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Ford Focus ST-225 Mk2 Block Mod (Does It Work?)

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Last Updated on: 7th September 2023, 12:47 am

Mk2 Ford Focus ST225s that have been tuned or modified often encounter an issue where the cylinder liners crack.

To address this, there’s an engine modification known as the ‘block mod’ designed specifically for the Mk2 Focus ST225 and other cars with this 5 cylinder Volvo engine.

This problem is believed to result from a weak point between the liners, where “heat-swelling” could lead to minor fissures.

A block mod modification involves inserting small metal shims in the spaces between the cylinder liners, which reinforces the engine, reduces hotspots and prevents heat swelling, thus averting potential cracks.

Many tuning firms consider this modification to be crucial if you’re planning to enhance your Focus ST’s power beyond its original specifications.

It’s worth noting that cases of cylinder liner cracking have been reported even in vehicles that maintain factory power levels.

How the Block Mod Works

While the exact effectiveness of the block mod in entirely preventing cylinder liner cracks remains unclear, it is hypothesized that its success, if any, could stem from the reinforcement it provides to the liners.

The inclusion of metal shims within the tiny coolant channels nestled between the engine’s liners could enhance their durability and rigidity.

This prevents swelling and shifting, theoretically lowering the chance of the metal fracturing.

Nevertheless, there are opinions suggesting that inserting shims into the cylinder liners might actually be more detrimental than beneficial.

> Cracked cylinder liner signs and causes

mk2 focus st225 block mod

Shimming the Cylinder Liners to Fix a Cracked Liner?

A cracked cylinder liner typically necessitates an engine replacement or, at the very least, the installation of new cylinder liners, driving many to seek a more cost-effective solution.

The method of inserting shims into the cylinder liners not only adds resilience but also seals the coolant passage.

This blocks the coolant from leaking into the combustion chamber via the liner cracks, potentially providing a short-term remedy.

However, it’s uncertain if this truly offers a permanent solution.

Some individuals report that this method resolved their cracked liner problem, while others claim it was merely a stop-gap solution and the cracks continued to grow and spread.

st225 block mod metal shims

Is the Block Mod Worth Doing?

This largely depends on individual perspectives. Numerous esteemed tuning specialists advise putting in a block mod before altering or remapping your Focus ST.

Conversely, a considerable number of ST owners propose avoiding it.

It appears this problem is somewhat like a gamble, where some people manage to operate their ST225 at high power levels without any block mod and encounter no issues.

Meanwhile, others with standard power Focus STs experience cylinder liner cracking.

We strongly suggest that you delve into this matter independently before making a decision. Seek advice from forums and gauge people’s opinions.

How to Prevent a Cracked Liner

There are essentially two methods to almost completely ensure that you won’t face a cylinder liner crack.

  1. Implementing superior quality liners
  2. Exchanging with an RS engine or RS block

The optimal way to fully safeguard against cylinder liner cracks is either to install enhanced steel liners or to substitute the entire engine or engine block with the RS model.

Note: There have still been some instances of cracked liners in RS engine blocks, so this may not even be a fool-proof method either.

References & More Information

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  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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