the same hub kit and are interchangeable, and they also interchange with a lot of the Mercury outboard units. So if you're finding Gen I props hard to source, that's not a reason on its own to change drive generations.

Is my MerCruiser Alpha One a Gen I or a Gen II?

Alpha One Gen I runs roughly 1972-1990; Gen II is 1991 and newer. A quick tell is the water-pump housing: plastic = Gen I, metal = Gen II. If your year is right at the 1990-91 transition, send a photo or call and we'll confirm before you order.

Is my MerCruiser Alpha a Gen I or a Gen II?

1991 was the start of Gen II, so 1990 and earlier is Gen I. Another quick tell is the gear lube reservoir in the engine compartment, which indicates a Gen II setup. We always ask rather than assume, because a drive can have been changed at some point in its life - send us the drive serial number and we'll confirm it outright.

My Alpha One top cap kit came with a different size bearing - is it wrong?

Probably not. The Alpha Ones used both small and large bearings on the top caps; after 1995 it was all small bearing, and the small one is what we use. Any new top cap kit from us or from the OEM will come with the smaller bearing. The new smaller bearing is used with the new top cap, pressed onto your original stub shaft and then shimmed properly.

My drive is older than 1990 - is it still an Alpha Gen I?

Possibly not. There's an earlier design often called the "R" style gearcase, also known as an MC-1, which predates the Alpha Gen I and is sometimes loosely called a pre-Alpha drive. The internal parts are different even though some water pump components carry over. Send us a photo of the driveshaft splines sticking out of the lower - if there's a preload pin, you're on the older style and it takes a different water pump base.

Should my upper driveshaft be hollow?

On an Alpha Gen II, yes - the Gen II upper driveshaft is hollow. The Gen I is not. If you have a Gen I with a hollow-looking shaft, it's possible someone rebuilt it using the wrong seal at the bottom of the upper unit and plugged the shaft to work around it.

What is the difference between the SE116 lower and the SE116 HA lower?

It's the gear ratio. HA stands for high altitude gear set - the HA lower is used to make the 2.00 and 2.40 ratios and is fitted to the 3.0L 4-cylinder engines from 1998 onward. The standard SE116 lower covers the 1.47, 1.62, 1.81 and 1.94 ratios. Tell us your engine and we'll tell you which of the two you're on.

Part Numbers

Can you cross-reference an OEM part number to your parts?

Yes - any Mercury number can be entered into our website search and if we make it, it will show. For a bigger job, make a list of the OEM part numbers you need and either enter them on the site or send us the list and we'll cross-reference them for you. That's usually faster than trying to identify parts from a diagram.

How do I read an SEI Mercury part number like 90-216-A20?

The number decodes the unit for you: 90-216 is the SE216 family, and the suffix gives the ratio and shaft length - A20 being the 20-inch shaft. On that particular unit you're looking at a 13-spline driveshaft, 1.75 ratio, for the 3.0L application. If you have the number off the box you can confirm your shaft length from it without measuring.

What does the "S" mean in a part number like 90-218-AS?

The letters decode separately - "A" is the gear ratio and "S" means stub shaft. A stub shaft unit is the version used with two-piece driveshafts. If you're not sure whether you have a one-piece or two-piece driveshaft, drop the lower and tell us what you find and we'll confirm which version you need.

Serial Number

Can I find my SEI serial number without dropping the lower unit?

Often yes - remove the trim tab anode above the propeller and the 6-digit number is stamped into the recessed area underneath it. Getting to the red tag itself does require dropping the unit, but for most purposes that stamped number is all we need. Send it to us at 813-925-7127 and we can look up the build date, ratio and warranty status.

Can you identify my lower unit from the SEI serial number on it?

Yes - send us the 6-digit number and we can tell you the exact model, ratio and shaft length it was built as, plus when it was sold and whether it's still in warranty. That's often the fastest route when the engine's own ID tag is gone: several customers come to us having bought a boat with one of our units already fitted and no other information.

Does the serial number on my SEI lower unit tell you the gear ratio?

Yes - give us the serial off an SEI unit and our sales team can look it up and tell you exactly what it is, including the ratio it was built with and when it was sold. Call or text 813-925-7127 with the number. If the unit on your boat isn't one of ours, we'd need the MerCruiser drive serial number instead, or a tooth count from the two gears in the upper under the top cap.

I can't find my engine serial number anywhere - what else can you work from?

Try the freeze plugs on the powerhead, which often carry the number when the ID tag has been lost or painted over. Failing that, tell us the gear ratio, engine size in litres and driveshaft length and we can usually narrow it down. Photos of the gearcase and the bearing carrier behind the prop help too.

My drive serial number won't decode - what now?

Some older serials come back only as "MERCRUISER ALPHA" without giving the ratio or generation, and some stickers are simply unreadable. Two fallbacks: the number is also stamped into the gearcase in the same area as the sticker, and a wire wheel will often bring it up. Failing that, pull the top cap off the upper and count the teeth on the two gears - that gives us the ratio directly.

There's a casting number on my lower unit - does that mean it's an SEI unit?

Not necessarily. A number in the style of 1623-9413-C-S is a MerCruiser casting number, which means you're looking at a MerCruiser unit rather than one of ours. An SEI unit is identified by the serial on the red tags between the two halves of the drive. Send us a photo if you're not sure what you're looking at.

What's the difference between my transom serial number and my drive serial number?

They're different numbers and only the drive serial tells us your gear ratio - this is easily the most common mix-up we see. The transom assembly carries its own serial, so if the number you found is on the transom, we'll need you to go back and find the one on the drive itself, near where it says MerCruiser or Alpha. If the drive is already split into two halves, there are serial tags visible inside the cavities of both housings.

Where is the serial number located?

On a Mercury/Mariner outboard it's on a decal on the engine bracket that bolts to the transom, usually on the port side. On a MerCruiser sterndrive it's on the drive itself. On an SEI drive the serial is on red tags between the two halves - the drive has to be separated to see them. Send us a photo if you're not sure and we'll point it out.

Where is the serial number on a Bravo transom assembly?

It's on the inside transom plate, on a red tag carrying our logo. That's what we need to check warranty status on a transom assembly, so it's worth locating before you call.

Where is the serial number on a MerCruiser drive?

Look on the port side of the upper unit, on the MerCruiser sticker or underneath it. It's usually also printed on the plastic cover on top of the engine. Be careful not to confuse it with the transom assembly serial - that's a different number and it won't give us your gear ratio.

Where is the serial number on a Yamaha lower unit?

It's stamped where the anode sits above the prop - take the anode off and you'll see it. If the unit is already off the engine, there's also a red plate in one of the internal cavities carrying the same number.

Where is the serial number on an SEI drive or lower unit?

There's a red tag with our logo inside both the upper and the lower, in cavities you can't see while the drive is bolted together - we're looking for the 6-digit number on it. The same number is also stamped into the case: on the upper it's under the top cap, and on the lower it's on the underside of the cavitation plate, in the recessed area the trim tab anode covers. It's also on the original box if you still have it.