Bore Scoring in Hypereutectic Blocks

As manufacturers search to find ways to extend the life of the internal combustion engine, we are seeing a trend of engine downsizing. Reduction of cylinder count resulting in reduced displacement and engine weight coupled with forced induction is providing sizeable gains in fuel economy with equal improvement in horsepower. One of the biggest changes with modern engines has been the elimination of iron liners which allows for tighter bore spacing and larger cylinder bores as well as for a reduction in water jacket and total cooling system volume because of the increased thermal efficiency offered by an all-aluminum engine.

A great example of this is Mercedes M139 engine which makes 421 horsepower from 2.0 liters. To be able to accomplish this, an all-aluminum engine with optimized thermal and volumetric efficiency is required. With Toyota’s 2ZZ-GE engine they were able to increase horsepower by 26% and torque by 5% with no increase in displacement by eliminating iron liners. Not having a iron liner to impede heat transfer allows for improved performance. Like Toyota, Honda also developed their own hypereutectic aluminum engine block technology that was first used in the Prelude Si and subsequently in their NSX and S2000 models.

Manufacturers like Porsche have successfully used all aluminum engines for over half a century by using thin coatings applied to cylinder bores, starting first with hard chrome and later using Nikasil. However it was Chevrolet with the Vega where the first liner-less aluminum block was used. The Vega used Reynolds A390 aluminum (now known as Alusil) where through a special preparation process the silicon particles in the aluminum were exposed in the bores. This also required the use of an iron-clad to prevent galling from the aluminum piston running in a raw aluminum bore. The way this system works is the oil goes around the silicon particles and allows formation of the required tribofilm to support travel of the piston and rings in the aluminum bores. It is worth noting that shortly after the Vega, Porsche also used Alusil in both their 944 and 911 models with great success, with many of those engines still running flawlessly almost 50 years later. Many other manufacturers including Mercedes, BMW, and VW/Audi have and still use Alusil to this day.

However, when the coating on the piston is compromised or too many of the exposed silicon particles in the bores become damaged and fracture, the tribofilm can no longer support normal operation. When this occurs, bore scoring is the result. This is not to be mistaken for seizing of the piston in the bore, however there is often significant metal transfer between the piston, rings, and bores when bore scoring occurs. Symptoms typically include piston slap and significant oil consumption; increased iron, aluminum, and silicon oils as detected by using used oil analysis can also be early indicators of bore scoring which can be verified by bore scoping cylinders.

Starting with the Boxster in 1997, Porsche used a new process developed by the creators of Alusil called Lokasil. To simplify things, we refer to Lokasil as localized Alusil. Where normally a whole block is cast out of Alusil, Lokasil uses a freeze cast liner inserted into the mold where the pre-form has silicon particles embedded in resin. When the molten aluminum is injected into the mold, the resin is burned off, leaving a thin area of high silicon content aluminum centered around the cylinder bore. The cylinder is then prepared the same way you would with an Alusil bore to expose the silicon particles. This process was revised by Kolbenschmidt several times with Lokasil II and then Lokasil III, however problems with this process continued and Porsche went back to using Alusil in their sports car engines for the 2009 model year. These Lokasil blocks used from 1997 through 2008 can suffer a multitude of issues including cracking and scoring.

The big difference between the modern Lokasil and Alusil engines and older ones are in the process used to prepare the cylinder bores and with the coatings used on the pistons. Originally a lapping process with an etching paste was used to remove the aluminum from the bores, effectively exposing the silicon particles. The more common process used which is faster and more cost effective is a mechanical one, however care must be taken to not fracture too many of the silicon particles, which could be a reason why failures are more common with these newer engines. Likewise, the pistons used to get an iron clad plated which was later replaced with more environmentally friendly coatings that have not held up as well as previous processes. Another variable is the reformulation of oils and the push for thinner oils to improve fuel economy, all of which have a detrimental effect on wear.

Conventional thinking would say the more effective way to repair an engine with scored bores would be to put in an iron liner, however most modern engines with FEA optimized crankcases do not have sufficient parent metal in them to allow installation of an iron liner. With models where the cast in aluminum liner can be machined out and a wet liner fitted, other issues can occur stemming from dissimilar material expansion rates including head gasket failures, cracking of blocks due to the required press fit, and not least performance and cooling issues. Several companies including LN Engineering utilize nickel silicon carbide plated aluminum dry and wet sleeves to repair aluminum engines with bore scoring or other cylinder failures.

Where sleeving is not required, oversized pistons with the proper coatings can sometimes be sourced from the OEM that are specifically designed for use in engines with hypereutectic cylinder bores. Preparing these bores also requires special tooling and processes as they do not get honed conventionally. Alternatively the cylinder bore can be slightly bored out to allow for application of Nikasil plating. Processes now used by OEMS like APS and PTWA coatings will soon also be available to the aftermarket, providing more options to repair hypereutectic aluminum blocks without having to make compromises in the area of performance or longevity. To successfully repair and rebuild these modern engines it is ever more critical that modern processes and technologies are used to ensure satisfactory results.

The high cost of reconditioning aluminum engine blocks can be prohibitive, making replacement shortblock or longblock engines from the OEM often the most attractive option.

Unlike an aluminum engine block originally fitted with cast iron or steel sleeves from the factory which can be easily bored out and fitted with an oversized aftermarket piston, aluminum blocks require a special process before new pistons and rings can be fitted. Additionally, those pistons have to have a special ferrous coating or plating on the pistons to be compatible with the aluminum cylinder bores.

The majority of European automobile manufacturers using hypereutectic aluminum engine blocks rely on Kolbenschmidt’s Alusil. Reconditioning requires the cylinders to be bored out, honed, and then have the silicon particles exposed. When honing, it is not specifically to generate a cross hatch in the bores, but rather to size the cylinder and remove any damaged silicon particles generated from the boring operation. There should be no visible crosshatch when this has been completed. The last step of exposing the silicon particles can be done either with a mechanical or chemical etching process and is critical to making this all work properly. Skipping this step will result in engine failure. Specialized tooling is required to carry out all these operations which can be sourced from Kolbenschmit, Rottler, or Sunnen. Once this is done, if the OEM does not offer an oversized piston, you can contact Mahle Motorsport as they can supply a custom piston with the required Ferroprint coating on the pistons suitable for use in this application.

A more common practice is to use Nikasil or equivalent plating to repair an aluminum engine block. The cylinder bores have to be bored out approximately .008” to .010” to allow for build up of the nickel silicon carbide electro-plating onto the cylinder walls. If there are deep gouges in the bores, it may be possible to go to the next oversize or to dry sleeve the block with an aluminum sleeve back to stock before plating, assuming Nikasil compatible rings are available. After plating, the cylinder bores would then be diamond honed to provide the required surface finish, which is significantly different than how you would hone a conventional cast iron or steel cylinder bore. Total Seal is a good source for Nikasil compatible piston rings and for recommendations for appropriate ring end gaps; contact Total Seal first before making any plans for cylinder bore sizing or before ordering pistons. Another consideration is the availability of head gaskets if enlarging the bore size – Cometic can often supply gaskets with custom bore sizes and thicknesses (in case you need to resurface the deck).

When it comes to the piston, once the cylinder bore has been Nikasil plated, you are now free to use an aftermarket piston as long as you ensure the correct Nikasil compatible rings are used. If undamaged, the original piston can sometimes be reused if the iron clad coating is removed and pistons are recoated, but often a new aftermarket piston is the best choice. We would recommend sending a sample OE piston to whomever you use to make your pistons. It is critical that the correct piston to wall clearance is observed as traditionally most all aluminum engines run tighter clearances (and ring end gaps) than an engine with cast iron or steel sleeves. The choice of forging alloy also would affect this, as a 2618 forged piston often needs approximately .0005” more clearance than a similar 4032 forged piston which has high silicon content. A 2618 piston may be better for a performance application because it is stronger, but a 4032 piston will be closer to the OE piston in performance and longevity. Using an aftermarket piston may require slightly more clearance than what was specified by the factory however this should be discussed with your piston supplier to ensure you do not have excessive piston to wall clearance.

When it comes time for assembly, cleanliness is key to success. Any contamination in the bores can cause significant damage to the cylinder walls. Ultrasonic cleaning alone is not sufficient – we advise customer to use Kimwipes and denatured alcohol when cleaning their cylinder bores to ensure all foreign debris and contamination is removed from the walls. The Kimwipe should come out of the bores as white as they went in – only then do you know the bores are clean and ready for assembly. The appropriate assembly lube should be used on the pistons, rings, and cylinder bores and piston installation using a tapered sleeve ring compressor is recommended to prevent chipping of the Nikasil or broken piston rings.

The second most common error we see is the use of synthetic oil. Under no circumstances should synthetics be used anytime during the break-in process. Rings can be notoriously difficult to seat with Alusil or Nikasil bores, so a conventional non-friction modified oil must be used for break-in and for at least several thousand miles until the rings are fully seated.

For many rebuilders, once the engine is complete, that’s where their job stops, but not their liability as the builder has to warranty their work when problems arise. Often many of the issues with rebuilt engines stem from improper installation or break-in as well as faulty ancillary components. Qualifying used components to be reused or mandating replacement of these critical items is equally as important as how the engine is broken in. Before initial start, performing a vehicle handover or system adaptation reset to clear all the control modules and ECUs in the vehicle is equally as important as any learned behavior prior to the rebuild may cause issues upon initial start on the fresh engine.

For newly rebuilt engines, over-fueling is big issue, especially during initial run in. Running rich is often considered safe, but remember this – fuel is not a lubricant, especially with modern ethanol enriched fuels. Cylinder wash-down is often the leading contributor to issues with Alusil or Nikasil cylinders, so ensuring fuel trims and AFRs are correct is essential. Bad injectors, oxygen or MAF sensors, or vacuum leaks can all lead to over-fueling or cylinder wash down. Smoke testing the engine for vacuum leaks is also critical in this day and age of plastics being used throughout modern engines and should be carried out on any new engine installation.

Beyond initial run-in, when it comes down to maintenance, the importance of using Top Tier fuels and fuel system treatments including polyetheramine (PEA) as found in Driven Injector Defender ensure proper fuel system operation is as important as what oil is used and how often it is replaced. OEMs are pushing for lower viscosity oils to improve fuel economy and longer oil change intervals to reduce cost of ownership, both of which aren’t necessarily best for the engine. It is up to the builder to verify and set bearing clearances which ultimately will dictate the appropriate oil viscosity and to make a recommendation for what oil (we’re partial to Driven Oils) is best for the engine and intended use, so don’t blindly rely on OE guidelines for both. In our more than twenty years of experience with Porsche engines and with Alusil and Nikasil, we’ve learned what works and what doesn’t. Hopefully we’ve shortened the learning curve for you and demystified reconditioning of hypereutectic aluminum engine blocks so you don’t have to fear rebuilding a modern all-aluminum engine using these technologies.