Cummins 4BT Thermostat for 3.9L 4B and 4BTA Engines
4B3.9 · 4BT3.9 · 4BTA3.9
A Cummins 4BT thermostat should be selected by the original part number or the engine build record—not by “3.9L” alone. This page brings together the thermostat options in our store that publish 4B3.9, 4BT or 4BTA fitment, then shows what to verify before ordering.
Use the product list for a known replacement number. If the old number is unreadable, send the Cummins Engine Serial Number (ESN), CPL, application and thermostat photos for a fitment check.
Shop 12 Cummins 4BT Thermostat Options
The products below are current store listings whose published fitment data includes 4B3.9, 4BT3.9, 4BTA3.9 or another explicit 4BT reference. They are alternatives for different engine builds and applications, not twelve interchangeable versions of one thermostat.
4BT Thermostat Part Numbers Represented on This Page
This is a shopping index for the products shown above. “Published fitment” describes the information in the corresponding 4BTdiesel product listing; final compatibility still needs to be checked against the particular engine.
| Store listing / replacement number | 4-cylinder reference published in the listing | Important ordering distinction |
|---|---|---|
| 3917324 | 4B3.9, 4BT3.9 and 4BTA3.9 | This listing identifies thermostat 3917324, gasket 3920336 and bushing 3923331. |
| 3917324 / 3923331 kit | 4B3.9 | Check the kit contents against the seal/bushing pieces needed at teardown. |
| 4080631 | 4B3.9 | Also lists several later engine families; do not transfer that fitment to every early 4BT. |
| 3946849 | 4B3.9 | Application coverage is broad; verify the engine record rather than a vehicle year alone. |
| 3928499 | 4B3.9 in the product fitment description | The title emphasizes 6B/Dodge applications, so a 4BT buyer should confirm by ESN before ordering. |
| 5292744 / 3954830 | 4BT | Confirm whether either number is the current match for the original engine build. |
| 4900260 | 4B3.9 | Compare the physical thermostat and seal arrangement, not just its outside diameter. |
| 3864178 / 3925473 | 4B3.9 | Multiple cross-reference numbers are published; send the number found on the removed part. |
| 4929641 / 5292707 | 4B3.9 | The same listing covers many electronic B-series variants; check CPL/application carefully. |
| 3928639 | 4B3.9 | Do not select it solely because it also appears in a larger-engine listing. |
| 5292708 / 3974823 | B3.9 / 4B3.9 | The product is listed as a 76°C option. Use that temperature only when the original specification calls for it. |
| 5292738 / 4930315 | 4BT, 4BT3.9 and 4BTA in the product description | The title focuses on other applications; treat the 4BT reference as a fitment lead to verify, not automatic approval. |
Why “Cummins 3.9 Thermostat” Is Not Enough to Place an Order
4B, 4BT and 4BTA are useful engine-family descriptions, but the thermostat installation can still differ by CPL, application, housing arrangement, service replacement history and temperature specification. Repowered equipment adds another risk: the machine model may no longer identify the engine that is installed.
Cummins instructs parts users to look up engine parts with the Engine Serial Number or part number. Its QuickServe system includes an engine-specific parts catalog, dataplate information and part-number supersession. That is why this page does not claim one “universal 4BT thermostat.” See the Cummins Parts Information page and Cummins QuickServe overview.
Use the product page when you know:
- The complete old thermostat or kit number.
- The required opening-temperature specification.
- Whether the gasket, seal or bushing is included.
- The engine/application listed in the full fitment data.
Request a manual check when:
- The engine has been swapped or rebuilt before.
- The old thermostat has no readable number.
- Two listings appear to fit the same 3.9L engine.
- The housing or seal differs from the product photo.
What to Send for a Matched 4BT Thermostat Quote
- Dataplate photo: include the complete plate, not a cropped model name.
- Typed ESN and CPL: typed numbers help us read faded or reflective plates.
- Old part photos: photograph both sides, the flange, valve and all markings.
- Application: identify the generator, loader, excavator, forklift, marine unit or repower vehicle.
- Cooling complaint: state whether the engine runs hot, warms up slowly, fluctuates or is being serviced preventively.
- Needed pieces: thermostat only, thermostat with gasket/bushing, or an additional housing component.
If you need help finding the identification plate, use our Cummins engine serial number lookup guide. For other 3.9L cooling and engine components, open the Cummins 4BT parts hub. To browse all makes and engine families, return to the broader diesel engine thermostat category.
Before Installing the Replacement
Compare the new and removed parts before adding coolant: part number, overall dimensions, flange and valve layout, temperature marking and the supplied seal. Clean the housing sealing surface, replace damaged sealing pieces, orient any specified vent or jiggle valve according to the engine service information, refill with the correct coolant, remove trapped air and check for leakage.
A new thermostat is not a complete diagnosis for every overheating complaint. Restricted airflow, a loose or damaged belt, water-pump problems, coolant loss, combustion-gas leakage, plugged passages, incorrect cap pressure or an inaccurate sender/gauge can produce similar symptoms. Diagnose the cooling system before repeatedly replacing thermostats.
Cummins 4BT Thermostat Questions
Are all Cummins 4BT thermostats the same?
No. The store listings above show multiple part-number groups that publish 4B3.9 or 4BT fitment. Match the original number or use ESN/CPL and application data.
Can I order by 3917324 alone?
If 3917324 is clearly the current number on the removed thermostat or verified engine parts record, use the corresponding listing. Also check whether you need the gasket 3920336 and bushing 3923331 or a different seal arrangement.
Should I choose a lower-temperature thermostat to prevent overheating?
Not as a shortcut. A thermostat with a different rating does not repair poor airflow, coolant loss, a damaged water pump, combustion leakage or another cooling-system restriction. Use the temperature specified for the engine build and application.
Does a 4BTA use the same thermostat as a 4BT?
Some listings may cover both, but the names alone do not establish compatibility. Compare the part number and engine record because application and service changes can differ.
Is the thermostat gasket included?
Only when the individual product listing states that it is included. For example, the 3917324 set shown here identifies a gasket and bushing. Do not assume every single-thermostat listing contains those pieces.
Match the Part Before the Engine Is Refilled
Send the identifiers while the old thermostat and housing are still accessible. We can compare the store options with the information provided and put the selected part number in writing before shipment.
