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What are Piston Sleeves, and why are they Important in Performance Engines?

The only time a sleeve is needed is when the cylinder is cracked or insufficient material in the engine's cast to bore the cylinder. In either situation, for a sleeve that is interference fit, the cylinder that requires repair can be machined, which means that it will need to be pressed into the block of the cylinder.

If a cylinder has to be "bored out" to repair a cracked or otherwise damaged engine, sleeves can also be used to restore a specific bore size. To boost the engine displacement, some engine builders will also "overbore" their cylinders. Diesel engines are sleeved more frequently than gasoline, but there are also reasons to sleeve gas engines. High-performance diesel engines can generate significant pressure inside the bore so that cylindrical damage is relatively standard. The engine can be significantly protected from these extreme pressures by sleeves intended for high-performance applications.

Bore damage comes from extreme temperatures that may distort the pistons or melt them. When the piston is distorted to this extreme, the cylinder bore is galvanized to leave melted aluminum behind. This is generally referred to as a 'burnt hole' and is found in only one cylinder in most instances. The engine block can be machined to accept a replacement sleeve, repairing the lousy cylinder and requiring only one replacement piston, saving the rest of the block.

The main benefit of boring the cylinders and installing oversized pistons and rings is the cost savings of not having to replace the pistons and rings. Furthermore, many blocks are too thin to accommodate over boring without sleeves reliably. And if we're talking about an aluminum block with integral iron sleeves, the only way to save or modify the block may be by machining out the original sleeves and installing new ones.

Dry Sleeves

Most of the aluminum OEM automotive engine blocks use dry, gray iron cylinder sleeves in their engines. A dry sleeve is either cast into the block or pressed into the bore. Heat is transferred from the sleeve by the aluminum bore to the coolant that contacts the bore. 

Simply put, the coolant does not contact a dry sleeve. Instead, it is installed in the cylinder block on the wall of the cooling jacket. In contrast, the coolant in a wet sleeve comes into direct contact with the sleeve. Compared with wet sleeves, dry sleeves also tend to have thin walls. 

Wet Sleeves

Between the engine block and liner, wet sleeves often have a cooling water gap. They can also be produced for the integration of cooling passages. Sleeves can also be referred to as water-jacket sleeves with cooling passages. 

Wet sleeved cylinders have better cooling and a more even distribution of temperature due to being in direct contact with the coolant, but this design makes the engine less rigid as a whole.

Performance Applications

Sleeves also offer several advantages for high-performance applications. The distance between the bore centers and the casting thickness limits the displacement of an engine block. If the block is a weighty wall cast, you can increase the motor's displacement by boring the cylinders oversize and using a stroker crank. Stroker cranks are standard because they require fewer adjustments. For low rpm torque, long-stroke cranks are good, but a short stroke, large bore oversquare configuration is better for high revving power. Consequently, you may want to increase the bore size if you are building a high revving performance engine rather than relying so much on increased stroke to generate more power.

You can only remove so much metal before running out of block to support the larger cylinder sleeves if you are installing dry sleeves in a block to increase displacement. Doing a wet sleeve conversion is one way to overcome this limit. The existing cylinders are removed from the machine, and wet sleeves are installed in place. Installation involves extensive block modifications and requires accurate CNC machining so that wet sleeves can be accepted, but the results are worth it. Wet sleeves can typically handle much higher horsepower and heat loads because the coolant is in direct contact with the outside of the sleeve. Consequently, as well as more displacement, you gain increased strength and reliability. There are wet sleeve conversion kits for specific late-model import engines as well as domestic V8s available.

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