Resleeving is a term which deals with replacing sleeves within an engine, most commonly that of a diesel engine. "What", you exclaim, "I didn't know an engine wore clothing, especially anything with sleeves!" Fear not, the matter will become clear as we progress.
First we have to understand a bit about the internal combustion engine. Within the engine a mixture of fuel and air is combined within a cylinder where it is ignited by a spark or by the extreme pressure of compression. The cylinder is a round shaft within the engine block which both contains and focuses the force of the fuel/air explosion into driving a piston downward which then turns a crankshaft. The crankshaft rotates and causes, through an intricate system of clutches and gears, the rotation of the wheels. This series of events is what takes you to work, shopping, or whatever other destination you choose.
Though the number of cylinders/pistons vary greatly, they are similar in function. The number isn't important to this discussion, their function, however, is germane to understanding resleeving. The cylinder holds within its diameter, (the bore), a piston. The piston has a number of rings which contact the cylinder wall. Rings serve two main purposes, one of which is to contain the force of combustion within the cylinder (compression rings), the other to scrape oil from the cylinder wall back down into the oil sump (oil rings). The lubrication from oil is needed to reduce (friction) between the rings and the cylinder wall.
With the finest lubrication available there is still friction between the rings and cylinder wall. Over time this causes wear or erosion of both the cylinder wall and the rings.
Replacing rings is relatively inexpensive; replacing cylinders is very pricey. One solution is to re-bore the cylinders and replace the rings with ones of a larger diameter, creating a new environment which is as good as new with one exception. Rings can be replaced a hundred times, but re-boring cylinders can only be done a very few times. The re-boring removes metal from the cylinder wall, reducing its strength and eventually causing the block to crack under the massive pressure and heat of combustion. What ever is a mother to do?
There is a simple solution. A steel or alloy sleeve is fitted within the cylinder. The piston rides up and down within the sleeve instead of the solid cylinder. The sleeve becomes worn by the friction instead of the engine block itself. All one need do is pull out the worn sleeves, replace them with shiny new ones, replace the worn rings, reassemble and your engine is as good as new again. This process is called resleeving an engine and it makes the rejuvenation of an engine a simple process where before the development of sleeves, it was a very limited option.
It is possible to replace a single sleeve or the entire set. Replacing a single sleeve however creates an unbalanced engine in that the friction within a new sleeve is greater than that of its neighbors which are worn or 'broken in'. This unbalanced situation can create uneven wear in other components. I would recommend replacing parts like sleeves and rings as a set instead of as individual components. The initial expense is greater but in the long term pays for itself in better engine service and less repair.