Cummins ISX, QSK Oil Pump Technical Bulletin
Published on July 23, 2025
Over the last few weeks, we have discussed numerous items that we are supporting on the ISX / QSX Cummins platforms, and items required during engine overhaul/rebuild. So, oil pump questions have come up, and I would like to review them with you all.
The ISX has 2 distinctive oil pump designs. Although some intraplatform variants are available, the 2 distinct units can be identified by the type and style of the oil pump pressure regulator retention. In some cases, they are identified as Aluminum or Cast-Iron pumps.
Figure 1 has a regulator piston (Dog Bone) retention style known as the PINNED regulator plug pumps.
Figure 2 has a regulator piston (Dog Bone) retention style known as the THREAD-IN regulator plug pumps.
The 2 styles of oil pumps have separate installation processes that we will be reviewing TO ENSURE WE SUPPORT OUR CUSTOMERS AS A SOLUTIONS PROVIDER and can provide proper installation advice if requested.
Figure 1 reflects the oil pump version with the pinned regulator plug that retains the spring and dog bone piston/oil pressure regulator in place in the pump housing.
This pump must use shims upon installation
- Align the dowel pins on the new oil pump with the mounting holes in the cylinder block.
- Start with the shims that came on the original oil pump, measure them, and record your initial findings. Install the oil pump and torque the mounting hardware to 56 Nm (41 ft-lb).
- Using a dial indicator, check the gear backlash between the oil pump idler gear and the crankshaft gear. If the backlash is within the recommended specifications, continue the installation.
MINIMUM SPECIFICATION: 0.18mm (0.007 in)
NOMINAL SPECIFICATION: 0.32mm (0.013 in)
MAXIMUM SPECIFICATION: 0.46mm (0.018 in)
- If the backlash is outside of the recommended specifications, you must select the proper shims, based on your initial shim size, to calculate the backlash up to the recommended specifications.
Shims are available in SIX different thicknesses, and are identified by a color marked on the side of the shim:
Yellow: 0.35mm (0.014 in)
Black: 0.45mm (0.018 in)
Purple: 0.55mm (0.022 in)
Red: 0.65mm (0.026 in)
Blue: 0.75mm (0.030 in)
Green: 0.85mm (0.033 in)
- Install the required shim and re-check the gear backlash to be as close to nominal as possible without exceeding the Min and Max specifications.
Figure 2 reflects the oil pump version with the thread-in regulator plug that retains the spring and dog bone piston/oil pressure regulator in place in the pump housing.
This pump may or may not use shims upon installation
- Align the dowel pins on the new oil pump with the mounting holes in the cylinder block.\
- Install the oil pump and torque the mounting hardware to 56 Nm (41 ft-lb).
- Using a dial indicator, check the gear backlash between the oil pump idler gear and the crankshaft gear. If the backlash is within the recommended specifications, continue the installation.
MINIMUM SPECIFICATION: 0.08mm (0.003 in)
MAXIMUM SPECIFICATION: 0.57mm (0.022 in)
- The THREAD-IN regulator plug oil pump design should NOT NEED SHIMS. However, gear backlash MUST still be checked after installing the new oil pump and shimmed to be within the Min/Max specifications if necessary.
So, in closing, when we are selling an overhaul kit, recommending our customers to always replace the Oil Pump is a must! If our customer advises that they are experiencing low oil pressure at idle with a Cummins ISX engine, one of the potential causes could be a worn or malfunctioning oil pump. It might take a little more time up front to determine this, but you will be helping our customers to prevent further downtime and costs when they run into problems in the future by always recommending a pump replacement.
