Doing your mercruiser 5.7 exhaust manifold and riser replacement isn't exactly a fun way to spend a Saturday, but it's a whole lot better than the alternative—which is usually buying a brand-new long block because your old engine sucked in a bunch of seawater. If you own a boat with the classic 350 Mag or 5.7L carb engine, you probably already know that these exhaust components are essentially "wear items." They aren't meant to last forever, especially if you're running in salt water.
Most guys start thinking about this job when they see those tell-tale rust streaks running down the side of the manifold or, worse, when the engine starts idling a bit rough because of a tiny internal leak. It's one of those maintenance tasks that feels intimidating because of the weight of the parts and the fear of snapping a bolt, but if you take it slow, it's totally doable for a DIYer.
Why you can't afford to skip this
Let's be real for a second: the exhaust system on an inboard/outboard engine is a bit of a weird design. You've got hot exhaust gases and cold cooling water traveling through the same heavy iron casting, separated by nothing but a thin wall of metal and a gasket. Over time, that metal thins out due to corrosion.
In a salt-water environment, you're looking at a five-year lifespan if you're lucky. Fresh water gives you more breathing room, but even then, those gaskets eventually fail. When the "wall" between the water jacket and the exhaust passage fails, water trickles back into the cylinders through the exhaust valves. If that happens while the engine is sitting, you get a hydrolocked motor. If it happens while it's running, you get "milkshake" oil and a ruined afternoon.
Spotting the warning signs
You don't always get a clear warning before a riser fails, but there are a few things to look for. Keep an eye out for "weeping" at the joint where the riser (the top part) meets the manifold (the bottom part). If you see salt crystals or rust stains there, your gasket is toast.
Another trick is to just feel the risers after the engine has warmed up. They should be warm, but you should be able to keep your hand on them for a second or two. If one side is significantly hotter than the other, you've probably got a blockage in the cooling passages, which is a precursor to a total failure. If you haven't changed them in seven or eight years and you're in salt water, don't even look for signs—just change them.
Getting started and tearing things down
Before you even pick up a wrench, do yourself a favor and spray everything with a good penetrating oil. Not the cheap stuff, but something that actually eats rust. Let it sit overnight. The bolts holding the manifold to the cylinder head are notorious for snapping, and you really don't want to be drilling out a Grade 8 bolt in a cramped engine compartment.
When you're ready to start your mercruiser 5.7 exhaust manifold and riser replacement, the first thing you need to do is drain the water. There are usually blue plastic wing-nut plugs at the bottom of the manifold. If the water doesn't come out when you pull the plug, poke a coat hanger or a small screwdriver into the hole. Silt and sand like to settle there and block the drain.
Once it's dry, start removing the hoses. Keep track of where they go, maybe take a quick photo with your phone. Then, unbolt the riser from the manifold. These things are surprisingly heavy, so watch your fingers. After the riser is off, you can get to the manifold bolts. I like to leave one bolt in the middle loosely attached while I remove the others so the whole heavy chunk of iron doesn't just fall off and crack the fiberglass in my bilge.
The secret is in the cleaning
If you want this job to last, you have to be obsessive about the mounting surfaces. Once the old manifolds are off, you'll see a bunch of old gasket material and rust stuck to the cylinder head. You need to get that surface shiny and smooth.
Use a gasket scraper, but be careful not to gouge the head, especially if you have aluminum heads (though most 5.7s are cast iron). I usually finish it off with a bit of Scotch-Brite or some light sandpaper on a flat block. If you leave even a tiny bit of old gasket behind, the new one won't seal, and you'll be doing this whole job again in a month.
Dry joint vs. wet joint
Depending on the year of your Mercruiser 5.7, you might have the "Dry Joint" style or the older "Wet Joint" style. The older style has water passages that go right through the gasket along with the exhaust. The newer Dry Joint style keeps the water passages on the outside of the main seal, which is a much smarter design because if a gasket leaks, the water goes outside the engine rather than inside the cylinders.
Make sure you've ordered the right parts for your specific serial number. There's nothing more frustrating than having the boat torn apart and realizing your new risers don't match your exhaust pipes.
Putting it all back together
When you're installing the new manifold, it helps to use "studs" to help align things. You can take a couple of long bolts, cut the heads off, and thread them into the cylinder head. Then you just slide the manifold onto those studs. It saves your back and ensures the gasket stays perfectly aligned while you're fumbling with the real bolts.
Speaking of gaskets, pay close attention to the ones between the manifold and the riser. Some kits come with different options—some have four holes for water flow, and some are "restrictor" gaskets with only two small holes. You need to follow the Mercruiser diagram for your specific cooling setup (closed cooling vs. raw water cooling).
Don't go crazy with the sealant. Most modern gaskets are designed to be installed dry or with just a tiny bit of tack. Overusing RTV silicone can actually cause the gasket to "squish" out of place or block those narrow water passages.
Torque and the final check
Don't just tighten the bolts until they're "tight enough." Use a torque wrench. These bolts should usually be torqued in a specific sequence and to a specific foot-pound rating (check your manual, but it's often around 25-30 ft-lbs for the manifolds).
Once everything is buttoned up, it's time for the moment of truth. Connect your earmuffs to a garden hose, start the engine, and look for leaks. Let it get up to operating temperature. Sometimes a leak won't show up until the metal expands from the heat.
While the manifolds are off, it's also a great time to check your exhaust flappers (shutters) inside the Y-pipe. If they're burnt up or missing, your engine could suck in water when you're slowing down quickly. It's a "while you're in there" type of thing that can save you a huge headache later.
Wrapping it up
Completing a mercruiser 5.7 exhaust manifold and riser replacement is a big milestone for a boat owner. It's physical work, and you'll probably have some sore muscles the next day, but the peace of mind is worth it. Knowing that your engine is breathing clearly and that there's no salt water creeping into your cylinders makes those long runs out to the middle of the lake or the offshore fishing spots a lot less stressful.
Just remember to check the bolt torque again after your first couple of trips. The heat cycles can sometimes cause things to settle, and a quick "snug up" will ensure everything stays watertight for seasons to come. Happy boating, and enjoy the fact that you just saved yourself about a thousand bucks in mechanic labor!