we have some trouble with the gearbox fo the 915iS and can not remount it to the engine so we are looking for the reason to get back to service.

For the whole story, we had the 25hour checks for our new engine. Within this we found a significant amount of brass particles in the oilfilter. Analysis and contact with Rotax gave the result, it was just a sign of run in of the oil flange in the back of the propeller shaft (type 3 engine).

To confirm this, we removed the gearbox to see the oil flange which showed some wear, so everything is ok.

So far everything went fine.

During reassembly of the gearbox to the engine, there is about 1mm missing when sliding it back on the engine.

We know of the right position to assemble the overload clutch and this error would give about 7mm of the gear protuding out of the gearbox.

The overload clutch can be mounted in 2 positions only due to the number of teeth/splines in the overload clutch (44) and the torque rod (18).

Both positions checked, no difference.

The splines can be a bit rough as written in the maintenance manuals so to use a nylon hammer to slide in the overload clutch.
This also is not the reason.

There are dogs/claws on the clutchhub in the damper clutch and the overload clutch. They touch and are not between each other as the dogs in 912/914. So they seem to be used for setting the depth for the teeth of damper clutch into the overload clutch.

Damper clutch and Overload clutch a aligned with the marks.

when assembled into the gearbox, the main gear is not aligned with the sealing surface of the gearbox. it is about 0.5mm high.

So, where is the additional length of the assembly?

Three engineers looking for the knack, we can't find the last detail.


Any hints or any experience what we may have missed?


best regards,




  • Re: 915 Gearbox Assembly

    by » 3 months ago

    Hi Sreffen

    The gear dogs inside are not lining up.  There is a special tool that you need to use to twist the gear for the dogs to match up and this involves twisting the gear on the spline that engages the torsion shaft.  in some cases it will not simply drop into place.  I suggest you get someone with training to do the work. 

    Check the heavy maintenance manual.


    38788_2_install main gear 915 .jpg (You do not have access to download this file.)
    38788_2_main gear depth and tool required.jpg (You do not have access to download this file.)

  • Re: 915 Gearbox Assembly

    by » 3 months ago

    Hi, thing is, we have one guy who just had the line maintenance training but this was not taught.


    The named tool is used to align the teeth, but we do not have the shown situation. This is about 5 mm above surface.

    In our case it is only 0.5mm above which fits to the 4 dogs between overload and damping clutch being engaged or not.

    I guess the part with ‚moving the propeller to engage the dogs‘ is the part and leads to the idea those 4 dogs have not to be aligned, but engaged.

    I‘ll check tomorrow and show pictures.



  • Re: 915 Gearbox Assembly

    by » 3 months ago

    double post

  • Re: 915 Gearbox Assembly

    by » 3 months ago


    Without the tool you will not get the gear to drop.  it is a very slight twist of the gear, while the prop shaft is bolted down to a bench, to spring the torsion shaft enough and allow the gear to drop the amount you need.  (twist while pushing down or tapping with a soft hammer) Some gearboxes will simply drop together, others will need the tool and twist to get into alinement. 


  • Re: 915 Gearbox Assembly

    by » 3 months ago

    Hm, I am nearly sure we are talking of different things.

    As we understand the tool is intended to align the teeth of damping and overload clutch after reassembly of both clutches when the alignment of the torsion shaft does not allow the teeth to engage..

    see first picture

    Then you have the result as in your picture at the incorrect part.

    We are already in there and it is not like the incorrect part of the image, we are at the correct part with one last half millimeter to go.

    So my goal is to understand how the dogs (second attached picture) have to be positioned to each other.

    Do they have to be to be between each other so they kan work as clutches, claws or how how to name it. Or are they used as distance elements and they have to be one top of each other?

    Tests show, that with those dogs in between each other (see third image) will do the trick.

    So if I turn the dog hub within the damper clutch a bit, it would fit.

    if i just would understand what those dogs/claws are for, I would know what to do.
    if they have to be aligned and not engaged, I have no idea what sense they are for and no idea how to solve my problem, because there is no other gap to get those 0.5 mm.

    This all is very annoying to have no information to that in both manuals.


    best regards and thanks for your patience,


    38797_2_image_1.png (You do not have access to download this file.)
    38797_2_image_2.png (You do not have access to download this file.)
    38797_2_image_3.png (You do not have access to download this file.)

You do not have permissions to reply to this topic.