What to Do About Rust on Auto Engine Cylinder Walls?by William Zane
If an engine is removed from a car and improperly stored, rust will often appear on the cylinder walls. The cylinder walls are the surfaces of the holes in the block that are bored out for the pistons. Obviously, this is a crucial area of a motor, since the pistons and rings slide up and down against this surface as the motor is running, usually at many thousands of RPMs. Not only must this surface be completely smooth and clean, but it must also be free of rough spots and pitting.
How Much Rust Is There?
How much rust present in an engine's cylinder walls will determine how easy it is to remove the rust and whether the block will even be able to be used in a running motor. If there is simply surface rust that can be scrubbed off to clean metal, then the block can probably be used. If the rust has caused pitting, where there are pockmarks eaten into the metal, it may not be able to be used. There are a couple of reasons the block would not be usable. One is that machining the bores to the point where the pitting is no longer present may result in a bore that is too large for any available pistons. The other is that the amount of boring might be beyond the manufacturing tolerances and would be too close to the water jackets or even the outside of the block.
If there is minimal rust in the cylinder walls, it can probably be removed with a fine steel wool and naval jelly, which is a chemical for rust removal. Simply apply the naval jelly to the steel wool and gently rub it up and down the cylinder walls until the rust is gone. Once the rust is gone, clean and preserve the cylinder walls with WD40 or a light coat of motor oil.
If there is more rust than steel wool will easily remove, the next step would be to use a flex hone, which is a round wheel with aluminum balls on it. This attaches to a drill and cleans the cylinders quite well at a microscopic level without damaging the cross-hatching that machine boring provides.
The next step after flex honing is honing. This is similar to flex honing, but uses blades to remove more material. This method will get rid of rust, but it is easy to score the cylinder and ruin the cross-hatching, which can result in inconsistent compression and other problems once the engine is rebuilt and running. This is best left to the professionals unless you are experienced with this type of honing.
The most extreme method and the most sure-fire way to get rid of rust is by having the cylinders bored by a professional machine shop. This is also the only method that will work if there is any serious amount of pitting. Again, though, make sure the block will still be usable when it is bored out past the pitting. Most machine shops charge around $30 per cylinder for boring.