Getting a handle on your 855 cummins valve adjustment is one of those rites of passage for anyone who spends a lot of time around these legendary Big Cam engines. If you've spent any time in the cab or under the hood of a truck powered by an 855, you know that distinct sound—the steady, rhythmic mechanical clatter that tells you everything is working as it should. But when that clatter starts sounding a little more like a "clack" or your fuel mileage begins to dip for no apparent reason, it's usually time to pull the covers and set the overhead. It's not a difficult job, but it does require a bit of patience and a "feel" for the tools that only comes with practice.
The 855 is a workhorse, and like any workhorse, it needs a little TLC to keep pulling hard. Many folks put off the valve adjustment because it looks intimidating with all those pushrods and rocker arms, but it's actually a very logical process once you get the hang of the marks on the accessory drive.
Why you shouldn't skip the overhead set
I've seen guys run these engines for years without ever touching the valves, and while the 855 is tough enough to take a lot of abuse, it's not doing the engine any favors. When your valves are out of adjustment, you're essentially messing with the engine's breathing. If the clearance is too tight, the valves might not seat properly, which can lead to burnt valves—and that's a repair bill nobody wants. If they're too loose, you get that excessive noise, loss of lift, and general sluggishness.
When people talk about an 855 cummins valve adjustment, they're usually talking about "setting the overhead." This includes the intake valves, the exhaust valves, and the injectors. Because the injectors on these engines are mechanically actuated by the camshaft, they're part of the same maintenance cycle. Getting them all synced up is what gives the Big Cam its legendary throttle response and grunt.
Getting your tools and space ready
Before you even crack a bolt, make sure the engine is cold. I can't stress this enough. You want the engine at ambient temperature—basically, let it sit overnight. Metals expand when they're hot, and the specs for the 855 are based on a cold engine. If you try to do this while the block is still warm, your clearances will be all over the place once it finally cools down.
You won't need anything too exotic, but a few specific tools make the job a whole lot easier: * A good set of feeler gauges (specifically .011 for intake and .023 for exhaust, though you should always check your engine's data plate). * A barring tool for the flywheel or a way to turn the engine over via the accessory drive. * A torque wrench (the inch-pound variety is usually best for the injectors). * Basic wrenches and sockets. * A fresh set of rocker cover gaskets (don't try to reuse the old ones; they almost always leak).
Understanding the timing marks
The secret to a successful 855 cummins valve adjustment is understanding the "A, B, C" marks on the accessory drive pulley. These marks tell you which cylinders are at Top Dead Center (TDC) or the correct position for adjustment.
The firing order for the 855 is 1-5-3-6-2-4. If you look at the accessory drive pulley, you'll see marks for 1-6, 2-5, and 3-4. You'll align these marks with the pointer on the gear housing. It's a bit of a dance—you turn the engine, check the mark, adjust the specific valves and injectors for that position, then turn it again.
One thing that trips people up is knowing whether you're on the right stroke. Just because the "1-6" mark is lined up doesn't mean you're adjusting cylinder number one. You have to look at the rockers. If both the intake and exhaust rockers for cylinder one are loose (have play), you're on the right spot to adjust that cylinder. If they're tight, you're likely on the companion cylinder (number six) and need to rotate the engine another 360 degrees.
Diving into the intake and exhaust valves
Once you've got your cylinder positioned correctly, it's time for the feeler gauges. For most 855 Big Cams, you're looking for .011 inches on the intake and .023 inches on the exhaust.
Slide the gauge between the rocker lever and the valve bridge. You're looking for what most old-timers call a "slight drag." It shouldn't be so tight that you have to muscle the gauge in there, but it shouldn't be floppy either. It's like pulling a piece of paper from under a heavy book—you want to feel some resistance, but it should still move smoothly.
Hold the adjustment screw with a screwdriver while you tighten the locknut. This part is crucial because the screw likes to turn with the nut, which will ruin your perfect setting. After you tighten the nut, check the clearance again. It's common to have to do it twice to get it just right.
The injector adjustment: The tricky part
The injectors are where things get a bit more technical. On an 855, the injector is pushed down by a third rocker arm. There are a couple of ways to do this—the "torque wrench method" or the "dial indicator method." Most guys doing this in their own shop use the torque wrench method because it's more straightforward.
You'll tighten the injector adjustment screw until it bottoms out to squeeze the oil out of the injector cup, then back it off and torque it to the specific inch-pound setting listed on your engine's CPL (Control Parts List) plate. It's usually quite a small number, so don't go cranking on it with a big half-inch drive wrench. Accuracy is everything here. If the injector isn't set right, you'll get smoke, rough idling, or even engine damage over time.
Double-check and button it up
Once you've worked your way through the firing order—A, B, C and back again—it's a smart move to rotate the engine a full two times and double-check a couple of the cylinders. It only takes five minutes and it gives you peace of mind that nothing shifted while you were tightening locknuts.
Before putting the covers back on, take a look around. Make sure no tools are sitting in the head and that the pushrods are all seated correctly. Clean the mounting surfaces for the rocker covers thoroughly. Even a tiny bit of old gasket material can cause a persistent oil leak that will coat the side of your clean block in no time.
Install your new gaskets, bolt down the covers, and you're ready for the first start.
That first start-up
There's nothing quite like the sound of an 855 Cummins after a fresh valve adjustment. When you first fire it up, it might sound a little different than it did before—hopefully quieter and smoother. Let it get up to operating temperature and keep an eye out for any oil seeping from the gaskets.
A proper 855 cummins valve adjustment can really wake an old engine up. You'll notice better throttle response, and the engine just feels "happier" under load. It's one of those maintenance tasks that pays for itself in fuel savings and engine longevity. Plus, there's a certain satisfaction in knowing that the overhead is set perfectly and that old Big Cam is ready for another few thousand miles of hard work.
It might seem like a lot of steps the first time you do it, but keep your spec sheet handy, take your time with the feeler gauges, and you'll find it's one of the most rewarding jobs you can do on your truck.