The cleaning process should remove all of the dirt, grease, oil, rust, scale and carbon deposits that have accumulated over the years. With painted castings, the old paint also has to be stripped away so the surface can be repainted. Even if you’re working with brand new castings, a final cleaning is still necessary after you’ve completed your cutting, resurfacing and honing procedures to remove residual traces of oil, metal chips and honing residue.
Dirt, oil and grease are all soluble to some extent depending on the cleaning solution or chemicals that are used. Chemical solvents are very effective cleaning agents, but their use has been restricted in many areas by rules that limit the release of Volatile Organic Compounds (VOCs) into the atmosphere. Aqueous cleaning solutions that contain detergents or alkalines have replaced many solvents, and typically require a certain amount of heat (130 to 170 degrees F) and agitation to clean effectively.
Hard carbon deposits in the combustion chambers and exhaust ports are probably the most difficult contaminants to remove. Chemical solvents can soften and remove carbon deposits, but they can’t be used universally. If a solvent cannot be used, some type of media blasting may be necessary to dislodge heavy carbon deposits. Combining the scrubbing action of ultrasonics with water-based cleaners can also provide an effective means of removing hard carbon deposits.
Cleaning heads and blocks can be a challenge for a number of reasons. The castings have intricate contours, complex geometries and both interior and exterior surfaces that have to be cleaned. There are water jackets inside the heads and block that may be lined with a layer of rust, scale or sediment. There may be blind holes that trap gunk and debris. Oil lines inside the block may be lined or plugged with sludge and varnish deposits. Combustion chambers and ports are usually coated with hard carbon deposits. The exterior surfaces of iron castings usually have a layer of paint underneath the grease and dirt.
Whether the castings are iron or aluminum makes no difference. Both have to be cleaned down to a bare metal surface. However, the different metals may require somewhat different cleaning techniques depending on the method(s) that are being used. Aluminum and cast iron react differently to chemical cleaning solutions, abrasives and heat.
A highly caustic solution in a hot tank or spray washer can be very effective cleaning agent for removing most contaminants from iron heads and blocks. But if the same solution is used on aluminum, it may be too caustic and etch or discolor the metal. On the other hand, if a milder cleaning solution designed for aluminum is used to clean cast iron, it may take much longer to achieve the same results. There are cleaning solutions and chemicals that work effectively on both types of metal, but the method preferred by many shops is to use a dedicated cleaning process for each type of metal.
Aluminum is a much softer metal than cast iron. If the castings are being blast cleaned, the same media and/or pressure that works well on cast iron may be overly aggressive on aluminum. Likewise, a softer media that doesn’t etch or remove metal on aluminum parts may not be aggressive enough if used to remove carbon and rust on iron castings.
Cast iron heads and blocks can easily withstand 650 to 750 degrees in a thermal cleaning oven. The high temperatures are very effective at transforming oil and grease deposits into ash, which can later be removed by airless shot blasting, blast cleaning or washing. But aluminum can anneal (soften) if it is subjected to 500 degrees F or higher for more than an hour or two. For this reason, the temperature inside a cleaning oven must be limited if aluminum heads and blocks are being cleaned.
High temperatures inside a cleaning oven can also loosen the seats and valve guides in aluminum heads. If the seats and guides are being replaced anyway, it’s not a concern. However, if the original seats and guides are being reused, and you don’t want them to fall out, thermal cleaning may not be the best option.
Restoring a Like-New Finish
One of the biggest challenges with cleaning aluminum heads, blocks and other parts is restoring a like-new appearance. Aluminum stains and discolors rather easily. The layer of oxide that forms on the surface of the metal as soon as it is exposed to air protects the metal against further corrosion, so aluminum does not have to be painted. But over time, the finish turns dull and may even turn black if the part is exposed to overly caustic chemicals during the cleaning process.
Dry blasting aluminum with glass beads, aluminum oxide grit or steel shot (stainless works well but is expensive), can remove discoloration. But this requires an additional cleaning step to make sure no beads, grit or shot are left behind in any of the nooks and crannies of the casting.
Masking off the valve guide and cooling jacket openings prior to bead or grit blasting can reduce the risk of media being retained. Even so, bead, grit or shot blasting removes metal and etches the surface, giving the casting a matte and slightly dull appearance. Depending on what your customer wants (or expects), this may require yet another step in the refinishing process if you have to spray the casting with aluminum or silver paint.
The Benefits of Baking Soda
An alternative approach to cleaning aluminum and restoring a like-new appearance is to use baking soda media with either a wet or dry blast process. The advantages of baking soda compared to other blast media is that it is relatively soft (only 2.5 on the Mohs hardness scale), inexpensive and is water soluble (which distinguishes it from other soft media such as plastic beads or walnut shells). Baking soda is also a very effective cleaning agent that can blast away hard carbon deposits without damaging the metal underneath. It also does not require a prewash, which can save time and labor.
When baking soda hits the surface of the metal, it fractures as it knocks loose the surface contaminants. This creates a lot of dust, which must be contained and filtered within a blast cleaning cabinet. But there are no hard particles left behind to cause problems later on. The residual dust can be rinsed off the part with water, leaving a clean, bright surface that looks like a new casting.
With wet blasting, baking soda (or other media) is mixed with water to form a slurry. The slurry is then mixed with air and directed at the part. One manufacturer’s approach is to use a low pressure pump (only 25 psi) but a high flow rate (180 gallons per minute) with its cleaning equipment. The design of the nozzles, air flow and slurry mixture all affect the cleaning action and effectiveness of the slurry. In some cases, a small amount of aluminum oxide can be mixed in with the baking soda to create a more aggressive cleaning action.
The main advantage with wet blasting is that it eliminates the dust. It is a closed loop process. Consequently, the cleaning cabinet does not have to be located in a separate area to keep dust away from other machining or assembly operations in the shop which can save time and labor depending on how the work flow is organized and positioned.
Scrubbing with Bubbles
Another cleaning process that works well on both aluminum and cast iron is ultrasonic cleaning. Adding an ultrasonic transducer to a hot tank multiplies its cleaning effectiveness. The transducer generates high frequency sound waves that bounce back and forth throughout the cleaning solution. This creates tiny bubbles that implode against the surface of the metal, knocking loose and scrubbing away any contaminants on the metal.
The main advantage of ultrasonics is that it the cleaning action reaches into hidden areas of the casting such as blind holes, crevices and internal passageways. It does an excellent job removing most contaminants, and it does not etch the metal. However, it works best as a "fine" cleaning process. Heavy layers of dirt and grease tend to absorb the energy of the sound waves, so for best results extremely dirty parts may need to be prewashed in a spray cabinet.
The key with ultrasonics is using equipment that has the proper power density and frequency for the parts that are being cleaned. Your equipment supplier can provide guidance on what size tank, fixturing and power rating will best suit your needs.
Bust the Rust
Removing rust from cast iron heads and blocks usually requires a mild acid (such as citric acid or phosphoric acid). Ultrasonic cleaning can also dislodge iron oxide from the surface of castings. However, when a water-based cleaning process is used, some type of corrosion inhibitor should always be added to the solution to prevent rust from reappearing after the parts have been cleaned.
If iron castings will be stored for some time prior to painting, and rust is a concern (especially in damp climates), sealing the parts inside plastic bags that have been treated with special rust inhibiting chemicals can preserve their appearance. The air-tight bag seals out oxygen and moisture, while the chemicals inside form a barrier on the metal surface that inhibits oxidation.
In most cases, iron heads and blocks will be painted or powder coated after they have been cleaned and machined. Post-cleaning is important too, because the exterior surface of the casting must be dry and oil-free for good paint adhesion. Equally important is making sure any blast media, metal chips or honing residue have been washed away before the engine is reassembled. Solvents will not remove honing residue from cylinder bores.
The bores have to be scrubbed out with hot soapy water and a brush, or washed out in a spray cabinet using the proper detergents. Any honing residue that remains on the bore surfaces may damage the rings, bearings or other wear surfaces inside the engine.
Shot retention is a concern if steel shot (or glass beads) are used at any step in the cleaning process. Hard particles that are left inside a casting may end up in the crankcase where they will eventually be pulled into the oil pump with potentially catastrophic results.
An often overlooked area where contaminants can hide is inside the oil pump pickup tube and screen. Any hard particles here may come loose and be sucked into the oil pump. Ultrasonics can do a good job cleaning the inside of the tube and screen, but it’s difficult to see inside to inspect for any residual contaminants. For this reason, you should consider replacing the pickup tube and screen rather then reusing it.
Cleaning: Different Methods for Different Needs
Because of the range of materials used in today’s engines, the restrictions placed on shop owners by local and federal governments and the rapid pace of development in cleaning technology, there is more variety than ever in the products available to clean engine parts. Consequently, there are more questions than ever, too, about what’s best for certain situations.
Strangely, perhaps, despite the advances in technology, it is estimated that up to 70 percent of shops use a manual cleaning system to hand clean engine components. This may seem slow and tedious to some, especially in the face of the rapid change in the machining industry, but it’s often necessary quality control in order to remove all of the hidden particles that can cause damage to an engine and eventually lead to a comeback. Plus, it allows you to see what you are cleaning and to inspect for any other damage or debris that may exist.
To achieve the excellence for which you strive and your customers demand, there are a variety of cleaning methods employed by today’s engine building/rebuilding facility. Engine builders should review their cleaning procedures periodically to make sure they are working well and there is no debris left behind. You may find that you can save money or time or both by changing equipment, media or chemicals. But you must recognize the solution must fit the need.
Cleaning today’s bimetal engines creates special challenges for engine builders because of the difference components used. You will more than likely have to combine several processes to get the kind of clean that you demand. The methods that will work best will depend on what you’re cleaning.