Product Description
Cummins 3955121 — Genuine Rocker Lever Housing (Rocker Box) for ISB 5.9L
MPN: 3955121 | Also Known As: rocker box, valve cover spacer, rocker lever housing | Related Numbers (ID only): 3955120 (housing variant)
Genuine Cummins rocker lever housing that mounts between the cylinder head and upper valve cover. Direct-fit for 2003–2005 Ram 5.9L ISB common-rail. Also seen in select Tier 3 ISB/QSB 6.7 industrial applications—always confirm by ESN.
Features & Benefits
- OE cast-aluminum housing with precision-machined sealing surfaces.
- Direct-fit drop-in for 2003–2005 5.9L ISB common-rail Ram 2500/3500.
- Correct harness pass-through and baffle geometry for factory function.
- Restores proper sealing when paired with new rocker housing gasket.
- Backed by Cummins for fit, finish, and durability.
Verified Fitment (Automotive)
| Year Range | Engine | Chassis |
|---|---|---|
| 2003–2005 | Ram 5.9L ISB (24V Common-Rail) | Ram 2500/3500 |
Why we publish 2003–2005 for automotive: XDP lists 2003–2005 Dodge 5.9L for 3955121; Thoroughbred also markets this part specifically for 2003–2005 5.9L, matching real-world Ram fitment. :contentReference[oaicite:0]{index=0}
Industrial/Off-Highway: Cummins’ catalog notes 3955121 is used primarily on Tier 3 6.7L ISB/QSB engines—confirm by Engine Serial Number (ESN) for non-automotive installs. :contentReference[oaicite:1]{index=1}
Compatibility Notes:
- Not typical for 2006–2007 5.9L or most 2007.5+ 6.7L Ram trucks. Many 6.7L applications use different housings such as 3974981 / 3975944—verify by VIN/ESN. :contentReference[oaicite:2]{index=2}
- Always pair with a new rocker lever housing gasket for a proper seal.
Specifications
- Type: Rocker lever housing (rocker box / valve cover spacer)
- Material: Cast aluminum
- OE Part Number: 3955121
- Quantity Required: 1 per engine (sold individually)
Often Paired With
- Rocker lever housing gasket: Cummins 3963379 (head-to-housing, 5.9/6.7 coverage). :contentReference[oaicite:3]{index=3}
- Valve cover gasket (upper cover): Cummins 3954324 for 2003–2005 5.9L. :contentReference[oaicite:4]{index=4}
What’s In The Box
- (1) Genuine Cummins Rocker Lever Housing (3955121)
- Gaskets/hardware sold separately (see “Often Paired With”).
Installation Tips
- Clean and inspect mating surfaces; install new gasket(s) and torque fasteners to factory spec.
- Verify harness routing/clearance and check for leaks after initial warm-up.
- Good practice: recheck valve cover fasteners after heat cycle.
FAQ
WILL THIS FIT MY 2003–2005 5.9L CUMMINS RAM?
Yes—3955121 is listed for 2003–2005 Ram 2500/3500 5.9L ISB common-rail. Always confirm by VIN/ESN. :contentReference[oaicite:5]{index=5}
DOES THIS COME WITH GASKETS?
No—order the rocker housing gasket 3963379 (head-to-housing) and, if needed, the upper valve cover gasket 3954324 for 2003–2005. :contentReference[oaicite:6]{index=6}
WILL THIS FIT 2006–2007 5.9L OR 6.7L RAM?
Those applications commonly use different housings (e.g., 6.7L often uses 3974981/3975944). Verify by VIN/ESN before ordering. :contentReference[oaicite:7]{index=7}
IS THIS AN OEM CUMMINS PART?
Yes—genuine Cummins 3955121 rocker lever housing.
WHY DO SOME LISTINGS MENTION 6.7L INDUSTRIAL ENGINES?
Cummins’ catalog shows 3955121 used primarily on Tier 3 ISB/QSB 6.7 industrial engines. That’s why we advise an ESN check for non-automotive installs. :contentReference[oaicite:8]{index=8}
