915 gearbox backlash

Hi Brad,

I can’t really answer about the “free play,” but I can say that I generally idle between 2200 - 2500RPM to avoid gearbox rattle. It’s generally worse when it’s cold, so I stick closer to 2500 rpm then. When it’s warm I can maybe get as low as 2000 rpm before I hear the gearbox, but I try not to let it get that low.

I was going to refer you to the 912is manual but now I see that the 915 is a completely different design.
The 915 is driven by a Torque shaft with splines on either end vs. the 912is' Prop Shaft design, so there are at least 3 sources of backlash in the 915 design.

And as you stated, the 915HMM Manual does not address backlash anywhere.
It may not be as big of an issue as one might think.

Roger might have some insight.

Hi Bill,

I'm kinda like you and not enough experience here to help out. This answer most likely needs to come from Rotax.

Hi Brad

The question about the 915 gearbox is interesting and perhaps you need to find someone who can do a spectrum check on the engines vibrations.  A good technician on that can tell you if it is indeed from the gearbox or the engine itself.  The gearbox parts spin at 2.54 lower speed than the crank so they can be isolated from the crankshaft and other rotating parts inside.  

The gearbox backlash, which is not given as a factor in the new gearbox design, is never an issue unless the gearset itself fails.  This only measures the clearance of the pinion to driven gear in the box which are both case hardened materials.  Almost all wear you see on the magnetic plug is from the small wear points on the friction clutches inside the box and slight wear on the drive dogs of the friction clutch to the torsion shaft.  The wear can only be checked by someone with the tooling to remove the torsion shaft and clutch packs to measure the slip wear internally.  This must be done in a workshop with tooling as noted.  I believe each iSC in Canada and the Americas have all the tooling and training to accomplish this.  The investment in tooling is high so it is unlikely that repair stations or iRMT would have them.  

Download a copy of the latest heavy manual and you can see all the parts and even all the tooling used to check these.  It is most likely not the gearbox itself. As a note the gearbox on the 915 has several variants so you need to be careful to know which one you might have.  The same applies if you in the future get a type 916, essentially the same box but with some variations inside the gearbox.  

If you are used to the standard design gearboxes used on the 912iS engines you will note that the 915/916 is more clunky or loud on startup and shutdown.  This is due to the internal design of the clutches and the torsion shaft.  Think of it as a big spring inside that winds up with torsional moment and then as it does so its movement is dampened by friction clutches.  These same clutches have to also control the release of that energy as there is a rebound inside the box.  The old steel to steel ramp system is gone inside the gearbox with these new designs.  

Cheers