Getting your Cummins 855 valve adjustment right is one of those maintenance tasks that separates a smooth-running rig from a noisy, fuel-chugging headache. If you've spent any time around these old-school workhorses, you know they are basically the backbone of the trucking and construction world. Whether it's an old Big Cam or an NT855, these engines are built to last, but they aren't "set it and forget it" machines. They need a little love under the valve cover every now and then to keep them humming.
If you start hearing a bit too much "clatter" from the top end, or if your fuel mileage is starting to dip for no apparent reason, it's probably time to pull those covers off. Doing a Cummins 855 valve adjustment isn't exactly rocket science, but it does require a bit of patience and a steady hand. If you rush it, you'll end up with an engine that runs rough or, worse, a burnt valve.
Why You Can't Ignore the Valves
You might be thinking, "It's a Cummins, it's supposed to be loud." And sure, an 855 has a distinct sound, but there's a big difference between healthy mechanical music and the sound of components beating themselves to death. When the clearance (the "lash") gets too wide, the rocker arms start hammering against the valve stems. This wears out the parts faster and messes with your engine timing.
On the flip side, if the valves are too tight, they might not close all the way. That's even worse. If a valve stays cracked open just a hair during the combustion stroke, the extreme heat will start eroding the edge of the valve. Before you know it, you've got a "burnt valve," and that's a much more expensive repair than a simple adjustment. Plus, when the valves aren't opening and closing exactly when they should, your turbo isn't going to spool right, and your exhaust might start looking a bit darker than you'd like.
Getting Your Tools and Space Ready
Before you dive in, you'll want to make sure you have everything you need. There's nothing more frustrating than having the engine half-apart and realizing you're missing a specific feeler gauge. For a Cummins 855 valve adjustment, you'll obviously need a good set of feeler gauges. I personally prefer the ones with a slight bend in them—it makes getting into those tight spots over the cylinder head a lot easier.
You're also going to need a way to bar the engine over. You can usually do this at the accessory drive or by using a barring tool on the flywheel. A 5/8" and an 11/16" wrench are standard staples here, along with a flathead screwdriver for the adjustment screws. It's also a good idea to have some new valve cover gaskets on hand. It's tempting to reuse the old ones, but they're cheap enough that it's not worth the risk of an oil leak dripping down your clean engine block.
The Importance of the Crossheads
Before you even touch the valves, you have to talk about the crossheads. On an 855, each rocker arm actually pushes down on two valves at once via a bridge called a crosshead. If that crosshead isn't sitting level, your valve adjustment is going to be junk from the start.
You'll want to loosen the locknut on the crosshead and back the screw out. Then, apply light pressure to the top of the crosshead with your finger to make sure it's sitting square on the valve stems. Turn the adjusting screw down until it just touches the valve stem, then give it a tiny bit more—usually about 30 degrees or one "flat" of the nut—to make sure it's snug. Lock it down. If you skip this, you'll never get the lash right because the bridge will be tilted.
Finding the Marks
This is where people sometimes get tripped up. To do a proper Cummins 855 valve adjustment, you have to follow the firing order and use the timing marks. Usually, you'll find these on the accessory drive pulley or the flywheel. They're labeled A, B, and C.
These marks tell you which cylinders are at Top Dead Center (TDC) so you can adjust the valves when they are completely closed (on the "Inner Base Circle" of the camshaft). The 855 follows a 1-5-3-6-2-4 firing order. When you're on mark "A," you're looking at cylinders 1 and 6. But remember, only one of those will be on the compression stroke. You can tell by checking which rocker arms have a little wiggle in them.
Performing the Adjustment
Once you're on the right mark and you've verified which cylinder is ready, it's time to slide that feeler gauge in. For a standard N14 or 855, you're usually looking at around .011" for the intake and .023" for the exhaust. Always double-check your specific engine data plate, though, because some high-horsepower or specialized builds might vary slightly.
Slide the gauge between the rocker arm and the crosshead. You want it to feel like there's a slight drag—kind of like pulling a piece of paper out from under a magnet. If it's too loose, the gauge will slide around with no resistance. If it's too tight, you'll have to yank it out.
Hold the adjustment screw steady with your screwdriver while you tighten the locknut. This part is tricky because the screw likes to turn with the nut. After you tighten the nut, check the gap one more time. It's very common for the gap to close up slightly as you torque the nut down. If it changed, loosen it and try again. It takes a few tries to get the "feel" for how much to compensate.
Don't Forget the Injectors
While you're in there doing a Cummins 855 valve adjustment, you'd be crazy not to check the injectors. On these mechanical engines, the injectors are operated by the same camshaft. The process is similar, but instead of measuring a gap with a feeler gauge, you're often setting the "bottom out" or using a torque-wrench method depending on whether you have Top Stop injectors or the older style.
Adjusting the injectors ensures that each cylinder is getting the exact same amount of fuel at the exact right micro-second. If your valves are perfect but your injectors are uneven, the engine will still feel "limp" and might vibrate more than it should.
Double Check Your Work
Once you've gone through all six cylinders, bar the engine over two full rotations and check a couple of them again. It sounds like extra work, but it's a great way to catch a mistake. Maybe you read a mark wrong or got distracted halfway through. It's a lot easier to fix it now than it is to put all the covers back on, start the truck, and hear a "clack-clack-clack" that shouldn't be there.
Clean the mating surfaces for the valve covers really well. Any bit of old gasket or grit will cause a leak. Tighten the bolts down evenly—don't just crank one side down and then the other. You want even pressure across the whole rail.
The Result of a Job Well Done
After a successful Cummins 855 valve adjustment, you should notice a difference almost immediately. The engine should start a bit easier, and that harsh metallic knocking should settle into a steady, rhythmic thrum. On the road, you'll probably notice better throttle response.
It's one of those jobs that feels really satisfying because you can actually hear and feel the results of your labor. These 855s are legendary for a reason—they are tough, honest engines. Keeping the valves adjusted is just your part of the deal to keep that legend running for another few hundred thousand miles. It's a bit of grease under the fingernails, sure, but it's well worth the effort to keep your iron in top shape.