Cummins 4089950 — Genuine Standard-Bore Piston & Ring Kit for ISB 5.9L 305HP

MPN: 4089950  |  Also Known As: piston & ring kit, standard bore piston, HO 305HP piston  |  Cross-Refs (ID only): 4897512, 2243672WR

Genuine Cummins standard-bore piston kit engineered for the 5.9L ISB 305HP common-rail configuration. Includes matched rings and wrist pin hardware. Sold individually; 6 required per engine.

Features & Benefits

• OE piston/bowl design for correct compression and combustion on 305HP calibrations.
• Matched ring pack for proper sealing and controlled oil consumption.
• Direct-fit standard bore—ideal for stock rebuilds where cylinders measure within spec.
• Kit convenience: piston, rings, pin, and retainers in one box.
• Backed by Cummins quality for durability and performance.

Verified Fitment (Automotive)

Industrial/Off-Highway: Also found across select ISB/QSB 5.9 (and related platforms) and used primarily on certain Tier-2 builds; verify by ESN/CPL before ordering.

Specifications

• Type: Standard-bore piston kit (piston, rings, pin, retainers)
• OE Part Number: 4089950
• Bore Size: Standard (STD)
• Quantity Required: 6 per engine (sold individually)

Often Paired With

• Rod & main bearing sets (match to crank condition/undersize)
• Full gasket set & head bolts (single-use torque-to-yield where applicable)
• Ring compressor & bore gauges for accurate assembly and measurements

What’s In The Box

• (1) Genuine Cummins Standard-Bore Piston (4089950)
• (1) Wrist pin + (2) pin retainers
• (1) Complete piston ring set (matched)

Installation Tips

• Measure cylinder bore (taper/out-of-round) and deck protrusion; hone/boring as required. Use STD only if within spec.
• Check ring end-gaps and orientation per Cummins service procedures.
• Verify piston/bowl orientation and connecting-rod offset.
• Always confirm ESN/CPL and emissions family before ordering/installing.

FAQ

WILL THIS FIT MY 2003–2004 5.9L 305HP RAM?

Yes—this kit targets the 305HP ISB common-rail configuration (2003 HO and early 2004 Federal 305HP). California-emissions 2004 trucks use different pistons; verify by VIN/engine tag.

HOW MANY DO I NEED?

Six kits for a full engine (sold individually).

WHAT DOES “STANDARD BORE” MEAN?

It fits cylinders at OEM standard diameter. If your block is bored oversize, choose the matching oversize pistons instead.

WHAT’S INCLUDED IN THE KIT?

Piston, wrist pin, pin retainers, and a matched ring set.

DOES THIS WORK ON INDUSTRIAL ISB/QSB ENGINES?

Often, yes—this kit appears across select ISB/QSB 5.9 (and some 3.9) Tier-2 builds. Always confirm by ESN/CPL.

HOW DO I VERIFY HO VS SO?

Check the engine tag for horsepower and the 8th digit of the VIN; HO trucks correspond to the 305HP configuration.