Oddities that can ruin your day
By Steve Scott
Do you really want to own or rebuild Cummins ISX/QSX or X15 engines? If you’ve walked through a bone yard or scrap yard, you’ve likely seen these engines with holes through the side of the block.
But make no mistake, these are not “bad” engines. In fact, all the major Class 8 truck manufacturers offer ISX and X15 series engines in their heavy-duty and long-haul trucks. No doubt the old Cummins 855 series engines are legends in this industry, but the ISX platform is well on its way to earning their place in history books.
This article highlights some of the quirks — call them oddities if you will — that you will encounter in the dual and single cam ISX engines and the later X15 engines. So, find a comfortable spot, grab your favorite beverage, relax, and let’s dive into the details. Details may seem dull… until an engine fails.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2026EPQ1/#p=84
A Little Background
The ISX was originally introduced as the Signature 600. The earlier engines had non-drilled connecting rods, grooved topped cylinder liners, multiple piece head gaskets and articulated pistons. These older versions have mostly gone by the wayside, so we’ll leave them to the past. However, if you do find one of these early engines, it would be a good idea to research the updates and check part availability before investing in a rebuild.
Oddities (or just the strange and unusual)
Here are a few updates and changes you will find in today’s ISX and X15 engines:
Plug Them
It may seem odd, but before you lift the cylinder head off the engine, it’s a good idea to open the gasket kit and find these four plugs.
The OE has made several attempts to stress the importance of using these plugs to block debris from going into the oil ports at each corner of the cylinder block.
Of course, there are many more ports and openings to consider on an engine than where these plugs are used, and there are additional plugs used on the front and sides of the cylinder block.
Press Them, Don’t Beat Them
On dual-cam engines, the injector camshaft is massive compared to the valve camshaft with a journal diameter of approximately 85.0 mm (3.346”) compared to the journal diameter of approximately 65.0 mm (2.559”) of the valve cam. The injector cam weighs almost 60 lbs., versus 35 lbs. for the valve cam. While it may be common practice to drive the camshaft bushing in on other engines, the Cummins installation instructions specify that the cam bushings (injector and valve) are to be pressed in (not driven). Several cylinder head rebuilders have reported that they are convinced that due to the size and amount of retention (crush) holding the injector cam bushing in place, they cannot be driven in without distorting or damaging the bushings, which can cause a failure once the engine is put in service.
Lube, Lube, and More Lube
Common to most assembly instructions and practices, lubricating the camshaft bushings, journals, and pre-lubing an engine before startup is not new. In the dual cam engines, as mentioned, the injector camshaft is massive. It receives oil from the cylinder head at journals #1 and #7. Oil is then supplied to the remaining journals via the large oil port inside the camshaft. This internal oil port holds approximately 1.06 liters (36 oz.) fluid. That’s over a quart of oil in this camshaft alone! Considering the load that’s placed on these camshaft bushings and the time for pressurized oil to reach the bushings, this makes pre-lubing the engine even more critical.
Into the Heart of Things: Cylinder Liners
History of the liner updates
- 1998-2000: Engines used a grooved top liner and multiple piece head gasket.
- 2000-2010: The flat top liner (150mm OD) with no shim was released and a one-piece head gasket.
- 2010-2013: A 152mm flat top liner was introduced.**
- 2012-2015: A 150mm flat top liner with a replaceable shim was released.*
- 2015: A 150mm flat top liner with an Anti-Polishing Ring (APR) and replaceable shim was released.* (Note: there is also a 152mm liner with an APR.)
*Shimmed liners are backwards compatible with all flat top liners and can be mixed within an engine. The shims are replaceable but should not be reused.
**The introduction of a limited number of cylinder blocks released by Cummins in 2010 with a 152mm OD liner diameter was somewhat confusing and caught a few rebuilders off guard who installed a standard 150mm OD liner into the oversized block unknowingly. The same piston kits could be used in both the 150mm flat top liners (no shim and shimmed) and the 152mm liner, but the liners are not interchangeable.
This latest change involving the introduction of an APR also requires a piston with lower ring locations. This affects both the 150mm and 152mm cylinder block engines. The APR liner has a counter bore machined in the inside top of the liner to accept the APR. The APR ring is slightly smaller in diameter than the liner bore. The purpose of the APR ring is to scrape carbon off the side of the piston crown, thus limiting the amount of carbon that can build up on the piston and reduces liner wear and polishing. The APR ring is symmetrical and can be installed in either direction. A different ring compressor (or adapter) is required to install the piston assembly into these new design liners. While in the engine, APR liners can be identified by the two bands machined around the bottom of the liner.
Mixing an APR liner with non-APR liners within an engine is not recommended.
Worn or Worn Out?
Inspect the condition of the cylinder block before installing liners. The earlier liner designs did not have a replaceable shim, and the liner sat directly on the counter bore ledge of the cylinder block. Those designs were more susceptible to fretting and wear of the counter bore surface, but with each liner design, the condition of the counter bore is critical to the life of the engine. The OE service publication requires the counter bore depth (the distance from the surface of the block deck to the counter bore ledge) be measured in four locations, and the variance between those four measurements cannot be greater than 0.025mm (0.001”). The ledge must be flat (level) within 0.0127mm (0.0005”). The condition of the surface of the counter bore ledge is also very important. Machining lines should be visible all the way across and around the entire circumference of the ledge. There should not be any areas of fretting or erosion across the surface, as shown below. This ledge (and the dimensions of the liner) is responsible for achieving the correct liner protrusion and correctly supporting the liner.
The upper receiver bore of the cylinder block needs to be inspected as well. This seems to be an area the OE overlooked completely. The OE service manuals do a good job of detailing the acceptable conditions and dimensions for the lower counterbore in the ISX cylinder block and the required liner protrusion specs. However, it falls short of addressing wear on the upper receiver bore of the block that supports the top of the liner. The only reference we found is the simple note (shown above) taken from Cummins “Inspect for Reuse” guide that focuses on erosion. However, erosion and wear are two completely different conditions.
The problem occurs when the upper bore diameter of the block gets worn to the point where it cannot hold the liner from moving. The liner movement then causes the head gasket to fret into the surface of the cylinder head and the head gasket to fail.
The photos shown below are examples of the wear found around the circumference in the upper receiver bores of a high mileage ISX block. Several machine shops and rebuilders we have talked to about this condition have said this is becoming a more common occurrence. If severe enough, instead of replacing the cylinder block per the OE instructions, they make a custom repair sleeve or insert to repair the damaged area. When they install a repair sleeve, they tend to use a finish bore size of 6.339″ to 6.340″. A worn diameter of 6.344″ tends to be the max diameter for the block according to the rebuilders we spoke with before they fit the block with a repair sleeve. Cummins does not list a max diameter spec for this upper bore of the block.
Deeper into the Heart: Pistons and Rods
Pistons
Along with the changes in the liners, pistons have had a variety of updates over the years. ISX pistons have evolved from open to closed skirts (as shown in the picture below), and from non-APR to APR designs. These must be matched correctly with their corresponding liners. APR pistons cannot be used in non-APR liners and vice versa. These designs are not interchangeable and should not be mixed in an engine. Due to the location of the rings, Non-APR and APR pistons must be used with the appropriate Non-APR or APR liner.
Connecting Rods
Connecting rods are the next headache on the list. Apart from the early non-drilled rods, the two most common connecting rod designs used today in the ISX and X15 engines are “drilled “rods that are either “saw cut” or “fractured”. These terms describe the surfaces between the connecting rod and rod cap. Drilled rods have an oil passage that supplies oil from the rod bearing (big end) up to the rod bushing (small end).
“Saw cut” rods have been used for years, and the parting surfaces of the rod and cap are machined. “Fractured” rods are the newer technology. These rods are forged as one piece, and then the cap section is fractured from the larger rod section. Fracturing the sections leaves uniquely matched rough surface between the rod and cap.
Fractured connecting rods require a different rod bearing (big end) than the saw cut rod. “Fractured” and “saw cut” connecting rods can be mixed in an engine, so it’s possible that you may use some of each design bearing within one engine. Saw cut versus. fractured rods can also be identified by the design of the cap. The cap from a fractured rod has a hole and a saw cut cap does not. The fractured rod introduced a few “oddities” of its own. How the rod and cap are handled, cleaned and assembled is critical. The cap and rod are serialized to identify their orientation and keep them paired together. If the cap is mistakenly placed on the rod backwards or on another rod, or if either is dropped/impacted on the fractured surface, the rod must be replaced. The surfaces cannot be rubbed together or even slightly damaged. Refer to the OE service bulletins for more details regarding handling and cleaning this type of connecting rod.
Bearings
Another oddity, and possibly the most critical is cut versus fractured connecting rods bearings. Typically, the connecting rod bearing shells either have identical profiles, and can be installed in either the rod or cap, or they have an upper and a lower shell, where the locating tabs are spaced widely apart, assuring they can only be installed in the correct position. This is not so with the cut ISX and X15 rod bearings. The locating tabs are so closely positioned that they can accidentally be installed in the wrong position. Installing them incorrectly is noticeable, but the misalignment does allow enough clearance that the connecting rod can still be bolted in place on the crankshaft. However, this is a guaranteed failure as it blocks the oil passage in the rod from providing oil to the rod bushing (small end) and piston pin.
Introduction of the fractured rod and rod bearings overcame the positioning problem by centering the middle tabs and adding a notch and tab to the corners of the rod and bearing shells.
Unfortunately, this created the possibility of yet another problem. A cut rod bearing can be incorrectly installed in a fractured rod. The ID of both the cut and fractured rod bearings are the same however, a fractured connecting rod has a 0.032″ larger bore for the rod bearing than a saw-cut rod. The fractured connecting rod bore spec is 3.9221″ to 3.9236″. The saw cut connecting rod bore spec is 3.890″ to 3.891″. The free length of a cut rod bearing is just enough to lightly hold it in the fracture rod during assembly, but as soon as the engine starts, the 0.032″ added clearance allows the bearings to spin and the engine is doomed.
Crankshaft Thrust Bearings
All versions of ISX, ISX15, X15 and QSX15 engines
There are three main bearing sets used in these engines. While the main bearings themselves are the same across all sets, the thrust bearings (plates/washers) differ. The correct main bearing set is determined by the specific combination of the cylinder blocks and crankshafts. The engine serial number can identify the cylinder block, crankshaft and main bearing set originally installed at the time of manufacture. However, if the engine has been altered since its original release, the actual cylinder block and crankshaft numbers are required to identify the correct main bearing set or thrust bearings.
A chart showing the compatible component options and a chart showing the service part number for the cylinder blocks, crankshafts and main bearings can be found on the following page.
So, What Causes the Holes?
Let’s go back to where we started as to why so many of the Cummins ISX and X15 engines end up with a hole through the side of the block. That answer lays in the design of the cylinder block and each of the liner designs. ISX and X15 engines use a “mid-stop” liner, meaning the liner is held at the top and mid-way down on the liner. This design leaves the lower section of the liner unsupported by the cylinder block.
The OE and most liners in heavy-duty engines are made from cast iron. But when something goes wrong, that restricts the free movement of the piston and connecting rod, or if the cylinder begins to seize, the cast iron mid-stop liners in an ISX and X15 engine cannot withstand the sideload forces and at the bottom of the stroke causes the liner to break below the mid-stop. Then the piston and rod can no longer stay on their correct axis and are thrown out the side of the block. Arguably a steel liner option will endure these forces longer, but neither material can withstand these abnormal conditions indefinitely.
Embracing the Oddities
ISX and X15 are powerful, proven engines that will continue to dominate the heavy-duty long-haul industries for years to come. But like any powerhouse, they come with their quirks. Understanding these “oddities” and paying close attention to details during rebuilds, can be the difference between a reliable engine and a catastrophic failure. Hopefully these tips will give you more confidence when dealing with these engines and help you stop failures before they start.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2026EPQ1/#p=84