STW2 vs STW3: Understanding Fuel Filter Differences

When it comes to maintaining your Lister STW engine, understanding the specific components that keep it running smoothly is paramount. Among these, the fuel filter plays a critical role in ensuring that only clean fuel reaches the injection system, preventing damage and maintaining optimal performance. However, for owners of the STW range, a common point of confusion can arise when it comes to identifying the correct fuel filter. Specifically, there's a notable difference between the filters used on the STW2 and STW3 models. This article aims to demystify these differences, providing a clear guide to help you select and maintain the correct fuel filter for your engine.

The Importance of a Clean Fuel Filter

Before delving into the specifics of STW2 and STW3 filters, it's worth reiterating why fuel filters are so vital. Internal combustion engines, particularly diesel engines like the Lister STW, rely on a precise fuel injection system. This system operates under high pressure and involves finely machined components. Even microscopic particles of dirt, rust, or debris present in the fuel can cause significant wear and damage to injectors and pumps. A good quality fuel filter acts as the first line of defence, trapping these contaminants and ensuring that only clean fuel is delivered to the engine's heart. Neglecting to replace a clogged or damaged fuel filter can lead to reduced engine power, poor fuel economy, rough running, and, in the worst-case scenario, costly damage to the injection system.

Identifying the STW2 Fuel Filter

The STW2 model typically employs a specific type of fuel filter. As noted, this is characterised by a small paper element that is housed within a metal bowl. The securing mechanism for this bowl is also distinctive: it is fastened by a single bolt located at the bottom of the bowl. This design means that when it's time for a filter change, the entire metal bowl assembly, containing the paper element, is usually replaced or the element within the bowl is serviced. The simplicity of this design often makes it a more straightforward component to identify and replace.

Identifying the STW3 Fuel Filter

In contrast, the STW3 model features a different fuel filter design. This unit is characterised by a metal body filter element. This element is not typically housed in a separate, easily removable bowl like the STW2. Instead, it is sandwiched between two other components: the filter head and a lower bowl section. The entire assembly is secured by a single, central bolt that is accessed from the top. This "sandwich" style means that when servicing or replacing the filter, the top bolt is removed, allowing the filter head and lower bowl to be separated, revealing the sandwiched element for replacement.

Key Differences Summarised

To make the distinctions clear, let's summarise the primary differences:

Air Filter Considerations for STW Engines

While the primary focus is on fuel filters, it's also important to touch upon air filters for the STW range. The information provided highlights an interesting point: many STW engines, even if they possess an air filter housing, may not actually have a filter element fitted. This housing might be present simply to act as an intake silencer. If your STW engine does have an air filter element installed, it's crucial to note its dimensions when ordering replacements, as there can be variations. The availability of these can also vary, and sometimes a photograph can be helpful when trying to source the correct part.

Other Routine Service Items and Availability

Beyond fuel and air filters, a comprehensive service of an STW engine involves several other components, each with its own considerations regarding availability and maintenance:

• Belts: Availability is generally good, but it's essential to specify if your engine is 24-volt. If the existing belt has no visible numbers, providing the inside length and width is crucial. This is because different pulleys have been fitted to STW series engines over time, and ensuring the belt doesn't bottom out in the pulley groove is important for longevity.
• Fuel Pumps: These are available to order but are noted as being seriously expensive. Due to the complexity of their removal, refitting, and the subsequent need to check injection timing and pump balance, it's strongly recommended that this job is left to experienced individuals.
• Water Pumps: Generally good availability, with impellers and repair kits also being available. New drive shafts are also in stock.
• Exhaust Manifolds: Original manifolds are very difficult, if not impossible, to find. However, replacements can be supplied in stainless steel, made to order.
• Manifolds (other types): Similar to exhaust manifolds, other types may have nil availability of original parts, but stainless steel alternatives can be custom-made to pattern.
• Engine Components: Availability of some STW-specific parts can be patchy. New cylinder heads and barrels are generally unobtainable, and serviceable used items are rare and often suffer from similar issues. While pistons and bottom-end components are often common with the ST model and thus easier to find, cylinder head components are usually specific to the STW but are often available.
• Complete Engines: New engines are long unobtainable. Core units for rebuilding are sometimes available. If you are considering rebuilding your existing engine, an inspection is highly recommended due to potential corrosion issues.

Corrosion: A Critical Consideration for STW Engines

A significant piece of advice for anyone undertaking substantial work on an STW engine is to thoroughly investigate the degree of corrosion present before committing to expenditure. Corrosion, particularly from raw water cooling, can severely compromise the engine's integrity. The process involves carefully removing the water rail (taking care not to leverage it excessively if corrosion is present, as it may break) and inspecting the rail and the water passages in the cylinder head. If these show signs of concern, it’s essential to proceed to check the cylinder jackets by removing the cylinder doors. This will give a clear picture of the extent of any internal corrosion.

Lay-up and Operational Tips

Proper lay-up and operational practices can significantly extend the life of your STW engine and mitigate corrosion:

• For Raw Water Cooled Engines: When laying up a raw water cooled STW, it is crucial to drain the block and pump. Refilling with an antifreeze mix of at least 30% not only provides frost protection but also helps inhibit corrosion.
• For Skin Tank Cooled Engines: If your engine is skin tank cooled, maintain the antifreeze mix in the skin tank year-round for continuous protection.
• Fuel in Oil: A rising oil level and/or oil smelling of fuel are clear indicators that fuel is entering the sump. This is most commonly due to issues with the fuel pumps (requiring overhaul) but can occasionally stem from leaking internal pipes or a defective lift pump. It is imperative not to ignore this issue, as thinned oil loses its lubricating properties, potentially leading to severe engine damage. In extreme cases, there's even a theoretical risk of the engine running away due to fuel vapour in the sump.

Conclusion

Understanding the differences between the STW2 and STW3 fuel filters is a fundamental aspect of maintaining your Lister STW engine. The STW2's bowl-mounted paper element contrasts with the STW3's sandwiched metal body element, each secured differently. Beyond filters, being aware of the availability of other parts, the importance of correct belt sizing, and the critical threat of corrosion will ensure you can keep your STW engine running reliably for years to come. Always consult the specific parts manual for your engine model and, when in doubt, seek advice from reputable suppliers or experienced mechanics.

Frequently Asked Questions

Q1: How do I visually tell the difference between an STW2 and STW3 fuel filter if they are both installed?

A1: The easiest way is to look at how the filter is secured. The STW2 filter is typically in a metal bowl held by a bolt from the bottom. The STW3 filter is a more integrated unit, often with a bolt visible on the top of the filter assembly holding it together.

Q2: Can I use an STW3 filter on an STW2 engine, or vice versa?

A2: No, they are designed for different housings and securing methods. Using the wrong filter could lead to leaks or improper fitting, compromising the fuel system's integrity.

Q3: My STW engine housing is empty. What should I do?

A3: This often means the housing is used as an intake silencer. If you wish to fit an air filter element, you will need to determine the correct dimensions for your specific housing and source a compatible element.

Q4: What is the most common cause of fuel in the engine oil?

A4: The most frequent cause is issues with the fuel pumps, requiring them to be overhauled. Leaking internal fuel pipes or a faulty lift pump are less common but possible causes.

Q5: Is it safe to use the engine if the oil level is rising due to fuel contamination?

A5: No, it is not safe. Thinned oil cannot provide adequate lubrication, and running the engine in this state can lead to severe internal damage. It should be addressed immediately.