The Harley-Davidson 120ST engine was marketed as a high-performance upgrade, but many riders have encountered serious issues. If you’re considering this engine, it’s important to understand its common problems and potential alternatives.
1. Crankshaft Weakness
One of the biggest concerns with the 120ST is its stock crankshaft. Riders have reported that it doesn’t hold up well under stress, leading to failures similar to those seen in other Harley factory cranks. In some cases, the end of the crank gets damaged near the cam plate, causing a drop in oil pressure and internal wear. A few mechanics suggest that without reinforcement, the 120ST crank is just as vulnerable as the stock 103 and 110 engines.
2. Reliability Issues Over Time
Some riders who installed the 120ST noticed problems within just a few thousand miles. These include:
- Oil pressure drops due to internal wear
- Premature bearing failure, requiring expensive repairs
- Overheating concerns, especially in hot climates
While the engine offers more power, these issues raise concerns about long-term reliability.
3. Comparison to the 110 Tire Shredder Kit
A common alternative to the 120ST is the 110 Tire Shredder Kit, which is sometimes installed on a new short block instead of going with a full 120ST engine. On paper, both setups share similar components, but the 120ST has a reputation for weak lower-end durability.
Some mechanics recommend skipping the 120ST altogether and opting for a 124-inch S&S crate motor, which offers:
- A stronger crankshaft that outlasts stock Harley parts
- Better overall reliability, reducing the chance of expensive failures
- A warranty option through S&S-approved shops, making repairs easier if issues arise
4. The Cost Factor
If you’re already dealing with a damaged 103 or 110 engine, replacing it with a Harley short block and upgrading with a Tire Shredder Kit might save a few bucks upfront. However, many riders argue that it’s only a temporary fix. Over time, repairs on the 120ST can add up to the cost of an aftermarket engine that would have been more reliable from the start.
Before making a decision, riders should consider whether they want factory-backed support with potential reliability concerns or a stronger aftermarket engine that might require independent shop work.
5. Key Maintenance Considerations
If you decide to go with the 120ST, proper maintenance is critical to avoid common failures. Riders who have installed this engine strongly recommend the following steps:
- Replace the oil lines – Any debris from a previous engine failure can damage the new motor.
- Clean the oil pan – Metal shavings from a worn-out crankshaft or cam plate can circulate through the system if not properly cleaned.
- Flush the oil cooler – A contaminated oil cooler can introduce old debris into the fresh engine, leading to premature wear.
These precautions help prevent oil pressure issues and premature crankshaft failure, two of the most frequent problems reported with the 120ST.
6. Long-Term Reliability Concerns
Even with proper maintenance, some riders report that the 120ST’s crankshaft isn’t much better than stock Harley-Davidson components. Several experienced mechanics argue that if you’re upgrading, you might as well go with a stronger aftermarket crank or choose an S&S 124 crate motor instead.
One major issue is that Harley dealerships will only replace broken parts with the same stock components, meaning if the 120ST fails, you could end up dealing with the same problems again. Aftermarket options, on the other hand, allow for stronger parts that improve reliability and longevity.
7. Cost vs. Value: Is the 120ST Worth It?
When it comes to cost-effectiveness, the 120ST is a mixed bag. While it offers some improvement over stock Harley components, many feel that for the price, you’re not getting the long-term reliability that comes with high-performance aftermarket engines like the S&S 124.
If you’re on a budget and already have a bolt-on kit, pairing it with a new short block, upgraded heads, and cams can be a viable option. However, some dealerships may be reluctant to warranty this combination, especially if it’s not a full factory-installed engine.
Additionally, some riders recommend buying a new short block or opting for a full aftermarket crate motor, like the S&S 124, which boasts a better crankshaft, improved heads, and greater overall reliability. Though these options might come with a higher upfront cost, they could save you money in the long run by avoiding repeat failures.
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1. What are common Harley 120ST problems?
The Harley 120ST often faces issues like engine overheating, oil pressure drops, and crankshaft failures. These problems can lead to expensive repairs, so regular maintenance is crucial.
2. Is the Harley 120ST reliable for long-term use?
While the Harley 120ST is a solid option for riders seeking factory parts, many riders report reliability concerns, especially with crank failure. Consider upgrading to a S&S 124 for better durability.
3. Should I choose a Harley 120ST or S&S 124 engine?
If you’re looking for long-term reliability and performance, the S&S 124 is often preferred over the Harley 120ST. The S&S offers a stronger crank and more robust components, leading to fewer issues.
4. How can I prevent Harley 120ST engine failures?
To reduce the risk of failures in the Harley 120ST, maintain regular oil changes, replace oil lines, and clean the oil pan. Investing in a quality oil pump and cam plate can also help.
5. What is the best upgrade for a Harley 120ST engine?
Upgrading to a S&S 124 crate motor or adding a better aftermarket crank and heads can improve the Harley 120ST’s performance. This upgrade addresses many common issues, providing long-term reliability.
6. Does the Harley 120ST have a good warranty?
The Harley 120ST warranty coverage varies by dealer and region. However, some riders suggest opting for S&S engines if warranty and long-term support are important to you.
7. How much does upgrading to a Harley 120ST cost?
Upgrading to a Harley 120ST engine can be costly, especially when factoring in labor, parts, and possible replacements like the oil pump and cam plate. Some riders find it more cost-effective to go for a S&S 124 motor.
8. Can I trust a Harley 120ST with long-distance riding?
The Harley 120ST can be suitable for long-distance riding, but its engine problems may arise during extended use. Consider regular maintenance and possibly upgrading to S&S for better durability.
Final Thoughts
Ultimately, the Harley 120ST can work for riders who prefer to stick with factory parts and are willing to invest in maintenance to keep it running smoothly. However, for those seeking durability and long-term peace of mind, upgrading to a S&S 124 crate motor or similar aftermarket engine may prove to be a wiser investment.
Whatever you decide, make sure to carefully consider your needs, your riding style, and the importance of warranty coverage before making a final decision on your engine upgrade.
