The Difference Between Pink and Blue Orings in a Cummins ISX

Understanding 150mm vs 152mm Cylinder Liners (Before It Becomes an Expensive Mistake)

If you work on Cummins ISX engines, there’s one detail that can’t be overlooked during an in-frame or overhaul: the color of the cylinder liner O-ring.

That small pink or blue O-ring tells a big story — and choosing the wrong one can lead to coolant leaks, liner fitment issues, and costly rework.

Let’s break it down simply.

Why the Cummins ISX Has Two Different Liners

Cummins ISX 15L engines were produced with two different cylinder liner outside diameters:

• 150mm liners

• 152mm liners

The difference comes from changes in cylinder block bore size, especially on engines built or rebuilt after January 1, 2010. Some blocks were machined with a smaller bore, others with a larger bore — and the liners must match the block exactly.

This is where the O-ring color becomes critical.

The O-Ring Color Tells You the Liner Size

Cummins made identification easier by using different O-ring colors on the mid-stop liner seal.

🔵 Blue O-Ring = 152mm Liner

• Used in large bore cylinder blocks

• Larger outer diameter at the liner mid-stop

• Must be matched with a 152mm block bore

🌸 Pink O-Ring = 150mm Liner

• Used in small bore cylinder blocks

• Smaller outer diameter at the liner mid-stop

• Must be matched with a 150mm block bore

⚠️ Important:
The O-ring color is not cosmetic — it directly corresponds to the liner’s outside diameter.

Why This Difference Matters (A Lot)

Installing the wrong liner size can cause:

• ❌ Coolant leaks at the liner seat

• ❌ Improper liner protrusion

• ❌ O-ring damage during installation

• ❌ Repeat teardown and lost labor time

Even 2mm makes a major difference in sealing and long-term reliability.

How to Identify the Correct Liner

Before ordering liners or an in-frame kit, you should confirm one of the following:

Option 1: Check the O-Ring Color

If the engine is already apart:

• Blue O-ring → 152mm liner

• Pink O-ring → 150mm liner

Option 2: Measure the Mid-Stop Diameter

Cummins recommends measuring the liner mid-stop at the specified location using:

• Cylinder liner gauge: 5299587

This ensures correct identification, especially on engines that may have been rebuilt before.

Bottom Line: Don’t Guess — Verify

When it comes to Cummins ISX cylinder liners:

• There are two liner sizes

• The O-ring color matters

• Guessing can cost thousands

Before ordering parts, always confirm:
✔ O-ring color
✔ Liner size (150mm vs 152mm)
✔ Block compatibility

If you’re unsure, providing the O-ring color or liner part number is often enough to get the correct parts the first time.

Need Help Identifying the Right ISX Liners?

Let us know:

• The O-ring color

• Or the liner part number

• Or the engine serial number (ESN)

We’ll make sure you get the correct liners — no guesswork, no delays, no rework.