Updated: Feb 9
✔ This article has been fact checked.
A common issue with tuned and modified Mk2 Ford Focus ST225s is their cylinder liners cracking, it's theorised that this is caused by a weakness between the liners where "heat-swelling" can occur, causing a small crack.
The block mod is a modification for the Mk2 Focus ST225, the block mod places small metal shims between the gaps in the cylinder liners, strengthening the engine and lessening the heat spots and heat swelling, preventing cracks from occurring.
Some tuning companies class this modification as essential when pushing your Focus ST beyond stock power levels, cylinder liner cracking has even happened on stock vehicles.
How Does the Block Mod Work?
It's not fully understood if the block mod 100% prevents cracking in the liners, but if it does, it likely does so by strengthening the liners.
The metal shims that are added in the small coolant passages between the liners in the engine add strength and stiffness, preventing swelling and movement, theoretically reducing the likelihood of the metal cracking.
However, some people believe that shimming the cylinder liners may actually cause more harm than good.
Does Shimming the Cylinder Liners Fix a Cracked Liner?
Because a cracked cylinder liner usually means the engine has to be replaced, or at the very least needs new cylinder liners to be installed, many people want a cheaper fix to the issue.
Shimming the cylinder liners adds strength and blocks the coolant passage, preventing coolant from entering the combustion chamber through the cracks in the liner. This means it could fix the issue, at least temporarily.
However, whether or not this permanently fixes the problem is not clear, some people say this solved the cracked liner issue and others say it was only a temporary fix and the crack continued to propagate and get larger.
Is It Worth Block-Modding Your Mk2 Ford Focus ST?
This is down to personal opinion. Many reputable tuners recommend installing a block mod before remapping or modifying your Focus ST. However, many ST owners suggest to avoid it.
It seems to be more of a luck-of-the-draw type of issue, in which some people run their ST225 at high power without any block mod and have no issues, some people have stock power Focus STs' and their car gets a cracked liner.
We recommend to do your own research into the subject before making a decision, ask on the forums and see what people say.
Watch the video below to see how the Mk2 Ford Focus ST is block modded.
100% Preventing a Cracked Liner
There are only two ways to pretty much 100% guarantee you won't get a cracked liner.
Installing upgraded liners
Swapping with an RS engine or RS block
Installing upgraded steel liners or replacing the whole engine or engine block with the RS version is the best way to 100% guarantee you won't get a cracked liner.