Rotary Gear Woes

Mattman4494

New Member
I recently had a rotary gear failure after fully rebuilding my Rotax 720. For the rebuild I used brand new wsm parts, but left the rv shaft assembly original because it was still in good shape.

The rv gear was completely stripped after approx. 20 hours on the rebuild. I tore the engine apart and inspected everything. Unfortunately I couldn’t find a smoking gun so I ordered a new rv shaft assembly from WSM thinking that should take care of any issues…wrong.

After installing the WSM rv shaft the crank is now binding when I try to turn it over by hand. Brass Gear is cut off center by .004”. Call up WSM and apparently this is a known issue. Only advice is to return and buy a OEM rv gear. $85 for just the gear.. ouch.

So I purchase the oem gear and install it onto my original rv shaft (My original rv shaft was still perfectly straight). Reassemble the engine and it’s all turning smoothly now. Rv backlash seems consistent. I’m happy and put everything back together for one last ride before fall sets in here in Indiana.

Ski runs great on the lake. Probably ride it for about an hour before calling it a day. So far so good… then I get home and see some glitter in the rv oil lines.. drain some oil out and sure enough the brass glitter is back after only 1 hour of runtime.

Not sure what to do now. I’m suspicious of the WSM crank, but don’t really have any way to test it. I did measure the crank for deviation and that all checked out. My hunch is the rv gear on the crank is slightly off center or not cut right. Thoughts?
 
Something is not right,,,or ”off”
The crank will not be out of alignment,,,
Is the outer bearing pressed in far enough to the sleeve flat retaining
washer?
Is the rear bearing seated properly?
Just throwing out ideas,,,
 
I would definitely agree with your assessment that something is not right :)

I’m confident the bearings are seated properly. Not sure exactly what you mean by “pressed in far enough to the sleeve flat retaining washer”. Brand new bearings for the rv shaft too I should add.

Appreciate the ideas.
 
IMG_8186.jpeg
I was looking though the photos I took while working on this and noticed that the crank gear seems off center visually. Should the gear be perfectly centered between the bearings?

I’ll need to get a side action dial indicator to be able to say for sure, but I have a hunch I’m going to be buying a new crankshaft
 
View attachment 64302
I was looking though the photos I took while working on this and noticed that the crank gear seems off center visually. Should the gear be perfectly centered between the bearings?

I’ll need to get a side action dial indicator to be able to say for sure, but I have a hunch I’m going to be buying a new crankshaft
I would think a cheap micrometer that measure inside and outside would do for this measurement especially if it is supposed to be centered and is off as much as it appears to be in the photo
 
It’s too tight for digital calipers. I just took this picture of the original crank and it appears more centered to my eye, but I need to get some real measurements before I can say anything conclusive.

IMG_8289.jpeg
 
It looks centered on the old crank to me too but camera angle can be very deceiving, what about using some spacers and feeler gauges? Then measure them? Just throwing out some ideas so maybe don’t need to buy a tool that might never get used again
 
Took my feeler gages and measured .116” on both sides between the gear and seal lip (old crank).

I’m now remembering doing the same measurement on the new crank prior to rebuilding. The gap was not even on both sides, but I can’t remember exactly how much it was off. I can re-measure once I tear the engine back apart.

In hindsight that should have been a red flag.. I just couldn’t see the gear being off center like that especially on a brand new crank from WSM.

I think I’ll need a dial test indicator to get in around the seal and get a proper measurement.
 
Took my feeler gages and measured .116” on both sides between the gear and seal lip (old crank).

I’m now remembering doing the same measurement on the new crank prior to rebuilding. The gap was not even on both sides, but I can’t remember exactly how much it was off. I can re-measure once I tear the engine back apart.

In hindsight that should have been a red flag.. I just couldn’t see the gear being off center like that especially on a brand new crank from WSM.

I think I’ll need a dial test indicator to get in around the seal and get a proper measurement.
You know what your doing, I’ve never even had a crank out but to me it doesn’t matter how much it’s off center, just the fact that it’s off Center at all means it’s no good and what’s eating up the brass gear. I’m assuming you will be contacting WSM about this crank?
 
This is what the brass gear looked like after an hour.
IMG_8311.jpeg


I did some rough measurements with my feeler gages, measuring between the gear and inner seals on the crankshaft. On the new crankshaft, the gap between the gear and seal was .014” bigger on one side.

The gaps were even on the old crankshaft.

Old crank:
IMG_8313.jpeg
IMG_8314.jpeg

New crank:
IMG_8316.jpeg
IMG_8315.jpeg

If these measurements are accurate it seems like the gear is shifted .007” off center.


Im going to get better measurements in the future, but it definitely seems like something is up with the gear on the crankshaft.
 
Did you ever get anything resolved? I’ve built many of these 787’s and the last one I used a WSM crankshaft. Absolutely cannot keep a rotary gear. Has stripped 4 different brands. Gears and shafts. I contacted WSM but was told they had no reports of issues. I’ll be tearing the engine back down this week.
 
Did you ever get anything resolved? I’ve built many of these 787’s and the last one I used a WSM crankshaft. Absolutely cannot keep a rotary gear. Has stripped 4 different brands. Gears and shafts. I contacted WSM but was told they had no reports of issues. I’ll be tearing the engine back down this week.
Nothing new to report yet. I have access to a CMM machine and will be getting the original and wsm crank measured this week or next hopefully.

Does your rotary gear appear to be off center on the crank?

Definitely seems like there is something going on with these cranks…
 
I agree. There was another post just a couple of months ago I saw describing the same thing. I didn’t notice it being off center, but failed to take a measurement. I’ll update when I get it disassembled. Thanks for replying!
 
Not dumb at all! Yes, cavity is full from oil from oil tank. No restrictions, etc. It’s full of oil when I pull the rotary shaft out. This definitely looks to be a concern with these WSM cranks.
 
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