Considering a pre-owned modern Porsche? Please do your due diligence—and this isn’t limited to simply sourcing a car you believe to be mechanically sound. As enumerated in the prior deep dive into the history and facts of LN Engineering’s multi-pronged remedy for the critical intermediate shaft bearing problems plaguing the M96 and M97 engine family, understanding the “what” and the “why” of mechanical failures is your best guard against a massive financial mistake.
And in the case of Porsche’s equally infamous bore scoring issue, the fix is always a full engine rebuild—the cost of which sometimes eclipses the purchase price of the vehicle itself. It’s also nearly endemic; compared to the IMS issue’s relative niche in the Porsche 911 and Cayman/Boxster models, bore scoring is a critical issue affecting not just those with the M96/M97 engine, but iterations of the Panamera, Cayenne, and Macan vehicles as well as other European makes, using similar hypereutectic aluminum engine technology.
There’s a lot of ground to cover with bore scoring—we’ll start with the “what.”
What is Bore Scoring?
Fundamentally, bore scoring is the wearing down of an engine’s cylinder walls via abrasion from the piston that will eventually and inevitably lead to total engine failure if left unchecked. This most commonly occurs in engines using hypereutectic aluminum blocks/cylinders, where a specific ratio of silicon to aluminum is meant to aid in lubrication via exposure of silicon particles to the engine oil.
Though our focus is on a group of vehicles considered to be both advanced and high-performance, the use of this tech dates back to 1927 when Lancia introduced the hypereutectic material for its pioneering Lambda model line. However, linerless aluminum blocks first hit mainstream production cars in the early 1970s with the introduction of the Chevrolet Vega. General Motors worked with the Reynolds Metal Company to develop the proprietary Reynolds A390 material that later evolved into the trademarked Alusil.
Porsche and other European automakers quickly adopted Alusil with mostly problem-free results. (This is not to be mistaken with Nikasil, a cylinder plating comprised of nickel, silicon, and carbide, which was used by Porsche for decades.) Porsche utilized Alusil in some air-cooled 2.7 and all 3.0 SC models, as well as in the 944 and subsequent transaxle Porsche 928 and 968 cars. Rather than wearing a cylinder plating, Alusil cylinders have uncoated aluminum bores requiring the exposure of silicon particles in the aluminum matrix by means of chemical etching or mechanical exposure, thus allowing for oil film formation on the cylinder walls, which is required to support the operation of the piston and piston rings. This is coupled with pistons that are iron-plated or coated to prevent the inevitable galling that would occur if an uncoated aluminum piston ran in an uncoated aluminum bore.
View Price on AmazonLN Engineering’s Charles Navarro theorizes it was the switch to mechanical exposure that potentially predisposed the M96/M97 block to bore scoring, having potentially damaged the silicon particles rather than simply expose them. This is further exacerbated by the switch from iron-plated cast aluminum pistons to forged aluminum pistons, where the iron coating on the forged pistons has proven far less durable and prone to flaking, which, in turn, acts as a serious abrasive that leads to galling and scoring when there is aluminum-on-aluminum contact. Meanwhile, earlier Porsches—including the 911 SC, 944, and 928—have significantly less incidence of bore scoring, likely due to the combination of chemical exposure and iron-plated pistons.
The kill shot, however, arrived when Porsche switched from Alusil to Lokasil in 1997, a decision sustained through 2008. In contrast to Alusil’s uniform hypereutectic makeup, Lokasil localizes the silicon content to the cylinder bores and allows the rest of the block to be cast
Source: thedrive.com
