Porsche M96/M97 Bore Scoring issues explained
There are many articles online and various 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?
The M96/97 Crankcases use ‘Lokasil’ Cylinder liners (a flexible but hard-surface metal matrix composite) and 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. Another factor in the M96/97’s engine design is the difference in lubrication and cooling. The oil spray and coolant 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 hotter when thermal and mechanical stresses were applied across the two banks of cylinders. This in turn causes the sixth cylinder to become slightly oval in shape, causing more friction between the cylinder and the wall which results in the cylinder lining being rubbed away. Once the protective cylinder lining is damaged, the cylinder is in direct contact will the wall which causes scarring. In addition, lubrication of the cylinders is also compromised by a two year oil change interval. 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 localised overheating. A full in-depth article on this issue and other export opinions can be found here
Porsche models that could be at risk of bore scoring
All water cooled 911’s Boxster’s and Caymans from 2004 through to 2009 are at risk of suffering from bore score however, it would appear the percentage of engines actually affected is relatively low.
“Porsche GB advise that it sold 9710 E 996s and 997s in the model years 2004-2006. Some very wet-finger-inthe-air guesswork suggests independent specialist rebuilds and OPC warranty replacements combined may have reached 500 engines from that period, which indicates an overall percentage of around five per cent. excerpt: GT Porsche Magazine (above link)
Rear the full discussion on cylinder bore scoring with the M96/M97 Carrera engines in the March 2015 GTPorsche magazine, by Peter Morgan
Symptoms of Bore Scoring
- A rythmic ticking noise that sounds similar to that of a hydraulic lifter
- One exhaust pipe has more soot than the other
- Increased oil consumption
- Extremely black engine oil
- Pooling of oil within the cylinders
- Aluminum debris found in the oil filter, and engine oil sump.
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 inner city or predominantly short journeys (where the engine is always running with a rich mixture and excess fuel can dilute the oil). The 20K/2yr service intervals also haven’t helped with engine life when in such an urban environment, a diluted oil doesn’t get changed often.
We would recommend an annual oil change for these models especially if you use your car for regular short journeys.
Models that have been well maintained with a good service history and a healthy cooling system will reduce the chance of localised overheating.