Photo 2/14 | Cummins Diesel Engine | As you can see by the dirt and oil leaks, we figured it was probably a good time to check the valve lash on this 200,000-mile Cummins, as we’re sure it had been a while.
Photo 3/14 | Cummins Valve Cover Removal | First, we need access to the lower valve cover, which means taking off the oil filler cap and removing the top valve cover.
Photo 4/14 | Lower Valve Cover Removed | Once the shorter upper valve cover is removed, the lower one-piece cover that actually protects the rocker arms is also taken off. In addition to removing the bolts, a few hoses must also be disconnected so the valve cover can be completely removed.
Photo 5/14 | Rear Vlave Cover Bolts | If you’ve gotten this far, and you’re wondering why your valve cover won’t come off, you’ve probably forgotten the rear bolts. There are bolts under the cowl of the truck at the very rear of the engine that also need to be removed before the valve cover can be lifted off.
Photo 6/14 | Valve Cover Removal | With all the hoses and bolts disconnected, the valvetrain itself is visible. Now it’s time to turn the engine over, in order to find overlap on the front and rear rocker arm assemblies.
Photo 7/14 | Turning Over Engine With Alternator | The easiest way to turn the engine over is to spin it backward using a 24mm socket and the alternator. If this is impossible due to a worn-out belt, you’ll have to use a Cummins barring tool and turn the engine over using the teeth on the flexplate instead. It should be noted that 12-valve and VP44-pump 24-valve engines will need a 22mm socket instead of a 24mm.
Photo 8/14 | Exhaust Valve | If you’re unsure about which rocker is on the intake side and which is on the exhaust, now is a good time to get your bearings. As viewed from the side, the exhaust-side rocker will be right above the exhaust port, at about a 10-degree angle. Also notice that there’s only one rocker for every two valves, as the rockers use a bridge to open and close two valves at once.
Photo 9/14 | Finding Overlap | The next step is to find overlap so the lash can be taken care of. Since the engine is turning over backward, the intake rocker will be closing, and right before the top of its stroke, the exhaust rocker will start to move. As soon as the exhaust rocker moves, stop turning, as the cylinder (in this case #1) will be on overlap. To double-check overlap, grab both rockers and try to pull up and down. If there is any space at all, the rockers aren’t on overlap. Since both the intake and exhaust valve are open at the same time, there should be zero lash on both sides.
Photo 10/14 | Checking Alternating Cylinders | With #1 on overlap, it’s time to go through the sequence of adjustment and check the intake and exhaust sides on alternating cylinders. That’s the #2 exhaust, #3 intake, #4 exhaust, #5 intake, and both on #6. When checking lash, check between the tip of the rocker arm and the valve bridge. Lash should be 0.010 inch on the intake side, and 0.020 inch on the exhaust, although a 0.004-0.006-inch deviation is considered acceptable. Valves that are “off,” can read 0.040 inch or more on a feeler gauge.
Photo 11/14 | Adjusting Valve | If you do need to adjust the valves (in our case, there were three out of 12 that were off) then you can do so by loosening the lock nut on the rocker with a 14mm wrench and using a 5mm Allen wrench to tighten or loosen the lash. Once the center Allen screw is in its proper place, make sure to tighten the lock nut back down with 18 ft-lb of torque.
Photo 12/14 | Checking Valve Lash With Feeler Gauge | Remember that when using the overlap method, the engine will need to be turned over a second time, until the #6 cylinder is on overlap. After a few more cranks with the alternator, it was time to check the lash on both valves on #1, the #2 intake, #3 exhaust, #4 intake, and #5 exhaust. At this point, our last two intake and exhaust cylinders on #1 checked within specs (remember to use different-sized feeler gauges), and we were ready to button things back up.
Photo 13/14 | Lower Valve Cover Install | The lower valve cover went on first, and we were even able to use the same gasket. Remember to connect the hoses to the breather back up at this point, and also don’t forget the rear valve cover bolts.
Photo 14/14 | Upper Valve Cover Install | With the bottom cover on, all that’s left is reinstalling the top valve cover. An experienced diesel technician can knock this job out in about 45 minutes. Now that our Cummins’ valve adjustment is handled, we’re set for a good number of years before we’ll have to check the lash again.