Artie’s Answers: Adjusting The Valves On 24-Valve Cummins Diesel

There are a number of people, even non-diesel people, that portray a diesel engine as unbreakable. That isn’t the case; they still need to be maintained to keep chugging along. All aspects of maintenance are necessary including fuel filters, air filters, and frequent oil changes. While those are all exterior parts, what about the interior maintenance that needs to be done?

For example, valve lash is a part of maintenance that should be checked at least every 100,000 miles. Although 100,000 miles is traditional, there are many people who perform this operation sooner. Over time, it’s not unheard of to be out of adjustment after only 75,000 miles, so taking a half-hour to do a check-up isn’t a bad idea. Unfortunately, we were too late for this particular 5.9-liter Cummins, but now that it’s coming back together, we figured we might as well share how the process is done.

Our test subject was a local pickup that had recently blown up. After a new engine was built for the truck, it started driving for a few thousand miles. Just for assurance purposes, we wanted to dive in and give everything a check.

Figuring out when it’s time to adjust the valves before you reach a certain mileage mark is based on three things: the sounds they make, reduced power, or diminished fuel mileage. If you find these symptoms all at once, it’s time to take a peek.

The procedure normally takes about 30 minutes to an hour, depending on how quickly you’re working on it. At a shop, valve adjustment could cost a good amount, but we feel this is a simple enough task that it can be done on your own. In any case, it’s a good learning experience and good knowledge for people to use and share with others, while also saving money.

In order to access the valvetrain, all that needs removing are the valve cover and valve cover rocker box.

On a Cummins engine, there are mating cylinders that meet at top dead center simultaneously. While the cylinders will reach TDC at the same time, they are on opposite strokes: compression or power. Tooling the engine over to TDC is much easier done with a second person, but once the engine reaches TDC, the harmonic balancer will be labeled or marked TDC, pointing at 12:00.

With our battery disconnected, we removed our valve cover, exposing the overhead of the engine. We then detached and unplugged the rocker box that hooks to the fuel injectors. With nothing in the way of our workspace, we began to turn the engine to top dead center for valve adjustment.

The Allen wrench adjusts the lash. The wrench tightens it down.

Once the engine is set to top dead center, the number one cylinder rocker levers should be loose; if they aren’t, you haven’t reached TDC. So, having reached 12:00, the valve lash could now be measured at the Intake 1-2-4 and Exhaust 1-3-5. By using a feeler gauge, we gauged the gap between the rocker arm socket and the crosshead.

Now, we mentioned that this job isn’t necessarily hard, but keep in mind that the rear two cylinders are tough to get to. As long as you have patience, you can get it done.

A rule of thumb for an adjustment is 0.254mm (0.010 inches) on the intake rockers and 0.660mm (0.026 inches) on the exhaust rockers. You’ll check the first few rockers in the sequence. If there are adjustments that need to be made, you will loosen the lock nut on the rocker arm and turn the adjusting screw until the desired lash is in place.

With the valve adjustment made, you tighten the lock nut to 24 ft-lbs and then recheck the lash. Sometimes with the tightening of the lock nut, you can accidentally maneuver its placement, so be careful.

Now that we had all of the first sequenced rockers adjusted, that only left our intake 3-5-6 and exhaust 2-4-6. Just like before, you can turn the engine over 360º to align the harmonic balancer to TDC once more.

Fortunately, the truck we worked on was 100-percent in check. Since the engine had failed initially, a new engine was built and installed. Again, it’s just good practice to think about your valve adjustments, for as simple as it is.

With the valves adjusted, you should experience a smoother-running and better-performing engine.

Using the same method as before, we checked all rockers in the next sequence, determining that no adjustments were needed. Once you’re confident that all rockers are torqued properly and the adjustments have been made, you can replace your valve cover rocker box and valve cover, and then reconnect the battery to start your engine.

Make sure you properly installed the valve cover and check for leaks. Also, check for a smooth or smoother-running engine as you drive your truck. What are your thoughts on an at-home wrench experience? Do you think this was valuable information for future reference? Let us know in the comments below.

About the author

Artie Maupin

Artie Maupin is from Southeast Missouri and has an extreme passion for anything diesel. He loves drag racing of all kinds, as well as sled pulling competitions.
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