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Though thermal cleaning uses heat rather than che...
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Media and Solutions For Cleaning Engine Parts
Cleaning is necessary to restore the cosmetic appearance of parts, especially aluminum parts, that are discolored and stained. Restoring a like-new shiny metal appearance can sometimes be challenging.
By Larry Carley
One essential aspect of engine building and rebuilding that cannot be avoided is the need for cleaning. Used engine parts have to be cleaned before they can be inspected. Parts that will be reused also have to be cleaned to remove dirt, grease, oil, carbon, rust, scale and other gunk that could contaminate a newly rebuilt engine or cause problems down the road.
A variety of media, chemicals and equipment can be used to clean engine parts, including a manual or automated jet spray washer cabinet, hot tank, ultrasonic cleaning tank, dry blast cabinet, airless shot blasting system, wet blast cabinet, vibratory cleaning and/or thermal cleaning oven. Every engine builder’s needs are unique, so it often takes a combination of different cleaning processes with various media and/or chemicals to achieve the results you want on different kinds of parts and different metals.
Cast iron and steel are relatively hard, heat-resistant metals so more aggressive cleaning processes can often be used to clean cast iron engine blocks and heads, crankshafts, connecting rods, valves, springs and other valvetrain parts. This includes using caustic chemicals and strong detergents in hot tanks and spray washers, baking the parts at high temperatures to oxidize grease, oil and carbon, and shot blasting or airless blasting to remove ash, rust and scale.
Aluminum, by comparison, is a softer, less heat-tolerant metal. Cleaning aluminum blocks, heads, pistons and other parts requires cleaners that are less caustic, temperatures that won’t overheat and anneal the metal, and cleaning media that won’t etch, discolor or damage critical surfaces.
In recent years, a number of new technologies have emerged that are well-suited to today’s cleaning challenges. These include aqueous cleaning with water-based detergents that work well on both ferrous and aluminum engine parts, ultrasonic cleaning that uses high frequency sound waves to remove contaminants that other cleaning techniques often miss, vibratory cleaning that literally scrubs parts clean, and dry or wet blasting with baking soda instead of more traditional media such as steel shot or glass beads to remove dirt and gunk from aluminum parts.
Solvents have long been a mainstay in the automotive industry for cleaning greasy parts. Solvents are quick and very effective at dissolving grease, oil and varnish. But the days of the traditional solvent tank are probably numbered. California, for example, already has strict rules that limit the use of solvents, and a coalition of Eastern states is considering similar legislation.
The main issue with solvents are the Volatile Organic Compound (VOC) emissions they give off, which contribute to urban air pollution and smog. But there are also concerns about possible health risks that may be associated with breathing solvent fumes. That’s why many shops are looking for safer, more environmentally-friendly alternatives.
Used solvents must be collected and disposed of properly by a licensed hazardous waste hauler. Solvents can be recycled but the disposal costs are usually higher than with other cleaning methods. Cleaning small parts with solvents by hand can also be a time-wasting chore that reduces a shop’s profit potential. Those same man hours that are wasted cleaning parts could be better utilized on more profitable machine and assembly work in the shop.
Water-based cleaners have been around for many years, and have continually improved. The latest generation of water-based products can clean cast iron and aluminum just as effectively as traditional solvents but without the environmental issues, health concerns or disposal costs. Wastewater still has to be collected and disposed of properly because of the contaminants it contains, but because most of the liquid is water, disposal fees are usually much less than that for spent solvents.
Most water-based cleaners are concentrated, so the actual cost per gallon of cleaner is also less than that for solvent. Hot tanks that have oil skimmers help prolong the life of the cleaning solution, which reduces operating costs even more.
One thing to keep in mind with most water-based cleaners is that they work best with hot water. Some water-based cleaning solutions for immersion tanks work best at an operating temperature of 115 to 150 degrees F. Other water-based detergents that are formulated for spray cabinets deliver the best results at 170 to 180 degrees F. The temperature can make a significant difference in cleaning effectiveness as well as energy costs. You want it hot enough to get the best results, but not so hot that you are wasting energy needlessly. Follow the recommendations of the chemical supplier as to the best temperature to use, and monitor the water temperature in the equipment to make sure it is within the desired range.
Water-based cleaners can be used with a simple sink tank and drum cleaning stand, a jet spray cabinet (with the appropriate additives to control foaming), a hot immersion tank or an ultrasonic cleaning tank. The products are usually formulated for specific types of cleaning equipment, as well as for various types of metals. Some water-based cleaners contain chemicals that are too strong for aluminum while others are aluminum-safe. Since it isn’t always possible to have separate cleaning tanks for aluminum and cast iron, many shops go with a water-based cleaner that works well on both (though it may take longer to clean cast iron parts with a product that is designed primarily for aluminum).
Cleaning aluminum with water-based cleaners requires detergents that won’t etch or discolor the metal. When exposed to certain chemicals, aluminum can oxidize and turn black. That coating must be removed and prevented from reforming with special additives in the cleaner.
With ultrasonics, a high frequency transducer in the bottom of the tank generates 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 dirt and grime. Adding some agitation to the tank helps the cleaning action even more by scrubbing away the contaminants once they have been loosened by the bubbles. The cleaning process is also able to reach deep inside ports, bolt holes and oil galleries. This makes it ideal for difficult-to-clean parts where other techniques may not work (things like cylinder heads, even oil coolers and pushrods).
Proponents of ultrasonic cleaning say it has numerous advantages over other methods. It uses water-based detergents so there are no VOCs to worry about. The bubbles do the scrubbing, so there is no manual labor required to clean parts (other than loading and unloading the parts in the tank). Most parts come out clean after 20 to 30 minutes in the tank. Ultrasonics also works well with both large and small parts (assuming the tank is properly sized).
Ultrasonics is also a kinder, gentler method of cleaning since it uses no abrasives that could remove metal or etch the surface of the parts in the tank. This is an important consideration with aluminum heads that mate to the block with multi-layer steel (MLS) head gaskets. MLS head gaskets typically require a very smooth surface finish. If the head still has a good surface finish on it, you don’t want to alter during cleaning and have to resurface it unnecessarily.
The key with ultrasonics is to use equipment with 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.
Some users prewash unusually dirty parts in a spray cabinet to remove layers of surface gunk before parts go in an ultrasonic tank. Others say prewashing isn’t necessary if you are using the right type of cleaning solution, power density and frequency setting for the parts that are being cleaned.
Shot blasting is a cleaning and surface finishing process that directs some type of hard or soft media against the surface of the part that is being cleaned. When the blast media hits the surface, it knocks loose what ever is on the surface. The aggressiveness and abrasiveness of the process depends on the type and size of blast media used, its hardness and the force with which it is directed against the surface. Round blast media is less abrasive then angular blast media, while soft media such as plastic and ground walnut or pecan shells are virtually nonabrasive.
Blasting with steel shot is most often used to remove rust, scale and ash from cast iron and steel parts while glass beads are most often used to clean aluminum. In addition to cleaning, shot blasting can also be used to roughen the surface of parts for better paint adhesion if the part requires painting. Blast cleaning with round steel shot can also produce a peening effect that compresses the surface of the metal. This can increase its fatigue resistance and durability, and be beneficial on parts such as crankshafts and valve springs.
When used for cleaning or stripping, the grade or size of steel shot will determine the surface finish. The round ball shape of the shot produces a clean, smooth and polished surface through its peening action. Smaller diameter shot will produce a smoother and more polished finish while larger diameter shot will clean more aggressively but leave a rougher surface.
Steel shot usually produces little dust, and can be used with both air-powered and wheel blast equipment. The hardness and density of steel shot means it can be used repeatedly (up to 3,000 times) before it has to be discarded. Steel shot has a hardness of 40 to 51 HRC, with sizes ranging from S-70 up to S-780 (the larger the number, the larger the diameter of the shot). S-70 to S-170 is typically used for fine, smooth shot blasting, while S-230 to S-330 is used to clean rust and scale from iron castings.
If you want a more aggressive cleaning action, angular steel grit can be used. The angular surface of the grit has a cutting action that removes metal and helps prep the surface for painting. Sizes typically range from G-10 to G-120.
For blast cleaning aluminum, glass beads are commonly used. Glass shot can remove contaminants without etching or damaging the surface of the metal. It leaves a clean bright surface and usually requires no masking. It is often used to clean pistons and aluminum castings.
Glass beads can be reused up to 30 times, but with repeated use the glass tends to shatter and break down.
Aluminum Oxide and Silicon Carbide blast media have an aggressive cutting action that works well for removing heavy rust from iron castings and steel parts, while prepping the surface for paint. Both types of media resist breaking down, and can be used repeatedly. Grit sizes range from #16 to #140.
Cleaning parts by blasting the surface with sodium bicarbonate (baking soda) has become a popular alternative to other traditional blast media. Glass beads and metal shot do a good job of removing surface contaminants. But parts have to be dry and relatively grease-free so the blast media doesn’t stick to the surface or gum up the equipment. There’s also the worry that some of the beads or steel shot may be retained in blind holes, oil galleries or crevices of parts, which could cause serious problems if not removed by a subsequent cleaning process.
With soda blasting, there are no such worries because baking soda is water soluble. It’s also a relatively soft media (a hardness rating of only 2.5 on the Mohs scale) so it won’t etch or work harden the surface of the parts that are being cleaned. It is very effective on aluminum, and can remove dirt, grease, oil and even paint. It’s also an ideal cleaning material for removing carbon deposits from pistons.
Soda can be used with either a dry blast cabinet or in slurry form with a wet blast cabinet. You also do not have to prewash parts prior to soda blasting as you do if you’re cleaning with glass beads or similar media. Soda is environmentally-friendly, too. It leaves no residue on parts, it’s nontoxic, nonflammable and rinses right off.
Baking soda media is typically available in three sizes (small, large and extra large), and may be formulated with other additives to prevent clumping when used in humid environments. It can also be blended with aluminum oxide if a more aggressive cleaning action is required. Some shops also mix small sized stainless steel shot with soda if they want a shot-peening effect while they are cleaning.
Because soda is so soft, it shatters when it hits the surface of the metal. This creates a lot of dust and means it can only be used once (unlike glass beads and steel shot that can be reused many times). To improve operator visibility, some dry blast cabinets have a controlled air flow pattern that helps keep dust away from the window.
When used in a wet blast cabinet, soda is mixed with water and other ingredients to form a slurry. The slurry is pumped through a spray nozzle to clean the part, and is then recirculated allowing the material to be reused three or four times during the cleaning process. The slurry solution can be either used in a manually operated blast cabinet or in an automated system with multiple nozzles. The cleaning cycle is followed by a rinse cycle to remove all traces of the slurry.
One of the advantages of this approach compared to dry blasting with soda or other media that hits the surface and bounces off is that the slurry scrubs as it cleans. This allows for faster, more effective cleaning. And unlike bead or shot blasting, it won’t peen small cracks together that could hinder crack detection efforts later. This can be an important consideration when building a high performance engine.
Though thermal cleaning uses heat rather than chemicals or blast media to burn off grease and grime, parts still have to be cleaned after they come out of the oven to remove ash, rust and scale. An airless shot blaster can be used for this purpose, or any other type of dry blast equipment, or a spray washer or ultrasonic tank.
Special chemicals can be used for post thermal cleaning to remove rust from cast iron parts, leaving the surface with a like-new appearance.
The technology that works for your shop may be a single type of cleaning process or it may be a combination of systems. Your equipment dealer can help you decide.
For more information and cleaning equipment supplier contacts, click on the Engine Builders Buyers Guide link.