The 5 Most Common Ford 351 Cleveland Engine Problems

Ford’s 351 Cleveland engine is a small-block 5.8L V8 produced from 1969 until 1974. The 351C was built to be a high-performance engine. However, it was actually a bit less capable than the 351W, it’s less performance oriented brother. It also doesn’t quite have the same reputation for reliability.

The 351 Cleveland’s are known to suffer from cracked heads, bearing starvation, worn valve guides, weak blocks, and failed thermostats. We’re going to discuss each of these in-depth below as well as provide our thoughts and experience on overall reliability.

Ford 351 Cleveland Engine Problems

• Cracked Heads
• Worn Valve Guides
• Thin Cylinder Walls & Block Weakness
• Thermostat Failure & Overheating
• Bearing Failure from Oil Starvation

1. Cracked Head Problems

One of the most material improvements of the 351C over the 351W was the cylinder heads. Two versions of the cylinder head were produced. 351C’s with two-barrel carburetors had larger ports and valves. 351C’s with the four-barrel carburetor had even larger ports and canted valves. Both versions were flowed significantly more air than the 351W heads. This is the reason many enthusiasts combine the 351W block with the 351C head for performance applications, making a 351 Clevor.

Despite the performance prowess of the 351C heads, their weakness was cracking. However, in reality their weakness is overheating. Overheating is the most common cause of cracked heads in the 351C engine. While thermostat failure is a common cause of the overheating, bad radiators and radiator caps, failed water pumps, and coolant leaks are also common overheating causes.

351 Cleveland cylinder head cracks most commonly occur across the exhaust seat and in the lifter valley. While cracks frequently occur on bored engines running significant amounts of power, it is also a common problem on stock engines.

Cracked Head Symptoms

• Coolant leaks
• Engine overheating
• Cylinder misfires
• Poor acceleration and performance
• Rough idling

2. Worn Valve Guides

Oil starvation is a common problem on 351 Clevelands which can cause the valve guides to receive inadequate amounts of oil and therefore wear down more quickly than they normally would. If the guides are excessively worn the valves can get off-track and close against the seat at a slight angle, eventually leading to cracked valves.

While worn valves guides can be machined and repaired, it requires significant skill. Machinists frequently use a process called reaming in which they increase the valve guide bore and install thicker valve guides. Reaming can be a cheaper way to replace worn out valve guides compared to buying brand 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

3) Thin Cylinder Walls

The 351 Clevor is a moniker given by enthusiasts when the block of a 351W is coupled with the heads from a 351C. Blocks from the Windsor re preferred because Cleveland blocks are weaker and hold less power potential.

Despite having a bigger and heavier block, the 351C was cast with very thin cylinder walls. Because of the thinness of the walls, performance potential is limited. Overboring or trying to push too much power on a stock block can result in the cylinder walls cracking.

Cracked blocks is not an issue with stock engines. However, with the number of people modifying these engines I wanted to point out the power limitations of the block itself.

Maximum Overbore

Ford Overbore Recommendation: 0.030″ overbore maximum
Performance Community Recommendation: 0.040″ overbore maximum
Maximum Overbore: 0.060″ overbore (not recommended)

If you are concerned about over-boring or block strength, you should have the cylinder sonic tested. Sonic testing measures the depth of the cylinder walls. Minimum reliable cylinder wall thickness on a 351 Cleveland is 0.12″. This level is considered reliable within the performance community while anything more thin than that is likely to break in high horsepower applications.

4) Thermostat Failure & Overheating

The thermostat on the 351C is unique. Rather than flowing coolant through the intake manifold, it flows coolant directly from the head to the block. Coolant then flows into the chamber below the thermostat. The thermostat has a restrictor plate, or “bypass orifice plate” located underneath the thermostat. When the thermostat is closed coolant flows through this restrictor plate and into the water pump for recirculation.

Once the engine warms up, the restrictor plate closes and the majority of the coolant flows into the radiator to be cooled down before being recirculated. If the restrictor bypass plate is missing then the water will continue flowing directly to the water pump instead of the radiator causing constant overheating.

The thermostat itself is unique in that it is responsible for sealing the bypass valve when the thermostat is in the open position. If you use a standard thermostat on these engines, the bypass valve will not be blocked, leading to overheating. Additionally, even with the proper thermostat installed, if you pull the restrictor plate out, your engine will overheat.

Thermostat Failure Symptoms

• Engine overheating
• Coolant leaks around thermostat housing
• Rapid temp increases (thermostat stuck closed)

Replacement Options

The most important thing is making sure you replace the thermostat with the proper 351C thermostat. A standard Ford thermostat will not cut it. Additionally, do not remove the restrictor plate when you swap the thermostat. The restrictor plate looks like it impedes coolant flow which leads a lot of people to remove it. It is supposed to and will cause overheating if removed.

5) Oil Starvation & Main Bearing Failure

Ford’s engineers focused on cost savings with the 351C engine. One area that they determined they could cut costs was in the oiling system. While the 351W had 3 main oil galleries, the 351C was reduced down to 2. The design of the oiling system sent oil first to the cam bearings instead of the main bearings. While this generally wouldn’t be an issue on low-performance H-code variants, the high-performance variants were built for power and high RPMs. These high performance applications were where oil starvation was the greatest, resulting in poor oil flow to the mains and possible bearing failure.

To compound the problem, the lifter bores had excess clearance which would cause oil to leak from the lifter bores. This lead to further oil starvation issues as it worsened oil flow to the mains.

While oil starvation usually only happens at high RPMs in the 351C, it is a limitation worth pointing out for those looking to run high power applications. However, there are oiling system modifications that can allow these engines to run reliably at high RPMs. For those looking to delve further into engine lubrication, here is a great article on 351C engine oiling.

Main Bearing Failure Symptoms

• Noises from engine / rod knock
• Low oil pressure
• Metal shavings in oil

351 Cleveland Reliability

The 351 Cleveland has two achilles heels: heat and power. While a stock one is a reliable engine they start to become more problematic once additional power is added. The thin cylinder walls create potential reliability issues and block cracks with overboring, and the oiling system is inadequate without modifications. Additionally, the cooling systems on these cars are not adequate for significant power and will need to be upgraded for performance applications. With all that being said, these engines can be highly reliable even in high horsepower applications with adequate supporting mods.

Bone stock the 351 Cleveland is a stout engine and likely shouldn’t have any issue with cracked heads, blocks, main bearings, internals, etc.

Overall, the 351C is a reliable engine but will take a significant amount of modifications and additional $$ to be reliable in high horsepower applications.