If you've noticed a growing puddle of oil under your truck lately, a 5.9 cummins rear main seal install is probably sitting at the top of your to-do list. It's one of those jobs that every Dodge Ram owner dreads, not because the seal itself is expensive, but because of everything you have to move out of the way to get to it. We're talking about a ten-dollar part that requires about ten hours of sweating and swearing to replace. But honestly, if you want to keep your driveway clean and your oil levels steady, you just can't ignore it forever.
The Reality of the Job
Before you even crack open your toolbox, you need to understand what you're getting into. The 5.9 Cummins, whether it's the 12-valve or the 24-valve version, is a heavy-duty beast. Because of that, the components surrounding it are also heavy. To do a proper 5.9 cummins rear main seal install, you have to drop the transmission. If you have an automatic, that's a heavy hunk of metal; if it's a manual, it's even heavier.
You're also going to be dealing with the transfer case (if you're 4WD), the driveshafts, and the flywheel or flexplate. It is a lot of work for a single circular piece of rubber and Teflon. However, doing it yourself can save you a massive chunk of change in shop labor. Plus, you'll know it was done right, which is a big deal when it comes to seals that are notorious for leaking if they aren't installed perfectly.
Getting the Right Parts
Don't cheap out here. When you're buying parts for your 5.9 cummins rear main seal install, I highly recommend going with an OEM Cummins seal or a high-quality kit like one from Fel-Pro that includes the installation tool.
The installation tool is non-negotiable. It's a plastic or metal sleeve that ensures the seal stays square as it goes onto the crankshaft. Without it, you're almost guaranteed to flip the inner lip of the seal, and if that happens, you'll be doing this whole job again next weekend. Most guys prefer the "dry" seal technology used in modern Cummins kits. These seals actually rely on being installed completely dry—no oil, no grease—so they can "transfer" a layer of Teflon to the crank surface for a long-term seal.
Tearing It Down
The first few hours are just about making room. Get the truck up on sturdy jack stands if you aren't using a lift. You'll need to pull the driveshafts, disconnect the shift linkage, and unplug the wiring harnesses from the transmission.
Once the transmission is out of the way, you'll see the flywheel or flexplate. You'll need a good impact wrench to get those bolts out. Once that plate is off, you're finally staring at the rear of the crankshaft and that pesky old seal. It's usually tucked into a housing, and it's likely caked in years of road grime and leaked oil. Take a second to breathe—the hard part of the teardown is done.
Removing the Old Seal
This part is a bit nerve-wracking. You need to get the old seal out without scratching the crankshaft. If you nick or gouge that metal surface where the seal sits, you've created a permanent leak path that a new seal won't be able to fix.
The "screw method" is a classic for a reason. Carefully drill a couple of tiny pilot holes into the metal face of the old seal (be super careful not to hit the crank or the housing). Then, thread in some small sheet metal screws. Use a pair of pliers or a small pry bar against the screws to pop the seal out. It usually takes a bit of wiggling, but it'll eventually give way. Once it's out, you'll see the bare end of the crank.
Cleaning Is Everything
I can't stress this enough: your 5.9 cummins rear main seal install will fail if things aren't clinical-level clean. Grab some brake cleaner and a lint-free rag. You want to scrub the crankshaft surface until you could eat off it. If there's a groove worn into the crank from the old seal, you might need a repair sleeve (often called a "speedy sleeve"), but hopefully, it's just dirty.
Inspect the housing as well. Any leftover bits of rubber or old sealant need to go. If you use a Scotch-Brite pad, be very gentle. You're looking for a smooth, shiny surface. Once it's clean, don't touch it with your oily fingers.
The Main Event: The Install
Now it's time for the actual 5.9 cummins rear main seal install. If you're using the standard Cummins-style Teflon seal, remember the "dry" rule. Do not put oil on the seal lip or the crank. It feels wrong to an old-school mechanic, but that's how these are designed to work.
- Slide the pilot tool onto the crank. This provides a smooth ramp for the seal to slide over.
- Position the new seal. Place it over the tool and start pushing it toward the engine.
- Use the driver. Most kits come with a plastic puck or a metal driver. You'll use the flywheel bolts to slowly and evenly draw the seal into the housing.
- Go slow. Tighten the bolts in a crisscross pattern, maybe a half-turn at a time. This ensures the seal goes in perfectly straight. If it cocks to one side, you're in trouble.
- Bottom it out. The tool is designed to stop once the seal is at the correct depth. Don't try to "guess" the depth; let the tool do its job.
Once the seal is seated, pull the driver and the pilot tool off. Take a flashlight and look at the lip of the seal all the way around. It should look uniform and snug against the crank.
Putting It All Back Together
Reassembly is basically the reverse of the teardown, but with more torque specs to worry about. When you put the flywheel or flexplate back on, make sure you use a bit of blue Loctite on the bolts. These engines vibrate a lot—that's just the nature of a straight-six diesel—and the last thing you want is a flywheel bolt backing out inside your bellhousing.
Torque them down to the factory spec (usually around 101 ft-lbs, but check your specific year's manual). Then comes the heavy lifting: getting the transmission back into place. Having a transmission jack makes this a lot safer and easier. Line up the splines, bolt it down, and reconnect your driveshafts and electronics.
Final Checks and First Start
Before you fire it up, double-check your oil level. You probably lost a little bit during the process, or maybe you drained it earlier. Once you start the truck, let it idle for a while. Don't go out and do a boosted launch immediately. You want that seal to "mate" with the crankshaft.
Crawl back under there with a flashlight while it's running. It might be smoking a little bit as the brake cleaner and old oil burn off the exhaust, but you shouldn't see any fresh drips coming from the bottom of the bellhousing. If it's dry, you've successfully completed your 5.9 cummins rear main seal install.
Common Mistakes to Avoid
If you're doing this for the first time, keep an eye out for these pitfalls: * Forgetting the dust seal: Some kits come with a separate dust seal. Make sure you know which order they go in. * Using a hammer: Never try to "tap" the seal in with a hammer and a punch. You'll distort the metal casing of the seal and it'll leak within a week. * Ignoring the crank condition: If there's a deep groove in the crank, a standard seal won't work. You'll need the kit that includes the wear sleeve. * Rushing the cleaning: Most leaks after a replacement happen because a tiny bit of grit was left on the crank.
Taking your time is the key here. It's an exhausting job because of the heavy parts, but the actual seal installation only takes about fifteen minutes of that total time. Focus on the details during those fifteen minutes, and your Cummins will stay leak-free for another couple hundred thousand miles. It's a great feeling to look at the ground after your truck has been parked overnight and see nothing but dry pavement.