Porsche Bore Scoring
There are many articles online and posts in Porsche forums about bore scoring issues and engine failure! While cylinder bore scoring has been an issue on a relatively small percentage of 996 and 997 models, it isn’t a universal problem.
How does bore scoring happen?
Due to the layout of the open deck crankcase on 996/997 models, the cylinder banks are not rigidly supported like the first generation 928, 944 & 968 watercooled engines. The oil spray and cooling flow faces the lower side of the cylinders which means the piston thrust on the right side cylinder runs slightly hotter than the left side. This results in the sixth cylinder becoming oval in shape when thermal and mechanical stresses are applied across the two banks of cylinders.
In addition, lubrication of the cylinders can also be compromised if the oil is changed at two year intervals. When fuel and moisture contaminate the oil (due to low mileage short journeys), the oil becomes diluted which results friction between the piston and the cylinder bore wall and can cause overheating.
Porsche models that could be at risk of bore scoring
All water cooled 911’s Boxster’s and Caymans from 2004 through to 2006 are at risk of suffering from bore scoring however, it would appear the percentage of engines actually affected is relatively low.
“Porsche GB advise that it sold 971 E 996s and 997s in the model years 2004-2006. Some very wet-finger-in-the-air guesswork suggests independent specialist rebuilds and OPC warranty replacements combine may have reached 500 engines from that period, which indicates an overall percentage of around five per cent. “excerpt: GT Porsche Magazine”
Factors that make it less likely include a good service history, a healthy cooling system (to reduce the chance of localised cylinder overheating during the warm up or over-run) and a usage that isn’t predominantly short journeys (where the engine is always running with a rich mixture and excess fuel can dilute the oil). A regular 12 month oil change is advised, especially if you use your car for regular short journey.
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