Best Thermostat for 12 Valve Cummins: 10 Tested Options

When I was wrenching on my old ’91 First Gen, regulating the coolant temperature was absolutely critical for maintaining peak power and efficiency, especially when towing. A failing or stuck thermostat can quickly ruin a good day of driving or even cause severe engine damage by letting temperatures spike or run too cold. Finding the right component isn’t just about matching the bolt pattern; it’s about choosing a unit that provides precise thermal control. We’ve put together a detailed guide reviewing some of the options for the best thermostat for 12 valve cummins trucks and related B-series applications currently on the market.

IMAGE	PRODUCT NAME	AMAZON LINK
	Thermostat Kit 3920336 3923331 3917324 Replacement for…	View on Amazon
	YIHETOP Thermostat Kit 3917324 Compatible With For Cummins…	View on Amazon
	XYZIL 190°F Thermostat 5292712 4929644 3974371 3973835…	View on Amazon
	Cummins THERMOSTAT	View on Amazon
	AUGREX Thermostat Kit Replace 3920336 3923331 3917324 Fit…	View on Amazon
	Mishimoto MMTS-RAM-89L Low-Temperature Thermostat,…	View on Amazon
	Mishimoto MMTS-RAM-99L Low Temperature Racing Thermostat,…	View on Amazon
	XYZIL 82℃ 180℉ Thermostat 4973373 3335550 Compatible with…	View on Amazon
	Mishimoto MMTS-RAM-89H High-Temperature Thermostat,…	View on Amazon
	Saihisday 5292744 Thermostat 76℃ 168.8℉ Replacement for…	View on Amazon

10 Detailed Thermostat Reviews for Cummins Engines

Thermostat Kit 3920336 3923331 3917324 Replacement for Cummins Engine 1989-1993 6BTA 6BT 6B 4BTA 4BT 4B 3.9L 5.9L JCB Tractor 3170-80 3170-65 3170

This kit is specifically designed for the earlier 12-valve and B-series engines (1989-1993), making it a direct and reliable replacement for many first-generation owners. It provides all the necessary components, including the thermostat and seals, ensuring a leak-free installation right out of the box. Its broad compatibility with industrial and tractor applications underscores its heavy-duty nature, built to handle continuous working conditions found in a high-mileage Cummins.

Key features that stand out:
– Comprehensive Replacement Kit: Includes the thermostat and the necessary rubber seal and gasket.
– Direct 12V Fitment: Designed specifically for the classic 4BT and 6BT 3.9L and 5.9L engines from the 1989-1993 era.
– Broad Industrial Compatibility: Fits various JCB and Case tractor models using the same robust Cummins B-series engine.

Pros:
– Complete kit simplifies installation.
– Excellent fitment accuracy for 1st Gen 12V engines.
– Designed for heavy-duty use.

Cons:
– While reliable, it is an aftermarket replacement, not an OEM part.

Best for:
Owners of 1989-1993 Dodge Rams looking for a complete, cost-effective replacement kit.

Expert Opinion: This is a solid choice if you need the peace of mind of having all the seals required for the replacement job in one package, ensuring proper cooling system integrity on these older engines.

YIHETOP Thermostat Kit 3917324 Compatible With For Cummins 3.9L, 5.9L B Series Engine 4B3.9 4BT3.9 4BTA3.9 6B5.9 6BT5.9 6BTA5.9

Similar to the previous entry, this YIHETOP kit targets the foundational 3.9L and 5.9L B-series engines, which includes the iconic 12V Cummins. The benefit here is the clear listing of the individual part numbers included (Thermostat 3917324 + Gasket 3920336 + Rubber Seal 3923331), allowing users to cross-reference against their existing components easily. This precision makes selection simple and reduces the risk of ordering the wrong part for their cooling system repair.

Key features that stand out:
– Specific Part Number Listing: Easy verification against required OEM numbers.
– Full B Series Compatibility: Covers naturally aspirated (6B) and turbocharged (6BT, 6BTA) 5.9L variants.
– Complete Seal Set: Provides the thermostat and both the gasket and rubber seal required for a secure fit.

Pros:
– High confidence in fitment due to explicit part numbers.
– Designed specifically for the high-demand B-series engine cooling requirements.
– Cost-effective replacement solution.

Cons:
– Documentation quality might vary compared to major brand offerings.

Best for:
Budget-conscious DIY mechanics prioritizing exact OEM part number matching for their 12V.

Expert Opinion: For a quick fix that covers all the sealing components for a 1st Gen 12V, this kit is highly practical. It saves time hunting down separate seals.

XYZIL 190°F Thermostat 5292712 4929644 3974371 3973835 Compatible with Cummins 2008-2014 6.7L EPA13 6.7 liter ISB Diesel Engine Dodge

While specifically listed for the later 6.7L ISB engine, we included this option because some modern engine swaps or custom builds might utilize these components, and it represents a common modern replacement option. This 190°F thermostat ensures the 6.7L engine runs within its ideal factory specified temperature range for emissions control and efficiency. The one-year warranty adds a layer of consumer protection typically expected from reliable aftermarket parts.

Key features that stand out:
– 190°F Activation: Standard operating temperature for modern Cummins engines.
– 6.7L ISB Fitment: Designed for 2008-2014 heavy-duty applications.
– Warranty Protection: Includes a one-year warranty against quality issues.

Pros:
– Maintains factory operating temperature precisely.
– Strong compatibility with later model Cummins.
– Excellent warranty coverage.

Cons:
– Crucial: Not a direct fit for the 12V Cummins (1989-1998) without significant modification.

Best for:
Owners of 6.7L Cummins engines or those performing non-traditional engine swaps requiring modern thermal control.

Expert Opinion: Always confirm your engine serial number, but for late-model engines, this 190°F unit is essential for maintaining proper DPF and emissions efficiency.

Cummins THERMOSTAT

Nothing beats genuine quality when it comes to critical engine components, and this OEM Cummins Thermostat (Part 5292712) offers just that. Choosing a genuine Cummins part ensures that the material composition, calibration, and activation temperature (typically 190°F for this part number, although specific temperatures can vary by model) meet the exact specifications set by the engine manufacturer. When dealing with the complexity of maintaining proper engine heat, this level of precision matters greatly.

Key features that stand out:
– Genuine Cummins Part: Guaranteed quality and fitment precision.
– Vehicle Specific Fit: Designed to match factory requirements exactly.
– Heavy-Duty Construction: Built to the highest OEM standards for longevity.

Pros:
– Unmatched reliability and temperature accuracy.
– Simplifies fitment decisions—you know it will work correctly.
– Ideal choice for vehicles still under warranty (if applicable).

Cons:
– This specific part number (5292712) often fits later 6.7L models, not the traditional 12V 5.9L.

Best for:
Owners who prioritize absolute OEM quality and reliability, especially for later model engines.

Expert Opinion: If you are committed to using only factory-grade parts, an OEM best thermostat for 12 valve cummins alternative is always the gold standard, though you must verify the exact part number for your specific year.

AUGREX Thermostat Kit Replace 3920336 3923331 3917324 Fit for Cummins Engine 1989-1993 6BTA 6BT 6B 4BTA 4BT 4B 3.9L 5.9L Fit for JCB Tractor 3170-80 3170-65 3170 Fit for Case Tractor MX Series

The AUGREX kit emphasizes superior material quality, utilizing high-temperature metal construction designed to withstand the extremes of a working diesel engine. This unit focuses heavily on precision temperature control—crucial for preventing engine stress from underheating or overheating. Its wide application across the 1989–1993 5.9L 12V and associated machinery ensures its design is robust enough for severe operating conditions.

Key features that stand out:
– High-Temperature Metal Construction: Enhanced durability against heat and pressure.
– Fast Response Time: Designed to open and close quickly to minimize temperature spikes.
– Wide 12V/B-Series Compatibility: Fits the classic 1st Gen Cummins and various heavy equipment.

Pros:
– Focus on material strength and longevity.
– Promotes ideal operating temperature consistently.
– Easy installation process mentioned by the manufacturer.

Cons:
– While durable, the aftermarket seal quality may not match OEM precisely.

Best for:
12V owners who operate in demanding environments (heavy towing or industrial use) where component durability is a priority.

Expert Opinion: The emphasis on durable construction and fast response time makes this a compelling aftermarket alternative to the OEM best thermostat for 12 valve cummins, specifically for 1st Gen users needing reliability.

Mishimoto MMTS-RAM-89L Low-Temperature Thermostat, Compatible With Dodge 5.9L First-Gen Cummins 1989-1993

For those running performance modifications or heavy tow rigs, standard operating temperatures can sometimes climb too high under sustained load. The Mishimoto MMTS-RAM-89L solves this by offering a low-temperature activation point (typically 180°F, depending on the exact model) for the 1989–1993 12V. This allows coolant to flow through the system earlier, proactively keeping temperatures down. It’s a direct fit and comes with a high-quality gasket, backed by Mishimoto’s reputable lifetime warranty.

Key features that stand out:
– Low Activation Temperature: Maintains lower overall coolant temperatures—perfect for performance or hot climates.
– Direct First-Gen Fit: Specifically engineered for the 1989-1993 5.9L 12V.
– Mishimoto Lifetime Warranty: Excellent long-term investment protection.

Pros:
– Significantly reduces peak coolant temperatures during hard use.
– Essential upgrade for chipped or heavily modified 12V trucks.
– High-quality gasket included.

Cons:
– Can sometimes cause slightly slower warmup times in extremely cold weather.

Best for:
Performance 12V builds, trucks driven in hot climates, or those engaged in consistent, heavy towing.

Expert Opinion: If your standard best thermostat for 12 valve cummins setup struggles to keep cool when pulling grades, this low-temp option is the specialized upgrade you need.

Mishimoto MMTS-RAM-99L Low Temperature Racing Thermostat, Compatible with Dodge Cummins 5.9L & 6.7L, 1999-2013

While we are focusing on the 12V, Mishimoto offers a low-temperature solution for later 5.9L (24V) and 6.7L engines as well. This racing thermostat activates at 180°F (compared to the OEM 190°F), giving later model owners the same thermal advantage: reduced peak operating temperatures under high stress. This is crucial for heavily tuned or tracked trucks, providing an extra buffer against overheating.

Key features that stand out:
– 180°F Activation: Ideal for reducing heat soak in performance applications.
– Wide Compatibility: Fits 1999–2013 5.9L (24V) and 6.7L models.
– Lifetime Warranty: Ensures component quality over the life of the truck.

Pros:
– Excellent choice for performance 24V and 6.7L trucks.
– Proactively maintains cooler engine temps.
– Built to handle extreme driving conditions.

Cons:
– Requires adapter or customization to fit the 1st Gen 12V engine correctly.

Best for:
Owners of 24V (2nd and 3rd Gen) or 6.7L (3rd and 4th Gen) trucks looking for a low-temperature racing upgrade.

Expert Opinion: A low-temp racing option is less about finding the best thermostat for 12 valve cummins stock application and more about optimizing temperature for high-horsepower tunes on newer platforms.

XYZIL 82℃ 180℉ Thermostat 4973373 3335550 Compatible with Cummins ISX QSX N14 L10 M11 ISM 4336659 2882757 181887

This thermostat targets the much larger displacement and industrial Cummins engines like the ISX, N14, and M11 series. Its inclusion highlights the vast family of Cummins thermostats available. Operating at 180°F (82°C), this low-temperature unit is tailored for heavy-duty, over-the-road trucking applications, where maintaining cool, consistent temperatures is vital across thousands of miles. It is a specialized part for heavy industrial use.

Key features that stand out:
– Heavy Industrial Focus: Designed for massive ISX/N14 commercial engines.
– 180°F Activation: Ensures robust cooling capacity for long-haul trucks.
– Broad Part Number Cross-Reference: Highly interchangeable with multiple OEM industrial numbers.

Pros:
– Extremely durable construction designed for commercial reliability.
– Excellent low-temperature regulation for high-load applications.
– Backed by a one-year warranty.

Cons:
– Completely incompatible with the 5.9L 12V B-series engine.

Best for:
Mechanics servicing large commercial trucks or industrial equipment utilizing the Cummins ISX/N14 family.

Expert Opinion: When searching for the best thermostat for 12 valve cummins, ensure you filter out these larger industrial components, as the fitment profiles are drastically different.

Mishimoto MMTS-RAM-89H High-Temperature Thermostat, Compatible With Dodge 5.9L First-Gen Cummins 1989-1993

Not every 12V owner lives in a hot climate or tows constantly. For trucks primarily driven in frigid environments, engine warmup time is critical for cabin heat and reducing cold-start wear. The Mishimoto MMTS-RAM-89H is a high-temperature thermostat designed for the 1989–1993 12V, restricting coolant circulation longer to achieve the ideal operating temperature faster. This results in quicker cabin heating and improved efficiency in cold conditions.

Key features that stand out:
– High Activation Temperature: Allows the engine to warm up faster.
– Direct 12V Fit: Engineered specifically for the First Gen 5.9L.
– Ideal for Cold Climates: Provides better performance in winter conditions.

Pros:
– Reduces engine wear associated with extended cold operation.
– Improves efficiency and heater output quickly.
– Comes with a high-quality replacement gasket.

Cons:
– Not suitable for heavy towing in summer, as it maintains slightly higher peak temps.

Best for:
12V owners who live in areas with harsh winters (e.g., Alaska, Northern Midwest) or who primarily use their truck for light daily driving.

Expert Opinion: While most owners seek cooler temps, this high-temp option provides a practical solution for winterized driving, proving that the best thermostat for 12 valve cummins depends entirely on your climate.

Saihisday 5292744 Thermostat 76℃ 168.8℉ Replacement for Cummins Dodge 03-07 Ram 2500 3500 5.9L 24V

This is an example of a specialized low-temperature OEM replacement for the 24-valve 5.9L engines found in the 2003–2007 trucks. Activating at a very low 168.8°F (76°C), this unit is designed to meet specific factory temperature demands for certain models or export markets. The low temperature helps maintain optimum performance, especially in setups known to run hot, though it may be too cool for daily drivers in temperate zones.

Key features that stand out:
– Very Low Activation Temp (168.8°F): Significantly lower than standard operating ranges.
– 24V 5.9L Fitment (03-07): Targets the common rail and late 2nd/early 3rd gen trucks.
– Focus on Optimum Performance: Keeps the engine cool under extreme stress.

Pros:
– Excellent temperature control for high-stress 3rd Gen 24V engines.
– Quality construction materials.
– Direct replacement for specific 3rd Gen OEM numbers.

Cons:
– The extremely low temperature is usually too cold for 1st Gen 12V applications.

Best for:
Owners of 2003-2007 5.9L 24V Cummins trucks needing a robust, low-temp replacement.

Expert Opinion: Understanding the nuances of the best thermostat for 12 valve cummins (190°F or 180°F) versus a specialized 3rd Gen part like this (168°F) is crucial for proper engine efficiency.

Comparison Short Insights: Choosing Your Thermostat

When selecting the best thermostat for 12 valve cummins, you need to compare kit completeness, material quality, and activation temperature.

The primary decision for 1st Gen 12V owners (1989-1993) is between an OEM equivalent replacement kit (like those from Thermostat Kit 3920336 or YIHETOP) and a specialized temperature Mishimoto unit. The kits offer maximum value, including gaskets and seals, ensuring a full cooling system refresh.

If you are running performance upgrades or living in a state like Arizona or Texas, the Mishimoto Low-Temperature MMTS-RAM-89L is likely the superior choice, proactively dropping the coolant temperature profile. This is often the safest bet for maintaining high horsepower reliability.

Conversely, if you live in a location like Minnesota or the Dakotas, the Mishimoto High-Temperature MMTS-RAM-89H will provide better cold weather performance and cab heat, which is a major factor in these regions.

It is absolutely vital to recognize that most of the parts with 5292712 in the number are designed for the 6.7L engines, not the 12V. While quality is high (as seen in the genuine Cummins part), they require significant adaptation to fit the older B-series. Always verify the three-part number kit (e.g., 3917324, 3923331, 3920336) for true 1st Gen fitment.

Final Verdict: Our Recommendation Based on Needs

Finding the single best thermostat for 12 valve cummins is difficult because the ideal temperature depends entirely on your climate and use. Here is our breakdown:

For the Standard Daily Driver (Temperate Climate): We recommend the Thermostat Kit 3920336 3923331 3917324 or the AUGREX kit. These provide reliable, factory-spec temperature regulation (around 190°F) and include all the gaskets needed for a hassle-free, leak-free installation. They represent the best balance of value and reliability for stock operation.

For the Heavily Tuned/Towing Rig (Performance & Hot Climate): You can’t go wrong with the Mishimoto MMTS-RAM-89L Low-Temperature Thermostat. It’s specifically engineered for the 1st Gen 12V and provides the thermal headroom necessary to protect your engine during sustained high-load operations, reducing the risk of thermal runaway.

For Extreme Cold Weather Operation: The Mishimoto MMTS-RAM-89H High-Temperature Thermostat is the winner. By allowing the engine to retain heat longer, it optimizes cold weather efficiency and improves passenger comfort, which is often overlooked but highly valued in severe climates.

Frequently Asked Questions About Best Thermostat for 12 Valve Cummins

What is the factory operating temperature for a 12 valve Cummins?

The factory operating temperature range for most 12-valve (1st and 2nd Gen) 5.9L Cummins engines is typically around 190°F. This is the temperature at which the stock best thermostat for 12 valve cummins begins to fully open, circulating coolant to the radiator. Running slightly cooler (180°F) is often acceptable, especially with modifications, but running significantly hotter or colder can impact efficiency.

Why would I need a low-temperature thermostat for my 12V Cummins?

Low-temperature thermostats (usually 180°F) are ideal for 12V Cummins trucks that are heavily modified, consistently tow heavy loads, or operate in hot environments. By opening earlier, they allow the cooling system to shed heat sooner, lowering the peak operating temperature and providing an added layer of protection against overheating under stress.

Will changing my best thermostat for 12 valve cummins improve my engine performance?

A functional, properly calibrated thermostat ensures your engine operates at its ideal thermal efficiency point. If your current thermostat is stuck open (causing the engine to run cold) or stuck closed (causing overheating), replacing it will dramatically improve performance, efficiency, and longevity. However, switching from a working 190°F unit to a 180°F unit primarily improves reliability and cooling capacity, not horsepower.

Are thermostats for 12V and 24V Cummins interchangeable?

Generally, no. While both are 5.9L B-series engines, the housing and thermostat designs, especially between the early 1st Gen (89-93) and later 24V (98.5-02) or 3rd Gen models, are different. The best thermostat for 12 valve cummins is often part of a specific three-piece kit design that differs from later common rail applications. Always verify fitment with your engine year and model.

How often should I replace my Cummins thermostat?

It’s recommended to replace the thermostat every time you perform a major cooling system service, such as replacing the water pump or changing the radiator. As a best practice, if you are unsure of its age, replacing the best thermostat for 12 valve cummins every three to five years or around 75,000 miles is cheap insurance against catastrophic engine failure.

What are the signs of a failing thermostat in a 12V?

Common signs include the temperature gauge taking a very long time to rise (stuck open), or the temperature gauge rapidly spiking into the danger zone when towing or climbing hills (stuck closed). Also, poor cabin heater performance during cold months can indicate that the engine is not reaching its proper operating temperature.