Cummins/Onan G12 Engine

Tools: Torque wrench, ST-669 Tool, Wire gauge

1. Loosen the valve adjusting screw locknut and back off the screw one complete turn.
2. Use light finger pressure at the rocker lever to hold the crosshead in contact with the valve stem nearest the push tube.
3. Turn down the crosshead adjusting screw until it contacts the corresponding valve stem.
4. Torque the locknut to 34-41 N·m [25-30 ft-lb]; if ST-669 tool is used, tighten to 30-35 N·m [22-26 ft-lb]. Adjust both intake and exhaust valve crossheads.
5. Check clearance between crosshead and valve spring retainer with a wire gauge. Minimum .51 mm [.020 in.]; GTA19 minimum .64 mm [.025 in.].

⚠ Excessive turbo boost may cause an engine failure.

1. Refer to the RED tag attached to the controller specifying the factory setting for altitude and horsepower.
2. Make sure that the engine has been stopped.
3. Disconnect the hose located at the bottom of the controller.
4. Remove the end cap from the cylinder by pushing in and rotating in a counterclockwise direction.
5. Install or remove spacers between the cap and spring to achieve the proper setting (see site altitude Table #8-12).
6. Loosen the lockscrew on the Turbo Boost Controller butterfly lever and adjust to a fully opened position. Tighten lockscrew on lever.
7. Start the engine and record Turbo Boost at full load condition.
8. Repeat steps 6 and 7 every time a shim or spring is removed.

Tools: Feeler gauge, Torque wrench, ST-669 service tool

1. Loosen the valve adjusting locknut and back off the adjusting screw one full turn.
2. Use a feeler gauge of proper thickness for the valve being adjusted and for the specific engine model (see Table #15-03).
3. Insert the feeler gauge between the crosshead and rocker lever and turn the adjusting screw down until the lever just touches the feeler gauge.
4. Tighten the locknuts to 54-61 N·m [40-45 ft-lb]; if ST-669 service tool is used, tighten to 41-47 N·m [30-35 ft-lb].

⚠ DO NOT USE THE OUTER MEMBER of the Vibration Damper to bar the engine.

Tools: ST-747 Barring Tool

1. Bar the engine in the direction of rotation until the No. 1 VS mark appears on the accessory drive pulley (GTA19 marks are on the vibration damper).
2. In this position both intake and exhaust valves will be closed if the cylinder is in the firing position; if not, rotate the engine one complete revolution.
3. Continue to rotate the engine until the VS marks line up with the marker on the engine block or engine water pump capscrew (GTA28).
4. Adjust the crossheads first, then the valves, following the procedure in this section.
5. Continue turning the crankshaft in the direction of rotation, adjusting one cylinder at a time until all cylinders have been adjusted.
6. Two complete revolutions of the crankshaft are needed to adjust all intake and exhaust valves.

⚠ Verify that your batteries have a full charge before starting this check.

Tools: Compression gauge, Spark plug removal tool

1. Locate and shut off the gas fuel valve and tag the valve with a DO NOT OPEN tag.
2. Ground the ignition G lead to prevent any ignition spark.
3. Remove spark plug probes from the spark plug wells. Check for oil or water contamination and clean before continuing.
4. Remove the spark plugs from ALL cylinders using a proper spark plug removal tool.
5. Block the engine throttle wide open.
6. Install a compression gauge in #1 cylinder.
7. Crank the engine with the starter until the engine has rotated 6 to 8 revolutions; the gauge should be at a stable reading.
8. Record the reading in pounds per square inch. Release the gauge pressure and remove the gauge.
9. Repeat for each cylinder. All cylinder pressures should be within a 10 percent variance.

1. Set the engine with #1 cylinder at the desired firing point on the compression stroke and adjust timing per Table #9-09. On the GTA28 use #1 cylinder on the left bank.
2. Establish the rotation of the Altronic unit by examining the drive end of the unit.
3. Rotate the unit shaft until the red mark on the shaft lines up with the marks on the housing. The GTA28 uses the CCW mark and all other CNGE engines the CW mark.
4. Mount the unit on the engine keeping the two red lines together as close as possible.
5. If the two lines cannot be made to meet, pull the back cover assembly away approximately 2 in. and carefully tilt the top away, keeping the internal plug connected.
6. Rotate the distributor shaft until the marks line up, then reinstall the cover over the two dowel pins and engage the gears.
7. Securely tighten the four fastening screws.
8. Set final timing with the engine at operating speed, then tighten the two 3/8 in. bolts securing the unit to the engine.

1. Unscrew the spin-on filter from the filter head and discard the filter element.
2. Apply a light even coat of lubricating oil to the gasket sealing surface prior to installing the filter.
3. Position the filter to the filter head. Tighten by hand until the seal touches the filter head and then tighten one additional turn.
4. Run the engine, check for leaks, shut down the engine. Add oil as necessary to bring the oil level to the H mark on the dipstick.

⚠ Do not remove the radiator cap from a hot engine.

⚠ Mechanical overtightening can distort the threads or damage the filter head.

1. Remove and discard the filter. Clean the gasket surface.
2. Apply a light film of lubricating oil to the gasket sealing surface before installing the coolant filter.
3. Install the filter as specified by the filter manufacturer.
4. Open the shutoff valve(s), if equipped, and install the coolant system pressure cap.

⚠ Avoid direct contact of hot oil with your skin.

⚠ Fill the oil filters with clean lubricating oil to avoid lack of lubrication on start-up.

Tools: Oil Filter Wrench 3375049

1. Reduce the load and operate the engine until the water temperature reaches 60°C [140°F]. Shut off the engine.
2. Remove the oil immediately to make sure all the oil and suspended contaminants are removed.
3. Clean the area around the lubricating oil filter head. Remove the filter and clean the gasket surface of the filter head.
4. Fill the oil filters with clean lubricating oil.
5. Apply a light film of lubricating oil to the gasket sealing surface before installing the filters.
6. Install the filter as specified by the filter manufacturer.
7. Check and clean the oil drain plug threads and seal surface. Install and tighten the oil drain plug to 65 ft-lbs (90 N*m).
8. Fill the engine with clean oil to the correct level.

⚠ Clean the area of any lubricating oil spilled while priming or filling the crankcase.

⚠ Do not prime the engine lubricating system from the by-pass filter.

Tools: Hand or motor-driven priming pump

1. On turbocharged engines, remove oil inlet line from turbocharger and add 50 to 60 cc (2 to 3 oz.) clean oil to prelubricate bearing; reconnect supply line.
2. Fill the crankcase to the L (low) mark on the dipstick.
3. Remove the plug from the lubricating oil crossover passage (G855/GTA855), from the oil filter housing (GTA28), or from the front of the oil cooler housing (GTA19).
4. Connect a hand or motor-driven priming pump line from a source of clean lubricating oil to the plug boss in the housing.
5. Prime until a 207 kPa [30 psi] minimum pressure is obtained.
6. Check the oil reservoir on the oil lubricator if equipped with an air-gas starter.
7. Crank the engine at least 15 seconds with the ignition OFF while maintaining external oil pressure at a minimum of 103 kPa [15 psi].
8. Remove the external oil supply and replace the plug. Tighten to the proper torque value.
9. Fill the crankcase to the H (high) mark on the dipstick with oil meeting Section 17 specifications.

Tools: Spark plug socket

1. Inspect the spark plug seating area in the adapter and clean to insure a good seating surface.
2. Position the new spark plug gasket on the spark plug; insert the spark plug in the adapter and tighten to 28-30 ft-lb (3.18-4.15 k-gm) torque.

Tools: Extra long 13/16 in. deep-well rubber insert spark plug socket

1. Disconnect the spark plug wire at the coil.
2. Lift off the rubber cap with the extension.
3. Using an extra long 13/16 in. deep-well rubber insert spark plug socket, remove the spark plug from the adaptor. Lift out the spark plug and gasket.