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 23 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

[Area-51]

There is nothing uniquely magical about rotax or any other reciprocating engine ever made, the physics remains constant across the entire plethora of them all. A competent and very experienced engine machinist will know what working clearances applied work in the real world, and what clearances fail; repco produced a bible regarding all this in the mid 80's... In my personal opinion stretch to torque studs are a total failure in an environment of dynamic combined gross temperature variations and harmonic modal frequency changes. Within the past 40years i have seen all of them fail when used in a stressed engine scenario; but after a few factory or shop iterations the problem goes away, or the market shuns the product and goes away...

 

A greater number of applied stud dowling in split cases along with a sufficient interface seal is what prevents fretting and associated bearing component damage.

[Area-51]

2 hours ago, Blueadventures said:

what year build; pre 2007?

This engine is 1998 production unit; again have yet to split the cases and confirm existence of fretting.

[BurnieM]

What year did Rotax fix this problem ? ie if we have engine made after x then we can relax

 

[facthunter]

Dowels won't stop it They can even weaken the faces if you aren't careful. Nev

[facthunter]

Be nice to know what they did  I know some beefing up was done but aluminium has it's limits where rigidity and tensile strength are Involved. Nev

[Blueadventures]

1 hour ago, BurnieM said:

What year did Rotax fix this problem ? ie if we have engine made after x then we can relax

 

About 2006

[Area-51]

1 hour ago, BurnieM said:

What year did Rotax fix this problem ? ie if we have engine made after x then we can relax

 

This issue was rectified with the release of the Series-2 9XX engines around 2005. The crankcase fixings were revised to M10 main bolts with integrated threaded heads and threaded M10 crankcase inserts that the cylinder studs now screwed into. Rotax offered retrofit upgrading of Series-1 crankcases at the time for a fee...

 

Series-2/3 spec'd crankcase engines do not suffer from fretting.

[skippydiesel]

Speculation - I don't think all erly 912ULS engines suffered from this. Seems to me mounting type, management, airframe and prop type, likly all had an influence.😈

[Area-51]

1 hour ago, skippydiesel said:

Speculation - I don't think all erly 912ULS engines suffered from this. Seems to me mounting type, management, airframe and prop type, likly all had an influence.😈

Agree; not sure about percentages but many Series-1 units are still flying on condition without any issues.

[Red]

Haven't seen this in the wild for years now.

I doubt there are many with the problem still

out there as it usually became apparent at fairly modest hours.