If you’ve got a Mercury Verado and you’re noticing oil in the water around your lower unit, or you can see it weeping right onto the lower unit itself, you’re dealing with one of the most common failure points on these engines: the drive shaft seals under the power head. I ran into exactly this on a 41-foot Sea Hunter running triple 350 Mercury Verados — the middle engine was losing oil, and I’d already traced it back to failed drive shaft seals during a previous look. That means pulling the entire power head. It’s not a small job, but it’s one you can do in an afternoon if you know the order of operations.
Why It Matters
Oil loss on a Verado isn’t something to put off. If you’re seeing oil in the water or on the lower unit, the drive shaft seals have likely already failed, and running the engine in that condition risks water intrusion into the power head or oil starvation — both of which turn a seal job into a much bigger repair bill. Especially here in Florida, where these engines are running in saltwater most of the year, catching this early and doing the job right saves you from a much more expensive teardown down the road.
Products
- Drive shaft seal — the top seal that fails and lets oil escape past the drive shaft
- Water seals (x2) — sit below the drive shaft seal, also prone to failure and worth replacing together
- Base gasket — new gasket needed when reseating the power head
- Chain tensioner and oil pump drive gear — removed and reinstalled as part of accessing the seals
- Clean engine oil — used to lubricate the new seals before installation
Step-by-Step Guide
Diagnose Before You Pull Anything
Before removing the lower unit, drain the oil and check the drive shaft for wear marks — grooves, scoring, or grinding at the top of the shaft. If the shaft is clean, you’re safe to proceed with a standard seal replacement.
Remove the Lower Unit and Side Chaps
Tilt the engine up and pull the lower unit. On the 350, remove the two Allen bolts and the twelve 5/16″ bolts holding the side chaps on. With those off, take out the ten 15mm nuts holding the power head down.
Work the Port Side
Trim the engine back down. Remove the exhaust plenum (two 5/16″ bolts, then push in and release the yellow tab). This gets you to the two 13mm nuts on the back of the power head. Disconnect the trim sender, the fuel line to the fuel rail, and don’t forget the hose routed behind the charge air cooler — it’s easy to miss.
Work the Front Side
Remove the 10mm nuts and bolts on the front panels, then the two 12mm bolts under the shift actuator. Spin the pin, catch the washer with your fingers, and pull the pin free. There’s a 12mm bolt attaching the actuator to the shift crank — it’s usually torqued down hard, so leave the actuator connected while you break it loose. On a 300 and under you’d disconnect a shift switch here, but the 350 doesn’t have one — instead, disconnect the water line cooling the supercharger.
Work the Starboard Side
Pull the top cover and disconnect the thermostat dump hose. Trim the engine level so the power head comes straight up when you lift it. Then de-rig: 14-pin main harness, power steering cable, water sensor, boat harness, clean power, fuel hose, and power cables. On triples or quads, tape off the terminals where your terminator resistor normally sits. Disconnect the trim pump, unplug the FSM wire, and get the two 10mm bolts on the side — one sits behind the thermostat dump hose and is tight to reach. Pull the telltale by hand and push the fitting into the hose so it doesn’t get lost.
Lift the Power Head
With everything disconnected, hook up a lifting hook and cable and lift straight up and off.
Replace the Seals
Using a 6mm Allen wrench, remove the chain tensioner and the small plate behind it (5/16″ bolts). Hold the shaft with an 18mm wrench and remove the nut on the oil pump drive gear with an 18mm socket. Pull the gear and drive chain to expose the seals — one drive shaft seal on top, two water seals underneath.
Clean the bore out, lube the new seals with clean engine oil, and press them in. The water seals go in lips-down. The drive shaft seal goes in lips-up.
Reassemble
Reinstall the oil pump drive gear and chain. Stick a screwdriver into one of the power head bolt holes to hold the 18mm wrench steady, then torque the drive gear nut to 59 ft-lbs. Reinstall the chain tensioner — the spring’s angled piece needs to rest toward the aft side. Put the metal plate back on.
Set the Power Head Back Down (Solo Job Trick)
Get the base gasket in place. If you’re working alone, tie a string to a bolt on one side of the power head, loop it around to the bolt on the opposite side, then use the strings to guide the power head down and line the bolts up with the holes as you lower it. Make sure the two spaces on the drive gear line up with the two pins on the bottom of the power head, and seat it all the way down.
Pro Tips
- Tape your power cables before de-rigging. On triples or quads, the terminator resistor terminals are exposed once you pull the harness — a stray touch there can cause problems you don’t want to chase down later.
- Don’t forget the hose behind the charge air cooler. It’s the one connection on the port side that’s easy to overlook, and leaving it attached will fight you when you try to lift the power head.
- The string trick actually works. If you’re doing this without a second set of hands, tying strings to opposite bolts to guide the power head down is a legitimate way to get the holes aligned without fighting the weight solo.
Final Verdict
This is a real job — pulling a power head off a 350 Verado is not a weekend-warrior five-minute fix, but it’s also not beyond a boat owner who’s comfortable with wrenches and patient about keeping track of bolts and hose routing. If you’ve got the tools (Allen wrenches, the right metric sockets, a lifting hook and cable) and you’re not in a rush, doing your own seal replacement saves a real chunk of labor cost versus a shop. Just don’t skip checking the drive shaft for wear marks first — if that shaft is scored, you’re looking at a bigger repair than a seal swap.
FAQ
How do I know if my Verado’s drive shaft seals are failing?
The clearest sign is oil visible in the water around the lower unit, or oil actually weeping out onto the lower unit itself. That’s the classic symptom of failed drive shaft seals on a Verado.
Do I need to replace the water seals at the same time as the drive shaft seal?
Since you’re already in there and the power head is off, yes — replace both water seals along with the drive shaft seal while you have access.
What torque spec does the oil pump drive gear nut need?
59 ft-lbs, using an 18mm wrench to hold the shaft and an 18mm socket on the nut.
Check Us Out!
We hope that this has been insightful and allowed you to understand a little bit more about outboard engine maintenance and what goes into keeping your boat running right.
If you have more questions or want to learn more about your boat and its different systems, consider joining our Boating Academy where we have created HUNDREDS of video courses teaching you basically everything about your boat!
You can also find even more helpful videos on our YouTube channel where we create even more boating videos!
And if you would like to support us to continue bringing you great content, please click the link below to Amazon where we get a commission from anything you are already going to buy! Click Here To Amazon!