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Cleaning Cylinder Heads and Blocks
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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.
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