Big Foot
How do I tell if my Mercury outboard is a Big Foot?
Measure the round part of the gearcase (the bullet, near the prop) outside-to-outside. 4-1/4 inches = Big Foot; smaller than that = standard. On the 40-60 HP motors, SEI's lower unit is the Big Foot version.
How do I tell whether my Mercury outboard is a Big Foot?
Measure the round part of the gearcase - the bullet, up near the prop - outside to outside. 4-1/4 inches means it's a Big Foot; anything smaller than that is a standard foot. If you'd rather not guess, send us a photo of the lower unit and we'll identify it for you. Big Foot gearcases are most often found on pontoons and heavier vessels rather than lighter runabouts.
Is my Mercury a small case or a large case?
This is the check that applies to the SE206 family. Measure the bullet - the round part of the gearcase by the prop - outside to outside: 4.25 inches is the small case and 4.75 inches is the large case. It matters because it rules ratios in and out. Send us the measurement along with your engine serial number and we'll tell you which unit applies to your motor.
My gearcase bullet measures 4.25 inches - what does that mean?
That measurement separates the small case from the large case (4.75 inches), and it's the check that matters on the SE206 family and on the SE304 and SE306 units. Which way it points you depends on what your engine was built with, so send us the figure along with your engine serial or model number and we'll confirm the right unit - and tell you whether a prop change comes with it.
What does a 4-1/4 inch versus 4-3/4 inch bullet diameter tell me?
It tells you which gearcase family you have, and it rules ratios in or out. On the Mercury outboards, the 1.78 ratio exists only in the small case, which measures 4.25 inches across the bullet outside to outside - if yours measures 4.75 inches you have the large case and it can't be a 1.78. Measure before ordering, because forum advice about "all of them being the same" frequently isn't right.
Driveshaft
How do I know whether I need to buy a new upper driveshaft?
It depends on your engine model, and we publish a chart showing it - a "No" in the "must purchase upper driveshaft" column means your existing shaft is interchangeable with our unit. In that case you remove it from your old gearcase and install it into ours by lining up the splines and tapping it in with a rubber mallet. Send us your model number and we'll check the chart for you.
How do I tell whether I have a one-piece or two-piece driveshaft?
The engine model number tells us, so send that over. On the two-piece applications you can reuse your existing driveshaft in our new unit. Be careful comparing measurements - a driveshaft measuring 27.25 inches from the top of the splines down to the edge of the case corresponds to what we call a 25" driveshaft, so a longer measurement than the label doesn't mean it's the wrong part.
My E-TEC driveshaft has clips on it - will it work in your gearcase?
Yes, the driveshaft will work in ours, but you need to remove the clips from your old shaft first. There's a note about the driveshaft clips on the application tab of the relevant product page. Our gearcase already has a new retaining clip pre-installed waiting for the shaft to drop into it.
My unit has a two-piece driveshaft - do I need a new one?
On the two-piece applications you reuse the top part of your existing shaft, so there's nothing extra to buy. Our stub shaft units (the "S" in a part number such as 90-218-AS) are the versions made for two-piece driveshafts. Tell us your serial number and we'll confirm which configuration your motor uses.
Rotation
Can I convert one of my twin standard drives to counter-rotation?
Yes. You'd fit a counter-rotation drive or gearcase, change to a left-hand propeller, and reverse the shift cable at the helm control on that side so it throws reverse correctly. Alternatively you can leave the setup exactly as it is and simply order another standard rotation unit - nothing else then needs to change.
Can I use a standard rotation lower unit on a counter-rotation motor?
It can be done, but only if you change to a right-hand propeller and check the shift linkage at the same time. Be aware this is serious: if you run a standard as a counter - or a counter as a standard - without changing the prop and, on a digital throttle boat, reprogramming the throttle, the gearcase will destroy itself. We see the specific damage that causes, so please don't skip either step.
Can a Bravo drive be run as either standard or counter-rotation?
Yes - that's one of the nice things about the Bravo. What dictates the rotation is which way the shift cable moves, combined with running the correct propeller for the direction you want. So if you have one left-hand and one right-hand prop on a twin setup, you already have one side running counter.
Do I need a standard or a counter-rotation unit?
On a single-engine boat you want standard rotation with a right-hand prop. On twins, the starboard engine is standard and the port engine is normally counter-rotation with a left-hand prop, which cancels out torque steer. The quickest check is simply which prop is fitted - send us a photo of the prop and the back of the boat and we'll confirm. On older twins it wasn't unusual to find two standard drives rather than a standard/counter pair, so it's worth verifying rather than assuming.
How can I tell whether a lower unit is counter-rotation?
You can't tell by shifting it on the bench - a standard and a counter will both feel the same there. Take the prop off and look down into the exhaust cavity at the prop shaft bearing carrier, where you'll see a visible difference between the two. We also have a counter-rotation identification guide we can send you.
I have a single engine with a counter-rotation lower unit - should I change it?
We'd advise switching back to a standard rotation unit with a right-hand propeller if that's an option for you. We're not big fans of single-engine counter-rotation setups: counter-rotation lower units aren't built quite the same way and frankly aren't quite as strong as the standard rotation units. On a single-engine boat there's no torque-steer reason to run one.
Is the powerhead different between standard and counter-rotation Yamaha outboards?
No - Yamaha changes direction in the lower unit, not the powerhead. Both engines' flywheels spin the same way when running and both shift the same direction; what differs is which way the prop shaft turns inside the lower unit. That's why a rotation change on a Yamaha is a lower unit change.
My engine plate says LH rotation - do I need a counter-rotation drive?
No, and this one trips a lot of people up. All the MerCruiser sterndrive engines were left-hand rotation, exactly as they were in automotive applications - that refers to the engine, not the drive. A single-engine boat is standard rotation with a right-hand prop. Right-hand rotation engines do still exist in the field but they're rare and were generally inboard boats.
What's the difference between standard and counter-rotation, and which do I need?
A single-engine boat uses standard rotation (right-hand prop). On twin engines, one runs standard and the other counter-rotation (left-hand prop) so the boat tracks straight. Counter-rotation uses a different lower unit - confirm your rotation before ordering, and on a twin setup make sure you're matching the correct side.
Which Mercury lower units are standard and which are counter-rotation?
They're separate part numbers rather than a configurable option - for the 1.75 ratio 25" application, for example, the SE216 is standard rotation and the SE218 is the counter-rotation unit. Send us your engine serial number and we'll give you the exact standard or counter part number for your motor.
Shaft Length
Does my engine model number tell me the shaft length?
Usually yes - an "L" in the model number indicates a 20" driveshaft and an "X" or "XL" indicates 25". That holds as long as nothing has been changed on your motor, and conversions between lengths do happen. It costs nothing to confirm by measuring, so we'd recommend doing both before ordering.
How do I measure my driveshaft length correctly?
Measure from the top of the driveshaft down to the gearcase mounting surface - the flat part that meets the midsection, past the water pump. About 23 inches means a 20" shaft, about 27 inches means a 25", and about 33 inches means a 30". Measuring from the cavitation plate instead will give you a different number and is the most common reason people order the wrong length.
How do I measure my shaft length?
Measure from the top of the driveshaft down to the mating surface past the water pump. About 22.5 inches corresponds to a 20" shaft. If the number you get sits between sizes, send us the measurement along with your engine serial number and we'll tell you whether you need the 20", 25" or 30" unit.
My model number says 20" but the OEM part number comes back as 25" - which is right?
Measure it, because either could be correct. Motors do get converted between shaft lengths, and once that's happened the model number no longer reflects what's fitted. Measure from the top of the driveshaft to the gearcase mounting surface past the water pump: around 23 inches is a 20", 27 inches is a 25" and 33 inches is a 30". Send us the figure and we'll match it.
What measurement corresponds to each Mercury shaft length?
Measure from the top of the lower unit case to the top of the shaft: 22 to 23 inches is a 20" unit, 27 to 28 inches is a 25", and 32 to 33 inches is a 30". The letter in your model number is a good cross-check - L indicates 20" and XL indicates 25" - but if the motor has been converted between lengths, only the measurement is reliable.
Spline Count
Did the Alpha driveshaft spline count change over the years?
No - the splines are the same from 1972 through 1990, so there's no concern there when fitting an Alpha One replacement across that range. Spline count is worth checking on the Mercury outboards, where 8 versus 13 does matter, but not on the Alpha drives in that period.
Do I have an 8-spline or a 13-spline driveshaft?
Those are the only two options, and you'll need to drop the current lower unit and count the splines on the end of the driveshaft that goes into the engine. Year is a reasonable guide - later motors are 13-spline - but it should be confirmed. If you find 8 splines you'll need to source that upper driveshaft from Mercury, and we'd strongly recommend locating it before ordering the lower unit.
I counted the splines on the shaft - does that tell me the gear ratio?
No. Splines on the driveshaft or prop shaft have nothing to do with the gear ratio - that's a common mix-up. The ratio is set by the gears inside the unit, so you need a tooth count on the pinion and forward gear, or a spin test. Spline count matters for fitment, just not for ratio.
Which prop hub kit does my replacement lower unit take?
It takes the same hub kit as your original drive, so if the prop was working on the old unit the same hub kit carries over. Source it from whoever made your propeller, or from a Mercury dealer on the MerCruiser applications - we don't hold those hub kit part numbers. The exception is where you're changing case type, which can change the prop shaft and therefore the hub. Tell us the unit and the prop you're running and we'll tell you whether it carries over.