Product Description
CUMMINS 4089950 OEM PISTON KIT (STANDARD) – ISB 5.9L 305HP (2003–2004)
The Cummins 4089950 OEM piston kit is the correct standard-bore piston and ring package for the 305HP ISB 5.9L common-rail engine used in 2003 High Output and early-2004 Federal-emissions Ram 2500/3500 trucks. This genuine Cummins kit is built to factory specs so you can restore compression, durability, and cold-start performance on stock rebuilds without guesswork.
- Genuine Cummins standard-bore piston kit for ISB/QSB 5.9 platforms using the 4089950 slug.
- Engineered for 305HP HO (2003) and early-2004 Federal-emissions 5.9L truck calibrations.
- Matched ring pack, wrist pin, and retainers for one cylinder—order six kits for a complete engine.
- Ideal for stock or near-stock builds where the block measures within STD bore specification.
- Backed by Cummins engineering and testing for long service life and consistent cylinder sealing.
Fitment Details
Primary Automotive Fitment (Ram Trucks)
- 2003 Dodge Ram 2500/3500 – 5.9L ISB common-rail, 305HP High Output (HO) only.
- Early 2004 Dodge Ram 2500/3500 – 5.9L ISB common-rail, 305HP Federal-emissions trucks (not for most California-emissions engines).
Industrial / Off-Highway & Other ISB/QSB Applications
- Used across select Tier 2 ISB/QSB 5.9 and some 3.9L engines—verify by ESN/CPL before ordering.
- Common on engines with CPLs that match the 305HP automotive configuration or equivalent industrial builds (consult Cummins QuickServe or your Cummins dealer for exact CPL coverage).
Important Compatibility Notes
- 2003 model year: only HO trucks are rated 305HP; Standard Output trucks use a different piston spec.
- Early 2004 Federal trucks (305HP) are typically correct; many 2004 California-emissions trucks require a different piston design—always verify by VIN/ESN.
- Always measure cylinder bore for taper and out-of-round; use STD only when the block cleans up within standard size.
What’s Included
- (1) Genuine Cummins piston – standard bore (P/N 4089950).
- (1) Wrist pin.
- (2) Pin retainers/locks.
- (1) Complete piston ring set matched to the piston and bore size.
Cross-Reference & Related Cummins Parts
Cummins and OE ID Cross-Refs (ID numbers only)
- Piston body / assembly cross-ref commonly associated: 4897512.
- Other catalog cross-ref IDs sometimes listed with this kit: 4089259, 2243672WR (Mahle reference; not a Cummins-boxed part).
Helpful Cummins Companion Parts on CPP Diesel (all genuine Cummins)
- Standard bore kit for the same 305HP engine (this product): CUMMINS 4089950 OEM PISTON KIT (STANDARD)
- .020" oversize piston kit for 305HP 5.9L: CUMMINS 4089976 OEM PISTON KIT (.020 OVER) – ISB 5.9L 305HP
- .040" oversize piston kit for 305HP 5.9L: CUMMINS 4089977 OEM PISTON KIT (.040 OVER) – ISB 5.9L 305HP
- .040" over piston kit for 2004.5–2007 ISB 5.9L: CUMMINS 4089671 OEM ENGINE PISTON KIT (.040 OVER) – RAM 5.9L
- Matching overbore ring set for 2003–2007 5.9L: CUMMINS 3802234 OEM PISTON RING SET (.040 OVER, 1.00MM) – RAM 5.9L
- Wrist pin bushing used in many 5.9L rods: CUMMINS 4891178 OEM CONNECTING ROD WRIST PIN BUSHING – RAM 5.9L
- Upper engine gasket kit for 2003–2007 ISB 5.9L: CUMMINS 4089819 OEM UPPER ENGINE GASKET KIT – ISB 5.9L
Installation Notes
- Measure each cylinder for taper, out-of-round, and size before deciding on STD vs oversize.
- Check ring end gaps in the actual bores and set according to Cummins service procedures.
- Verify piston orientation, bowl direction, and connecting-rod offset when assembling.
- Always confirm ESN/CPL and emissions family with Cummins or QuickServe before ordering.
Specifications for cummins 4089950
| Attribute | Value |
|---|---|
| Brand | Cummins (Genuine OEM) |
| Part Number | 4089950 |
| UPC | Not published by Cummins (barcode varies by packaging; verify from box label if required). |
| GTIN | Not specified by manufacturer in public catalog data. |
| Engine Compatibility | ISB 5.9L 24-valve common-rail (305HP HO/early Federal truck), select ISB/QSB 5.9/3.9 Tier 2 industrial/marine applications (verify by ESN/CPL). |
| Configuration | Piston kit with rings, wrist pin, and pin retainers (per-cylinder service kit). |
| Bore Size | Standard (STD) – factory cylinder size. |
| Kit Contents | (1) piston, (1) wrist pin, (2) pin retainers, (1) matched ring set. |
| Cross Reference IDs | Common related IDs: 4897512 (piston body), 4089259, 2243672WR (Mahle reference – not Cummins-boxed). |
| E-Codes | Not applicable – internal engine component without E-mark lighting or exterior equipment requirements. |
| Shipping Weight | Approx. 3.05 lb per kit (Cummins catalog reference; final ship weight may vary with packing). |
| Package Dimensions | Approx. 5.13" W × 4.88" H × 5.00" D (CPP Diesel shipping data). |
| Shipping Method | Ships from CPP Diesel via standard ground service; typically qualifies for free shipping in the contiguous U.S. |
| Country of Origin | Not specified by manufacturer (Cummins global sourcing; check box label for exact origin). |
| Prop 65 Warning | Automotive service parts can expose you to chemicals including engine exhaust and related compounds known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov. |
FAQ
Will this piston kit fit my 2003–2004 5.9L Cummins?
This kit is correct for 305HP ISB 5.9L engines—2003 HO trucks and most early-2004 Federal-emissions trucks. If you have a 2004 California-emissions truck or a Standard-Output rating, you may need a different piston. Always confirm the horsepower on the valve cover tag and check the 8th digit of the VIN before ordering.
How many Cummins 4089950 kits do I need for a full rebuild?
Each 4089950 kit services one cylinder. A 5.9L inline-six requires six kits for a complete set of pistons and rings.
What’s the difference between 4089950 and 4089976 / 4089977?
Part 4089950 is the standard-bore piston kit. Part 4089976 is .020" over (+0.50 mm), and 4089977 is .040" over (+1.00 mm) for blocks that have been bored and honed oversize. Use 4089950 only when your cylinders remain within standard size; choose the appropriate oversize kit if the block has been machined.
Does this Cummins kit include rings, pin, and locks?
Yes. The 4089950 kit is a complete per-cylinder package with the piston, matched ring pack, wrist pin, and pin retainers so you don’t have to source those parts separately.
Can I use this piston kit on ISB/QSB industrial or marine engines?
In many cases, yes—Cummins lists 4089950 for certain Tier 2 ISB/QSB 5.9 and 3.9L applications. Because industrial and marine CPLs vary widely, you must verify compatibility using the engine serial number (ESN) and CPL in Cummins QuickServe or with your Cummins dealer before ordering.
Do I need machine work when using the standard-bore piston?
You still need to measure and hone the cylinders, but if the bores clean up within standard dimensions and the taper/out-of-round are within spec, you can run the 4089950 standard-bore kit. If the block is worn beyond spec, plan on boring and using an oversize piston such as 4089976 (.020") or 4089977 (.040").
