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 March 12 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]

  On 12/03/2025 at 11:03 PM, Blueadventures said:

what year build; pre 2007?

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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]

  On 13/03/2025 at 1:59 AM, BurnieM said:

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

 

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About 2006

[Area-51]

  On 13/03/2025 at 1:59 AM, BurnieM said:

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

 

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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]

  On 13/03/2025 at 3:41 AM, 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.😈

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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.

[Area-51]

Agree. Maybe someone out there knows what percentage failure rate triggers an AD and lesser SB situation by an OEM.

[facthunter]

There used to be rules, but NOW TBOs etc are "all over the Place". You can almost CLAIM anything. The lighter you build the motor THE more stress the Parts are being subject to. . ALL Engines have a life and for a plane It's NOT when it's HAD IT in the normal sense.  . Nev

[Moneybox]

Perhaps it's a simple as propeller balance and the associated vibration. My engine is 2004 so obviously within the suspect engine range however I've submitted an application for the MARAP program. Engines and other items on this program will be monitored to see if the lifespan applied by the manufacturer is accurate.

 

My LAME chose to not rebalance my carburettors and propeller even though he brought the gear with him. He said it was a smooth and quiet as any Rotax so best not stuffed around with. If it's vibration related which it must be then I may have dodged that one.

[Moneybox]

From the Rotax forum;

 

Gentlemen

Before everyone assumes in all cases crankcase fretting is strictly a Rotax design issue, please consider there are many influencing factors that ultimately can lead to a certain destructive harmonics being transmitted through the engine, resulting in loss of crankcase stud torque leading ultimately to crankcase fretting!

I've seen this issue from time to time in our Diamond Katana Commercial Flight School fleet and in all cases we found it attributable to a series of compounding issues which if not resolved ultimately lead to the fretting issue. Things such as proper carb synchronization, intake induction design influence on mixture settings, worn carb parts such as slide needles, slide bores, float arms, etc., bad engine mounts(worn out, too stiff, too soft, wrong design for isolating first order natural frequency of engine), poor engine mount design(again fails to totally isolate engines first order natural frequency), propeller balance and tracking, mass inertia, blade flex influence etc., gear box maintenance including friction torque settings, gear dog spring pack pressure etc., and gear box gear wear(pitting caused by poor oil quality with inadequate zinc content to prevent gear tooth pitting etc.).

As well, and as important is prop loading. Many people are over propping their engines by adding too much pitch, in essence running to high a gear(too much load), for the amount of throttle they are loading the engine too! I've seen a lot of chat on this site about prop loading and can tell you one rule of thumb that works for one aircraft/prop/engine combination may not neccesarily work for another as there are many floating variables here also.

Too many people run over square on engine load versus throttle setting like running your car in fourth gear with a boot full of throttle trying to climb a steep hill when they should be in second gear! The Rotax 912 series can run it's entire life wide open throttle at 5500rpm and will love you for it and at this loading has the absolute least natural engine vibration!

Lastly, if you are sure you have discovered a safety issue with your engine, you can file a Customer Service Information Report(CSIR) by going to the "Service" button on this web site, then scroll down and click on "File a Customer Service Information Report". Make sure you fill in completely this form with all required information and then submit it. Copy of your submission will be sent to the factory and the other to the applicable Rotax Distributor in your area but only if it is deemed legitimate and not a fart in a mitt! Upon submission you may be further contacted for more information and follow up depending on the issue and circumstances. The BIG advantage is the system is designed to pick up repetitive problems allowing the factory to track trends and act on them! I've used this CSIR reporting system many times and it is the absolute best way to report issues and get action on them!

In Canada here where I operate we're pretty lucky to have the Rotax Distributor Rotech, these guys are responsible for all "Certified" Rotax engines in North, Central and South America, most of which are run in high utilization flight schools meaning they see many problems and find solutions to them in most cases years before the non-certified fleet does! They also have a Hugh store of Used serviceable parts including crank cases, gear boxes, cylinder heads etc., for those looking for saving some money, as well they have the best Rotax factory approved engine overhaul shop in the industry. Their web site is www.Rotech.ca , I use these guys all the time to solve weird problems and they always seem to have the answers, many which I've reflected in this writing!

Hope this info helps.......

[skippydiesel]

  On 16/03/2025 at 5:47 AM, Moneybox said:

 

My LAME chose to not rebalance my carburettors and propeller even though he brought the gear with him. He said it was a smooth and quiet as any Rotax so best not stuffed around with. If it's vibration related which it must be then I may have dodged that one.

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No offence Moneybox but your LAME is conning you.

 

Humans do not possess the sensitivity to judge, without instruments, the need or not, to balance the carburetors or propeller.

 

Your engine should have the carburettor balance checked & adjusted, if necessary. (This should be a routine check every 100 hrs) Do this before prop balance. Should include idle speed adjustment (about 1600 - 1800 rpm). You can do it yourself with a cheap motorbike twin carb balance kit. You will, need extra vacuum tube.

 

Your engine & propeller may benefit from a dynamic propeller balance check and adjustment, if necessary. Static (blades weight matched) first. Dynamic balanced down to .05 ips.(almost turbine smooth)😈

[Moneybox]

  On 16/03/2025 at 6:49 AM, skippydiesel said:

No offence Moneybox but your LAME is conning you.

 

Humans do not possess the sensitivity to judge, without instruments, the need or not, to balance the carburetors or propeller.

 

Your engine should have the carburettor balance checked & adjusted, if necessary. (This should be a routine check every 100 hrs) Do this before prop balance. Should include idle speed adjustment (about 1600 - 1800 rpm). You can do it yourself with a cheap motorbike twin carb balance kit. You will, need extra vacuum tube.

 

Your engine & propeller may benefit from a dynamic propeller balance check and adjustment, if necessary. Static (blades weight matched) first. Dynamic balanced down to .05 ips.(almost turbine smooth)😈

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Yes Sean I agree however it's not that I didn't do it.

 

 

The may be a little old school but I trust my mercury level more than I trust the modern electronic device although if your gauges are balanced it should do well. Each intake manifold is connected to one end of the tube filled to the required level with mercury. All I have to do is level the mercury so I have it pretty close.

[Area-51]

All those points above should be noted for consideration.

 

One thing i will say is, after installing this 100hp unit that replaced the 80hp, on first start up session to check and balance carburettors the engine sounded a bit rattlee; after carbi balance it settled right down... The source of the rattle was traced to the spur gear backlash in the gearbox; at higher rpm it is still traceable as vibration... Out of all the meshing spinning and reciprocating components of these units the spur gear backlash is active to some degree. I am left believing that this is the source of first order harmonic input leading to crankcase fretting. In short it sounds bloody horrible through the stethoscope. If it was an engine that i knew nothing about and somebody brought to me to diagnose i would say that sounds f'd, needs opening up for further assessment... But alas, that's how it is, and it works, and in the bottom of the oil can we will find powdery fine magnetic dust that can be mined and turned into bespoke expensive metallic fleck paint for a custom car paint shop 👨‍✈️

[Moneybox]

  On 16/03/2025 at 8:52 AM, Area-51 said:

All those points above should be noted for consideration.

 

One thing i will say is, after installing this 100hp unit that replaced the 80hp, on first start up session to check and balance carburettors the engine sounded a bit rattlee; after carbi balance it settled right down... The source of the rattle was traced to the spur gear backlash in the gearbox; at higher rpm it is still traceable as vibration... Out of all the meshing spinning and reciprocating components of these units the spur gear backlash is active to some degree. I am left believing that this is the source of first order harmonic input leading to crankcase fretting. In short it sounds bloody horrible through the stethoscope. If it was an engine that i knew nothing about and somebody brought to me to diagnose i would say that sounds f'd, needs opening up for further assessment... But alas, that's how it is, and it works, and in the bottom of the oil can we will find powdery fine magnetic dust that can be mined and turned into bespoke expensive metallic fleck paint for a custom car paint shop 👨‍✈️

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Anything over a fast idle should have sufficient load from the propeller to take up the slack. Some backlash is need to allow space for lubrication and expansion but any sort of load should prevent the components separating enough to rattle. Most new components will have a certain amount of metal from the machining process and run-in collected in the first filter change but that shouldn't continue to show up.

[Area-51]

  On 16/03/2025 at 9:52 AM, Moneybox said:

 

Anything over a fast idle should have sufficient load from the propeller to take up the slack. Some backlash is need to allow space for lubrication and expansion but any sort of load should prevent the components separating enough to rattle. Most new components will have a certain amount of metal from the machining process and run-in collected in the first filter change but that shouldn't continue to show up.

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That is the assumption flying around the entire rotax community... Another assumption is the one regarding violent power pulses from the little sewing machine motor revving its guts out while producing output to the gearbox; hence gearbox dog gear function... Morse chain would be the better choice for this unit...

 

Easier to just pay money, service machine, pay money, fix machine...

[onetrack]

Get some helical gears cut - end of backlash problem associated with crude spur gears.

[Moneybox]

  On 16/03/2025 at 10:54 AM, onetrack said:

Get some helical gears cut - end of backlash problem associated with crude spur gears.

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Helical gears produce thrust, it might just be the start of another set of problems.

[onetrack]

I don't see where the thrust problem couldn't be addressed with a bit of housing reinforcement, such as through-bolts close to the gears.