The Jabiru engines mandate a pretty regular re-torquing of case bolts and head bolts. My take on this, which could be entirely wrong, is that builders that want to treat their Jabiru like a Lycoming or Continental (meaning with little maintenance other than oil changes) are the ones that are seeing issues with case fretting. In fact, that's exactly what happens when the through bolts stretch and/or are allowed to run a bit loose if one doesn't stick to the torque schedule in the maintenance manual. Through studs do stretch quite a bit, which is why Jabiru has a torquing schedule in their maintenance manual.
The center mains on the VW engines are often times fretted and narrowed. I have seen the same on the center mains on many air cooled engines, including Lycoming and Continental if they aren't torqued correctly. That's the chief cause for DIVCO milling and line boring used cases is to fix the narrowed center main due to case fretting from improperly torqued through bolts. -Jeff Scott Los Alamos, NM > > > What is "crankcase fretting?" > > One example of crankcase fretting is wear that occurs when the case > halves that make up the "crank cradle" rub against one another, because > operational forces momentarily (during each piston throw) relieve the > bolt tension that normally holds the two halves tightly together. The > displacement of the material at the joints allows the case halves to > clamp together tighter when the engine isn't running, which has the > effect of tightening the main bearings against the crank, in this example. > > -- > Mark Langford > ML at N56ML.com > http://www.n56ml.com
