The Ford 351 Cleveland engine, a 5.8-liter small-block V8, was built from 1969 to 1974 in small blocks by Ford. The engine is a 90-degree overhead valve V8 from Ford’s “335” engine family. It was only made until 1974, despite the 335 engine family being in production until 1982. The 351C was designed from the ground up to be a high-output engine.
As emissions regulations tightened and the heyday of high-performance muscle cars came to an end in 1974, the 351C’s appeal waned. It was eventually retired.
The 351C is referred to as the Cleveland because it was made in Brook Park, Ohio, a suburb of Cleveland, where it was designed. Despite coming from different engine families, the 351 Cleveland and 351 Windsor are strikingly similar. Despite the fact that the 351 Cleveland and other 351 engines had many similarities, such as the same 351 cubic inch displacement, bore spacing, and cylinder head bolt pattern, the 351 Cleveland and other 351 engines had many differences.
The 351C weighs more due to the additional two inches on the block. In comparison to the 351W, the block had a completely different water pump housing that was integrated into the block. Additionally, the intake manifold is dry, as opposed to the Windsor’s wet one, and the spark plugs are smaller, as well as the rocker cover having eight bolts rather than six like on the Windsor.
Ford moved production of the 351 engines from Windsor to Cleveland when the Windsor plant hit capacity constraints. Freer flowing heads were among the numerous changes made by Ford to the engines built at the Cleveland plant.
351 Cleveland Engine Variations
Numerous high-performance engines were derived from the 351 Cleveland’s basic design, in addition to the base model. In contrast to the H-code engine, which was designed for low-performance base models, the other engine variants were typically found in high-performance vehicles.
Each of these 351 Clevelands has its own unique combination of combustion chambers, compression ratio, tappet height and duration as well as different valve train components.
There is a difference between the 351 M-code and the 351M engine. This 6.6L engine was introduced by Ford in 1969 as the Ford 400, a pushrod V8 built on the 351 Cleveland’s 4.0-inch bore/stroke ratio and 402 cubic-inch displacement.
Ford de-stroked and shortened the crankshaft stroke on the 400 engine in 1975 to create a 5.8L, 351 cubic inch unit. The 351M is a different engine from the 351C “M-code,” which was also known as the 351.
|351 H||1970-1974||Low, open chamber||Base, low performance engine used in most non-performance Ford’s|
|351 M||1970-1971||High, closed chamber||Torino, Mustang, Cougar, Montego|
|351 R “Boss 351”||1971||High, closed chamber||Boss 351 Mustang only|
|351 R “HO”||1972||Low, open chamber||1972 Ford Mustang|
|351 Q “CJ / Cobra Jet”||1971-2974||Low, open chamber||Ranchero, Torino, Montego, Cougar|
Ford 351 Cleveland Engine Problems
- Cracked Heads
- Worn Valve Guides
- Thin Cylinder Walls & Block Weakness
- Thermostat Failure & Overheating
- Bearing Failure from Oil Starvation
1. 351 Cleveland Cracked Heads
The cylinder heads were a significant upgrade from the 351W in the 351C. The cylinder head was produced in two variations. The ports and valves on 351Cs equipped with two-barrel carburetors were larger. Ports and valves on 351Cs equipped with a four-barrel carburetor were even larger. Both models flowed significantly more air than the 351W counterparts. That’s why a lot of enthusiasts build a 351 Clevor by pairing up a 351W block with a 351C head for high-end performance.
Despite the 351C heads’ high performance, they had a flaw: they were cracking. However, their main flaw is that they tend to overheat. When it comes to 351C engine cracked heads, the most common problem is from overheating. When it comes to overheating, a faulty thermostat is a common culprit. Other common causes include faulty radiators and radiator caps, broken water pumps, and coolant leaks.
Cracks in the exhaust seat and lifter valley of the 351 Cleveland cylinder head are the most common occurrences. Cracks are more common in bored engines making a lot of power, but stock engines can also have them.
Metal has a thermal expansion/contraction relationship. The heads can only withstand a certain amount of heat. Exceeding this temperature range can cause microscopic cracks to form, which can worsen due to internal engine pressure and additional heat.
351C Cracked Head Symptoms
- Coolant leaks
- Engine overheating
- Cylinder misfires
- Poor acceleration and performance
- Rough idling
Cracked heads can only be repaired by buying new ones. This is the only option for cracked heads. Aluminum, the most common material for modern heads, has a higher heat tolerance than steel. The Edelbrock RPM Performance Heads are popular among owners who want to increase their engine’s power.
2. 351C Worn Valve Guides
There is a valve guide for each valve in the cylinder head. Valve springs and guides work together to keep the valve in proper contact with the valve seat when the valve is open. The guides are pressed into the cylinder head and are made of cylindrical metal pieces. Once inside the valve guide, the valves move up and down as needed. Additionally, the guides keep gases and oil out of the combustion chamber by ensuring proper seat contact during intake strokes.
Valves wear out quickly due to the constant contact between the valve stem and the valve guides. Having insufficient oil in the valve guides can cause them to wear out faster than they otherwise would. This is a common problem on 351 Clevelands. When the valve guides are worn out, the valves can become misaligned and close at an angle to the seat, resulting in cracked valves.
It is possible to repair worn valve guides, but it takes considerable skill. Increase the valve guide bore and install thicker valve guides are two common machining techniques used by machinists. Replace worn out valve guides by reaming instead of buying new heads.
Worn Valve Guide Symptoms
- Smoke coming from exhaust (caused by oil leaking into combustion chamber)
- Excess oil consumption
- Cylinder misfires
- Poor performance
- Lack of acceleration
The most obvious symptom is black exhaust smoke. Most of the time, this happens when the engine is still warm from storage. When the engine is turned off, oil will leak into the combustion chamber and be burned as soon as the engine is started.
The majority of the smoke will disappear once the engine has warmed up. The second most obvious sign is high oil consumption, which is accompanied by thick black exhaust smoke.
3. Thin Cylinder Walls – 351 Cleveland
When a 351W block is combined with 351C heads, it’s dubbed the 351 Clevor by enthusiasts. Because 351C blocks are less powerful and hold less power potential, people prefer 351W blocks.
Despite the fact that the 351C had a larger and heavier block, it had a cylinder with extremely thin walls. The performance potential is restricted due to the thinness of the walls. Cylinder walls can crack when you overbore or try to make too much power with a stock block.
With stock 351C engines, cracked blocks are not a problem. I felt it was important to point out the limitations of the block given the large number of people who modify these engines.
351 Cleveland Maximum Overbore
Ford Overbore Recommendation: 0.030″ overbore maximum
Performance Community Recommendation: 0.040″ overbore maximum
Maximum Overbore: 0.060″ overbore (not recommended)
Due to the thin cylinder walls, Ford has publicly recommended an overbore of no more than 0.030′′. Although we’ve heard of bores up to 0.060″, the aftermarket community generally recommends sticking with a 0.040″ overbore. For the avoidance of doubt, we do not recommend going over the generally accepted maximum of 0.040 inches.
Having the cylinder sonic tested is a good idea if you’re worried about over-boring or block strength. The thickness of the cylinder walls is measured using sonic testing. A 351 Cleveland’s minimum reliable cylinder wall thickness is 0.12 inches. In the performance community, this level is considered reliable, but anything thinner than that is more likely to break in high horsepower applications..
4. 351C Thermostat Failure & Overheating
The thermostat for the cooling system keeps track of engine temperature and regulates coolant flow. The thermostat is located halfway between the radiator and the engine. When the engine is cold, the thermostat is in the closed position, recirculating coolant through the engine and preventing it from entering the radiator to quickly heat the engine. To cool down, the coolant must first pass through the radiator and be cooled before it returns to the engine. Once the engine has warmed up, the thermostat will open.
The 351C has a one-of-a-kind thermostat. The 351C does away with the intake manifold and uses a direct head-to-block coolant flow. After that, coolant is pumped into a chamber beneath the thermostat and circulated there. Underneath the thermostat on the 351C model is a “bypass orifice plate,” which serves as a restrictor. Recirculation of coolant occurs when the thermostat is closed and this restrictor plate is in place.
A restriction plate closes after engine warming, allowing coolant to flow to radiators where it can be cooled before returning to the engine. Water will flow directly to the water pump instead of the radiator when the restrictor bypass plate is missing, resulting in constant overheating.
When the thermostat is in the open position, it is responsible for sealing the bypass valve. The bypass valve will not be blocked if you use a standard thermostat on these engines, resulting in overheating. If you remove the restrictor plate from your engine while it has the proper thermostat installed, your engine will overheat and fail.
351 Cleveland Thermostat Failure Symptoms
- Engine overheating
- Coolant leaks around thermostat housing
- Rapid temp increases (thermostat stuck closed)
351C Thermostat Replacement Options
The most critical step is to use a 351C thermostat to replace the old one. The thermostat from a regular Ford won’t do the trick. When changing the thermostat, leave the restrictor plate in place. Many people remove the restrictor plate because it appears to obstruct coolant flow. If you remove it, your computer will overheat.
5. Oil Starvation & Main Bearing Failure – 351 Cleveland
The 351C engine was designed with cost savings in mind by Ford engineers. The oiling system was one place where they found savings to be had. Unlike the 351W and Windsor engine families, which had three main oil galleries, the 351C only had two. The oiling system was designed in such a way that oil was sent to the cam bearings before the main bearings.
When it comes to low-performance H-code versions, this isn’t a big deal, but when it comes to high-performance 351C versions, this is a big deal. Oil starvation was most severe in these high-performance applications, resulting in low oil flow to the mains and the possibility of bearing failure.
To make matters worse, the lifter bores had too much clearance, allowing oil to escape through the holes. As a result, there was a worsening of oil flow to the mains, which led to additional oil shortages.
If you plan on running high power applications, keep in mind that oil starvation only occurs at high RPMs in the 351C. However, the 351C can be made to run reliably at high RPMs by modifying the oiling system. If you’re interested in learning more about 351C engine lubrication, check out this informative article.
Main Bearing Failure Symptoms
- Noises from engine / rod knock
- Low oil pressure
- Metal shavings in oil
351 Cleveland Reliability
Heat and power are the 351 Cleveland’s downfalls. While a stock 351C is a reliable engine, adding more power makes things more complicated for the engine. In addition, the oiling system is insufficient without modifications because of the thin cylinder walls, which can cause reliability issues and block cracks during overboring.
Furthermore, the cooling systems in these vehicles are inadequate for vehicles with significant amounts of power and must be upgraded for high-performance use. Having said that, with the right modifications, these engines can be extremely dependable even in high horsepower applications.
The 351 Cleveland is a tough engine, and it’s unlikely that cracked heads, blocks, main bearings, internals, or anything else will cause problems.
Despite its reliability, the 351C engine requires a lot of work and money to be used in high horsepower applications.