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, engineered to maintain proper rocker arm geometry, injector harness clearance, and long-term oil sealing on your 2003–2005 Ram 5.9L ISB, as well as select ISB/QSB 6.7 industrial applications (verify non-automotive usage by ESN).
If your 5.9L Cummins has oil seepage around the rocker box, stripped threads, or casting damage from previous repairs, replacing the housing with a new OE Cummins casting is the right way to restore top-end reliability and prevent repeat leaks or valvetrain issues.
Fitment – Where 3955121 Is Used
- 2003–2005 Dodge Ram 2500/3500 with 5.9L ISB common-rail Cummins (24V), all cab and transmission configurations.
- Also used on select Tier 3 ISB/QSB 6.7 industrial/off-highway engines – always verify with the full Engine Serial Number (ESN) before ordering.
Typically NOT correct for:
- Most 2006–2007 5.9L Ram trucks (different rocker housing casting in many cases).
- Most 2007.5+ 6.7L Ram pickups (commonly use other rocker housings). Always confirm by VIN/ESN.
For industrial, agricultural, construction, or generator sets, confirm usage by ESN in Cummins QuickServe or with your engine tag before purchase.
Why Buy Genuine Cummins 3955121 From CPP Diesel?
- OE casting and machining: Correct rocker arm and injector alignment compared to off-brand housings or welded repairs.
- Better leak control: Clean, flat sealing surfaces help cure recurring top-end oil leaks when paired with fresh Cummins gaskets.
- Direct bolt-in: No modifications needed on listed ISB applications – simply swap over your existing rocker assemblies and hardware.
- Backed by Cummins: Genuine Cummins part number 3955121, supplied through the Cummins distribution channel.
Recommended Cummins Companion Parts
- Rocker box gasket (head-to-housing): CUMMINS 3963379 OEM ROCKER LEVER COVER GASKET – 5.9L/6.7L
- Upper valve cover gasket: CUMMINS 3954324 OEM VALVE COVER GASKET – RAM 5.9L (2003–2005)
- CCV crankcase breather filter: CUMMINS 3957987 OEM CRANKCASE BREATHER FILTER – ISB 5.9L (2003–2005)
- Rocker lever assemblies (if your rockers are worn): CUMMINS 4995602 OEM ROCKER LEVER ASSEMBLY – 5.9L/6.7L
- Top-end valve components:
- Cooling support while the top end is apart: CUMMINS 5292744 OEM 190° THERMOSTAT – 5.9L ISB (2003–2007)
All links above go to live genuine Cummins or Cummins-related product pages on CPPDiesel.com, making it easy to build a complete top-end refresh kit.
What’s Included
- (1) Genuine Cummins 3955121 rocker lever housing (rocker box / valve cover spacer).
- Gaskets, breather filter, rocker levers, bolts, and other hardware are sold separately (see recommended parts above).
Installation Notes
- Remove the upper valve cover and old rocker box; clean all sealing surfaces on the cylinder head.
- Install a new rocker box gasket (3963379) and upper valve cover gasket (3954324) during reassembly to prevent repeat leaks.
- Torque all fasteners to factory Cummins specifications in the recommended sequence.
- Reinstall injector harness, CCV components, and breather filter; check for oil leaks after the first heat cycle.
Specifications for cummins 3955121
| Brand | Cummins – Genuine OEM |
| Part Number | 3955121 (SKU: 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 (ESN verification required) |
| Material | Cast aluminum |
| Quantity Per Engine | 1 |
| Cross Reference | 3955121, CUM3955121, related casting 3955120 |
| UPC | Not published by Cummins |
| GTIN | Not published by Cummins |
| E-Codes / Emissions Notes | Used primarily on Tier 3 ISB/QSB applications; verify local emissions compliance where required |
| Approx. Shipping Weight | ~9.2–9.8 lbs (packaged) |
| Approx. Package Dimensions (L × W × H) | Approx. 31.0 in × 10.9 in × 9.6 in |
| 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 (varies by production batch) |
| 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 listed for most 2003–2005 Ram 2500/3500 trucks equipped with the 5.9L ISB common-rail diesel. Always verify by VIN/ESN if your engine has been replaced or modified.
Does this rocker box come with gaskets?
No. Gaskets are sold separately. For best results, pair this housing with a new rocker box gasket ( Cummins 3963379) and valve cover gasket ( Cummins 3954324).
Will this work on my 6.7L Cummins?
This part number is primarily used on 2003–2005 5.9L ISB automotive engines and certain Tier 3 ISB/QSB 6.7 industrial engines. Most 6.7L Ram pickups use different rocker housings. Confirm the correct part by VIN/ESN before ordering.
Is this a genuine Cummins part or aftermarket?
This is a genuine Cummins OEM rocker lever housing, not an aftermarket copy. It is supplied through the Cummins distribution network and meets all factory material and machining specifications.
Should I replace my rocker arms when I install a new housing?
If your existing rocker levers show excessive wear, pitting, or noisy operation, it is smart to replace them while the top end is apart. A popular OE option is Cummins 4995602 rocker lever assemblies (typically six per engine).
