Crankcase Fretting - Rotax & Other Stuff

[Area-51]

 

Ok, i think i finally understand this rotax crankcase fretting stuff from a practical perspective; here is my take... The crankcase starts fretting due to stretching of the cylinder retaining studs and modal harmonic vibration at particular rpm ranges; the longer any of the modal frequency bands are maintained the more fretting takes place... As the fretting continues it is wearing away a patch of metal between the two cases; so oil begins weeping and the cylinder studs around the area loose more tension, so combustion induced waste oil also later begins weeping from between head and cylinder joint... The engine remains in free rotation until such time the loose cylinder studs are again pretensioned... then the crankcases are squeezed back together, however the distance at the crank main journal and bearing shell is now reduced thus causing excessive tightness and friction of the free rotation, forcing out the otherwise retained viscous oil film... slacking off the cylinder stud tension again relieves the excessive grip at the main  journal allowing the engine to free rotate without effort again...

 

Repair of the fretted crankcases is only possible by refacing the parting surfaces and line boring the camshaft and crankshaft bores true again.

Edited 16 hours ago by Area-51

[Blueadventures]

what year build; pre 2007?

[facthunter]

The "crush" on the main bearing shells also being affected and the heat won't transfer out of them Properly. The stud tensions vary due to their Long length and the thermal expansion difference with the Alloy. IF they fret, there's also fatigue happening.  If any decking is done ,other distances change.. There would have to be specified limits to it. (There is on other motors). I doubt Rotax would like it being done. Nev