Product Description
Genuine Cummins 3958645 Thick MLS Head Gasket for 5.9L ISB
The Cummins 3958645 OEM cylinder head gasket (thick) is the genuine multi-layer steel (MLS) head gasket used on 2003–2007 Ram 2500/3500 trucks with the 5.9L ISB common-rail Cummins. This thick-grade gasket is engineered to restore proper piston-to-head and valve clearances on engines that have had the cylinder head and/or block deck resurfaced during machine work, while maintaining OEM sealing reliability under towing, work, or tuned power levels.
Why Choose Genuine Cummins 3958645?
- Thick-grade MLS design to compensate for milled head and/or block decks.
- Engineered by Cummins to match factory surface finishes and clamp loads.
- Proven reliability for stock, towing, and moderately tuned 5.9L ISB builds.
- Direct-fit for 2003–2007 Ram 2500/3500 5.9L ISB common-rail applications.
- Helps prevent combustion gas, coolant, and oil cross-leaks when installed correctly.
Nominal compressed thickness is approximately 1.25 mm (thick grade), making this a popular choice when machine work or higher cylinder pressures call for additional safety margin versus the standard-thickness gasket.
Fitment – 5.9L ISB Common-Rail & QSB5.9
This genuine Cummins head gasket is commonly used on:
- 2003–2007 Dodge Ram 2500 – 5.9L ISB 24-valve common-rail diesel
- 2003–2007 Dodge Ram 3500 – 5.9L ISB 24-valve common-rail diesel
Engine platform: 5.9L ISB (24-valve, high-pressure common rail)
Automotive use: 2003–2007 Ram 2500/3500 5.9L Cummins
Industrial/off-highway: Also found on select QSB5.9 and commercial applications – always verify by engine serial number (ESN) and CPL before ordering for non-Ram engines.
When to Use the “Thick” Head Gasket
- Head and/or block deck have been resurfaced (milled) during rebuild or repair.
- You need to restore factory piston protrusion and quench after machining.
- Builds running increased cylinder pressure (towing tunes, moderate performance) where you still want OEM reliability.
What’s Included
- (1) Genuine Cummins multi-layer steel thick head gasket – part number 3958645
No head bolts/studs or additional gaskets are included. Pair this gasket with the appropriate upper and lower engine gasket kits and crank seals for a complete reseal.
Recommended Genuine Cummins Companion Parts (Internal Links)
- CUMMINS 4089819 OEM UPPER ENGINE GASKET KIT – ISB 5.9L (2003–2007) – complete top-end gasket kit including standard-thickness head gasket configuration.
- CUMMINS 4089173 OEM LOWER ENGINE GASKET KIT – RAM 5.9L (2003–2007) – lower-end gasket set for oil pan, gear housing, and related seals.
- CUMMINS 3864458 OEM ENGINE OIL COOLER GASKET (COOLER TO FILTER HEAD) – RAM 5.9L (2003–2007) – cooler-to-filter head gasket often serviced during rebuilds.
- CUMMINS 3937111 OEM FRONT CRANKSHAFT SEAL SERVICE KIT – 5.9L CUMMINS (1989–2007) – OE front main seal kit with installer components.
- CUMMINS 3978517 OEM CYLINDER HEAD GASKET (.020 MARINE) – RAM 5.9L ISB (2003–2007) – alternative +.020" marine-thickness head gasket option.
Quick Specs
- Part number: 3958645
- Type: Multi-layer steel (MLS) cylinder head gasket – thick grade
- Nominal compressed thickness: ~1.25 mm (grade B / thick)
- Quantity required: 1 per engine (sold individually)
- Primary application: 2003–2007 Ram 2500/3500 with 5.9L ISB common-rail Cummins
- Secondary applications: select QSB5.9 and commercial engines (verify ESN/CPL)
Specifications for cummins 3958645
| Attribute | Value |
|---|---|
| Brand | Cummins – Genuine OEM |
| Part Number | 3958645 – Thick MLS cylinder head gasket |
| Engine Compatibility | 5.9L ISB common-rail (24V) – 2003–2007 Ram 2500/3500; select QSB5.9 applications (verify ESN/CPL) |
| Gasket Type | Multi-layer steel (MLS), thick grade |
| Nominal Compressed Thickness | Approx. 1.25 mm (thick grade) |
| UPC | 193807139377 |
| GTIN | 193807139377 |
| Shipping Weight | Approx. 1.8 lb (boxed gasket) |
| Typical Package Dimensions | Approx. 32.0" L × 10.0" W × 0.25" H |
| Shipping Method | Standard ground / small parcel; typically qualifies for free shipping from CPP Diesel where indicated |
| Cross Reference (ID Only) | Cummins 3958645; Cummins 3954876 (ID only); PAI 131711 (aftermarket equivalent ID only) |
| E-Codes / Emissions Notes | Supports 2003–2007 light-duty diesel applications using 5.9L ISB common-rail Cummins; also used in certain QSB5.9 off-highway engines. Always match by ESN/emissions family. |
| Prop 65 Warning | Yes – This product can expose you to chemicals including lead and other substances 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. |
Cross-Reference & Related Genuine Cummins Parts on CPP Diesel
For full engine reseals or top-end work on your 2003–2007 5.9L ISB, pair the 3958645 thick head gasket with these genuine Cummins parts on cppdiesel.com:
- CUMMINS 4089819 OEM UPPER ENGINE GASKET KIT – ISB 5.9L (2003–2007)
- CUMMINS 4089173 OEM LOWER ENGINE GASKET KIT – RAM 5.9L (2003–2007)
- CUMMINS 3864458 OEM ENGINE OIL COOLER GASKET (COOLER TO FILTER HEAD) – RAM 5.9L (2003–2007)
- CUMMINS 3937111 OEM FRONT CRANKSHAFT SEAL SERVICE KIT – 5.9L CUMMINS (1989–2007)
- CUMMINS 3978517 OEM CYLINDER HEAD GASKET (.020 MARINE) – RAM 5.9L ISB (2003–2007)
Installation Notes
- Always verify correct gasket grade by piston protrusion, ESN, and build specs.
- Surface prep is critical: follow Cummins RA and flatness specs for head and block.
- Use new head bolts or quality head studs and follow the factory torque sequence and stages.
- Re-check coolant and oil levels, and monitor for leaks after the first heat cycles.
FAQ
WILL THIS FIT MY 2003–2007 5.9L CUMMINS RAM?
Yes. Cummins 3958645 is the thick-grade OEM head gasket used on most 2003–2007 Dodge Ram 2500/3500 trucks equipped with the 5.9L ISB common-rail engine. Because Cummins used different gasket grades, we still recommend confirming by VIN and ESN before ordering.
WHEN SHOULD I CHOOSE THE “THICK” 3958645 GASKET INSTEAD OF A STANDARD GASKET?
Use the thick-grade 3958645 when the cylinder head and/or block deck have been resurfaced or when piston protrusion and valve clearances call for additional thickness to maintain safe quench and compression. It is also common on higher-cylinder-pressure towing or moderate performance builds where extra margin is desired.
IS THIS GASKET SUITABLE FOR QSB5.9 OR INDUSTRIAL APPLICATIONS?
This is a genuine Cummins head gasket and is also used in certain QSB5.9 and commercial applications. However, industrial engines can vary by CPL and emissions certification, so always verify by engine serial number (ESN) and parts catalog data before using this gasket in non-automotive applications.
CAN I REUSE MY OLD HEAD BOLTS WITH THIS GASKET?
We strongly recommend new fasteners any time a head gasket is replaced. Many builders take this opportunity to upgrade to quality head studs for improved clamping, especially on tuned or heavy-towing trucks. Reusing stretched or fatigued bolts can lead to premature gasket failure.
DOES THIS LISTING INCLUDE ANY OTHER GASKETS OR SEALS?
No. This listing is for the single thick MLS head gasket only. For a complete reseal, add the appropriate upper and lower engine gasket kits (such as Cummins 4089819 and 4089173), plus front and rear crank seals and any additional gaskets your build requires.
