The D51844 is a framing nailer capable of driving up to a 3 ½ inch long full round common nail. As with most air nailers, when properly maintained DeWalt framing nailers will give years of trouble-free service. The most common issues, other than simple troubleshooting, are caused by one of two parts, the driver cylinder or the trigger valve. Both of these units are sealed with rubber O rings that control the release of pressure from the nailer. Worn O rings or a damaged valve or driver cylinder can cause erratic firing, or cause the gun not to function at all. You can buy both the O rings and the entire assembly for both parts from DeWalt.
Disconnect the gun from the air hose. Locate the roll pin directly above the trigger. Lay the gun on its left side and use a small metal punch and ballpeen hammer to drive the roll pin out, until the trigger mechanism is loose. Remove the trigger to expose the front of the trigger valve.
Pull the valve assembly from the nailer's handle, pulling toward the tip of the gun. Inspect the valve and O rings, looking for wear on the rings and cracks or splitting in the plastic valve. If the valve appears undamaged and the trigger has been easily returning to its original position after firing, replace only the O rings. If there is apparent damage to the valve, or the trigger has malfunctioned, replace the entire assembly.
Fit new O rings onto the valve body and apply oil to the rings with a cotton swab, or apply oil to the rings on the new valve, depending on your repair option. Fit the valve back into the nailer handle, aligning the grooves on the valve with the opening for a proper fit. Press the valve all the way in.
Fit the trigger back in place and use the metal punch and hammer to reset the roll pin, with gentle taps, to move the pin slowly to avoid bending the pin or damaging the head. Reconnect the air hose and return the tool to normal use.
Disconnect the nailer from the air hose. Remove the four screws holding the square vent to the back of the gun. Use an Allen key and turn the screws counterclockwise to remove them. Lift the cover from the rear of the gun.
Push a thin metal rod through the fastener port in the tip of the gun to push the driver cylinder out the back of the gun. Inspect the cylinder and O rings as previously outlined for the trigger valve. Inspect the metal driver as well, looking for bends, grooves or rough edges. Replace the O rings, the O rings and cylinder or the O rings cylinder and driver, depending on the damage you find.
Insert the cylinder into the gun. Align the notches to the nailer body for correct fit and press the cylinder all the way home. Reinstall the rear vent and install the four mounting screws, tightening them with the Allen key. Reattach the air hose and return the gun to normal use.