Product Description
CUMMINS 3955121 OEM ROCKER LEVER HOUSING (ROCKER BOX) – ISB 5.9L (2003–2005)
The Cummins 3955121 OEM rocker lever housing (rocker box / valve cover spacer) is the factory aluminum housing that sits between the cylinder head and the upper valve cover on 24-valve ISB engines. This is a genuine Cummins service part, not aftermarket, ensuring proper geometry for rocker arms, injector wiring clearance, and long-term sealing on your 2003–2005 Ram 5.9L Cummins ISB as well as select ISB/QSB 6.7 industrial applications (verify by ESN for non-automotive use).
If your 5.9L Cummins has oil seepage around the rocker box, stripped threads, or impact damage from past repairs, replacing the housing with a new OE casting is the right way to restore factory reliability and avoid repeat leaks or valvetrain issues.
Fitment – Where 3955121 Is Used
- 2003–2005 Dodge Ram 2500/3500 with 5.9L ISB common-rail Cummins (automatic & manual).
- Used on many ISB/QSB 6.7 Tier 3 industrial/off-highway engines – always verify by Engine Serial Number (ESN) before ordering.
Not typically correct for:
- 2006–2007 5.9L Ram trucks (transition years with different castings).
- Most 2007.5+ 6.7L Ram pickups which commonly use other rocker housings (e.g., 3974981 / 3975944 – verify by VIN/ESN).
Always confirm by VIN/ESN for trucks and by full ESN/application data on industrial equipment.
Why Choose Genuine Cummins 3955121 From CPP Diesel?
- OE geometry & machining: Maintains proper rocker arm and injector alignment compared to unknown reman or aftermarket cores.
- Leak control: Clean sealing surfaces for the cylinder head and rocker box gasket help cure top-end oil leaks when paired with fresh gaskets.
- Drop-in replacement: Direct bolt-in for the original housing – no fabrication or modification required for listed applications.
- Backed by Cummins: Genuine part from the Cummins distribution channel, not a generic copy.
Recommended Cummins Companion Parts
- Rocker lever housing (rocker box) gasket: CUMMINS 3963379 OEM ROCKER LEVER COVER GASKET – 5.9L/6.7L
- Upper valve cover gasket: CUMMINS 3954324 OEM VALVE COVER GASKET – 5.9L (2003–2005)
- CCV crankcase breather filter: CUMMINS 3957987 OEM CRANKCASE BREATHER FILTER – ISB 5.9L (2003–2005)
- Rocker lever assemblies: CUMMINS 4995602 OEM ROCKER LEVER ASSEMBLY – 5.9L/6.7L (1998.5–2018)
- Valves & top-end hardware:
- Cooling support for 5.9 top-end work: CUMMINS 5292744 OEM 190° THERMOSTAT – 5.9L ISB (2003–2007)
These direct links take you to live Cummins product pages on CPPDiesel.com, making it easy to build a complete top-end refresh kit around your new 3955121 rocker housing.
What’s Included
- (1) Genuine Cummins 3955121 rocker lever housing (rocker box / valve cover spacer).
- Gaskets, breather filter, rocker levers, and hardware are sold separately (see recommended parts above).
Installation Notes
- Remove upper valve cover and old rocker box; clean all sealing surfaces on the cylinder head.
- Install a new rocker box gasket (3963379) and valve cover gasket (3954324) during reassembly to prevent repeat leaks.
- Torque all fasteners to factory specifications in the recommended sequence.
- Re-route injector harness and CCV plumbing as originally installed and check for oil leaks after the first heat cycle.
Specifications for cummins 3955121
| Brand | Cummins (Genuine OEM) |
| Part Number | 3955121 (also referenced as CUM3955121) |
| Description | Rocker lever housing (rocker box / valve cover spacer) for ISB platform |
| Engine Platforms | 5.9L ISB common-rail (2003–2005 Ram 2500/3500); select ISB/QSB 6.7 industrial engines (verify ESN) |
| Material | Cast aluminum |
| Quantity Required | 1 per engine |
| Cross Reference | 3955121, CUM3955121, 707145AT, associated casting variant 3955120 |
| UPC | Not published by Cummins (service part) |
| GTIN | Not published by Cummins (service part) |
| E-Codes / Emissions Notes | Primarily Tier 3 / EPA-compliant ISB/QSB applications; verify by ESN for industrial/off-highway engines |
| Approx. Shipping Weight | ~9.8 lbs (packaged) |
| Approx. Package Dimensions (L × W × H) | 31.0 in × 10.9 in × 9.6 in (shipping carton) |
| Shipping Info | Ships from CPP Diesel; typically ground service with free shipping in the continental U.S. (see cart for options). |
| Country of Origin | As marked on Cummins packaging (USA or global Cummins supply chain) |
| Prop 65 Warning | This product can expose you to chemicals including engine oil and related compounds, which are 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 fit my 2003–2005 5.9L Cummins Ram?
Yes. Cummins 3955121 is the standard rocker lever housing for most 2003–2005 Ram 2500/3500 trucks equipped with the 5.9L ISB common-rail diesel. Always verify by VIN/ESN if your truck has had major top-end work or engine swaps.
Does 3955121 also work on 6.7L Cummins engines?
This housing is used on some Tier 3 ISB/QSB 6.7 industrial engines, but most 6.7L Ram pickups use different rocker housings. For 6.7L automotive applications, confirm the correct part number by VIN/ESN before ordering.
Is this a genuine Cummins part or aftermarket?
This is a genuine Cummins OEM rocker lever housing sourced through Cummins distribution, not an aftermarket or reman core. The casting, machining, and material spec match what was installed on the engine from the factory.
What gaskets do I need when replacing the rocker lever housing?
At minimum, install a new rocker box gasket (Cummins 3963379) and upper valve cover gasket (Cummins 3954324). Many owners also replace the crankcase breather filter (Cummins 3957987) while the valve cover is off.
Do I need to replace my rocker arms when installing a new housing?
Not always. If your rocker arms show normal wear and no looseness in the bearings, you can reuse them in the new housing. However, if you see scoring, excessive lash, or spalling, upgrading to new OE rocker lever assemblies like Cummins 4995602 is cheap insurance against future valvetrain noise or failures.
