Is a Ford 400 the same as a 351M?
No. The Ford 400 and the 351M are not the same engine; they are two distinct displacements within Ford’s 335-series family, with the 400 delivering 400 cubic inches and the 351M delivering 351 cubic inches. While they share design lineage, they are not directly interchangeable without significant modification.
This article breaks down what each engine is, how they differ, where they were used, and what you need to know about compatibility if you’re restoring a classic Ford or planning a swap.
Overview of Ford’s 335-series lineage
Ford 351M (Modified)
The 351M is a member of Ford’s 335-series V8 family. It represents a mid-range displacement option designed to offer more torque than the smaller Windsor-based engines while not going to the larger capacities of some other Ford big-blocks. The 351M shares the general block architecture of the 335-series but uses a distinct bore/stroke combination to reach 351 cubic inches, and it was offered with a variety of carburation and accessory configurations for different model lines.
Ford 400
The 400 is another 335-series V8, designed to deliver 400 cubic inches of displacement. Like the 351M, it uses the same broad family block but employs a different bore/stroke setup to achieve its larger displacement. The 400 was paired with several carburetion and accessory arrangements across Ford’s lineup and shares overall architecture with other 335-series engines, even though it is not a 351M in disguise.
Both engines sit within Ford’s 335-series family, which was popular in the 1970s for offering robust mid-sized and large-car performance. The differences between them are primarily in displacement, internal components, and accessory fitment, which influences compatibility when building or restoring a vehicle.
Key differences between the 351M and the 400
Below are the core mechanical and compatibility points that set the two engines apart. This list highlights what to expect if you’re evaluating parts or considering a swap between these two engines.
- Displacement and dynamic performance: 351 cubic inches versus 400 cubic inches, leading to different torque curves and horsepower characteristics.
- Internal components: The crankshaft, pistons, and connecting rods are matched to each engine’s displacement; these parts are not directly interchangeable without modification.
- Cylinder heads and intake hardware: Cylinder heads and intake manifolds are designed for their respective engines; cross-compatibility is limited and may require adapters or other changes.
- Block geometry and deck features: While both are part of the 335-series, there are differences in bore sizing, deck height, and cooling passages that affect machining and fitment.
- Factory applications and emissions era: The two engines were offered in different model lines and market conditions of their era, affecting availability of parts and compatibility with emissions equipment.
Understanding these distinctions helps buyers and builders anticipate what parts can be reused, what must be replaced, and what may require professional machining or fabrication.
In practice, these differences matter for reliability, performance, and restoration accuracy. They also influence how straightforward a swap or build will be for a given vehicle.
Interchangeability and swap considerations
If you’re contemplating swapping engines or rebuilding an existing setup, here are practical considerations to guide decision-making. The aim is to outline what is likely to fit and what may require more extensive work.
- Block and mounting compatibility: Although both engines share the 335-series lineage, exact casting numbers and mounting points can differ; verify block compatibility with engine mounts and chassis mounts for your vehicle.
- Crank and flywheel/balancer fitment: Matching crankshafts, flywheels, and harmonic balancers to the transmission and pulley system is essential; a mismatched setup can cause alignment and balance issues.
- Intake, exhaust, and carburetion: Intake manifolds and exhaust headers/manifolds may not line up without adapters or fabrication; carburetor or EFI systems may require adaptation.
- Intake and accessory drives: Accessory brackets, water pumps, and belt drives may differ between engines, potentially affecting accessory alignment and belt routing.
- Transmissions and bellhousing: The bellhousing pattern and transmission compatibility can differ; confirm that the chosen transmission mates correctly with the engine block and crankshaft.
- Emissions and electronics: Depending on the vehicle year and region, emissions equipment and engine-control interfaces can complicate a swap; carbureted setups may be easier than fuel-injected configurations from later eras.
For most projects, swaps between the 351M and 400 require a careful mix of compatible blocks, fabricated mounts, and sometimes custom exhaust and fuel-system work to ensure reliability and compliance with local regulations.
Summary
The Ford 400 and the 351M are not the same engine, though they share a common 335-series lineage. They differ in displacement, internal components, and fitment details, which means they are not directly interchangeable in most applications without significant modification. If you’re restoring a Ford from this era or planning a swap, verify exact block casting numbers, assess crank and piston compatibility, and consult contemporary manuals and community resources to confirm compatibility for your specific vehicle and configuration.
What is the difference between a 351M and a 351C?
The main differences between the Ford 351C (Cleveland) and 351M (Modified) are the block and bellhousing pattern, with the 351C using a small block pattern and the 351M using a big block pattern, which affects transmission compatibility. The 351M also has a taller deck, a different motor mount setup, and was primarily a low-compression, emissions-focused engine compared to the performance-oriented 351C.
This video explains the differences between Ford's 351C, 351M, and 351W engines: 1mMyvintageiron7512YouTube · Nov 17, 2016
| Feature | 351C (Cleveland) | 351M (Modified) |
|---|---|---|
| Block Design | Small block design, same as the 302/351W. | Taller "tall deck" block, shared with the 400 V8. |
| Bellhousing Pattern | Uses the small block pattern. | Uses the big block pattern, same as the 429/460 engines. |
| Transmission | Compatible with a small block transmission. | Requires a big block transmission. |
| Motor Mounts | Typically has a two-bolt motor mount setup. | Uses a three-bolt setup. |
| Intake Manifold | Wider intake manifold due to a lower deck height. | Wider intake manifold due to a taller deck height. |
| Performance Focus | Designed for performance with high-flow heads. | Designed for emissions control, with low compression, though it can be built for performance. |
| Crankshaft | Smaller 2.750-inch main journal diameter. | Larger 3.000-inch main journal diameter. |
| Pistons | Not interchangeable with the 351M. | Not interchangeable with the 351C. |
(this||self).Wufxzb=function(c,e,f,l,k){var d=document.getElementById(c);if(d&&(d.offsetWidth!==0||d.offsetHeight!==0)){c=d.querySelector("div");var g=c.scrollWidth-c.offsetWidth,h=Math.min(e?g:0,g);c.scrollLeft=e&&(l||f)?0:h;var a=d.getElementsByTagName("g-left-button")[0],b=d.getElementsByTagName("g-right-button")[0];a&&b&&(e=RegExp("\\btHT0l\\b"),f=RegExp("\\bpQXcHc\\b"),a.className=a.className.replace(e,""),b.className=b.className.replace(e,""),h===0?a.className="pQXcHc "+a.className:(a.className=
a.className.replace(f,""),k&&c.classList.add("pA30Ne")),h===g?b.className="pQXcHc "+b.className:(b.className=b.className.replace(f,""),k&&c.classList.add("FpCCub")),setTimeout(function(){a.className+=" tHT0l";b.className+=" tHT0l"},50))}};}).call(this);(function(){var id='_qdAlaZSkBryokdUPzbrA6QM_195';var rtl=false;var gecko=false;var edge=false;var soh=false;
(this||self).Wufxzb(id,rtl,gecko,edge,soh);})();
What does the M mean in a 351M engine?
There exists debate as to what Ford meant by the "M" designation of the 351M. Some claim the "M" stands for “Modified” - being modified from a 400-V8 with a shortened stroke - though others claim that the "M" refers to the Michigan Casting Center, where the 351M began production.
What Ford engine to stay away from?
Ford engines to avoid include the 6.0L and 6.4L Power Stroke diesels, known for catastrophic failures like EGR and head gasket issues. Other problematic engines are the 5.4L Triton V8 for its spark plug and timing chain problems, the 3.8L and 4.2L SXV6 engines for head gasket failures and timing chain issues, and certain EcoBoost models (especially early 2.7L and some 1.0L, 1.5L, 1.6L) that have faced problems with oil consumption, turbochargers, and coolant leaks.
This video discusses problematic Ford engines including the 6.4L Power Stroke: 1mAutotrendsYouTube · Jun 29, 2025
Diesel engines
- 6.0L Power Stroke (2003-2007): Prone to injector, EGR cooler, and head gasket failures.
- 6.4L Power Stroke (2008-2010): Suffers from fuel dilution, clogged DPFs, and cracked pistons.
V8 engines
- 5.4L Triton (1997-2010): Has issues with spark plugs breaking during removal, cam phaser failures, and timing chain stretch.
This video explains why the 5.4L Triton engine is considered one of the worst: 53sSeats & WheelsYouTube · Aug 10, 2025
V6 engines
- 3.8L SXV6 (1982-2008): Often experiences head gasket failures.
- 4.2L SX V6 (1997-2008): Has a problematic timing chain system with plastic guides that can fail and lead to catastrophic engine damage.
EcoBoost engines
- Early 2.7L EcoBoost (circa 2016-2017): Some models had problems with defective valve guides, turbocharger failures, and leaking plastic oil pans.
- 1.0L, 1.5L, and 1.6L EcoBoost: Some versions of these engines have reported issues with coolant intrusion into the engine block, overheating, and turbocharger failures.
- 3.5L EcoBoost: Common complaints include carbon buildup on intake valves, premature timing chain wear, and water pump failures.
This video explains some of the issues with the EcoBoost engine: 1mExoticCar PlayPlaceYouTube · Nov 15, 2025
Other engines to be cautious about
- Certain 2.0L, 2.5L, and 3.6L boxer engines: Some model years are less reliable.
What is the difference between the 351M and 400?
In fact, the only functional difference between the 351M and 400 is the crankshaft and pistons. Ford did use slightly different components in the 351M and 400, such as harmonic balancers, camshafts, and valve springs, but functionally, the engines were identical except for the crankshaft and pistons.
