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Regarding the bed mount versus the ring mount, a Rotax mechanic explained the benefits of the ring mount to me a long time ago and I can’t remember what he said, but the ring mount is considered a bit better than the bed mount. Even so, due to lower cost (the ring mount for the Rotax is $700) I initially was going to use the Sonex bed mounts and more or less adapt the Sonex engine mount to the Onex. I called Sonex to see if I could look at one of their Rotax engine mounts (I live just an hour away from the factory) but they make them to order and didn’t have any in stock. I did order the Sonex rail mounts that bolt to the side of the engine but then discovered that these mounts conflict with the new Rotax factory exhaust system. The Sonex mounts might be compatible with the old Rotax exhaust system, but Rotax now offers this new system which is fully welded up and requires less work than the old (less expensive) exhaust system that they still offer. It would not have been a big deal to weld a 1” extension into two of the rear exhaust pipes of the new system to get it to fit but at that point I thought the universe was trying to tell me to go to a ring mount so I bit the bullet and ordered the ring mount from Rotax and went that route. By the way my Sonex rail mounts are for sale in the classified section here.
As far as the CG considerations for the Rotax, I did move the engine forward 7 inches.
If anyone is considering making their own mount, this program is very helpful in cutting the tubing to fit all the weird angles:
http://www.metalgeek.com/static/cope.pcgiThe program outputs a paper template that you tape on the tube that shows you how to cut it to get the correct angle. I cut the tubing with an angle grinder.
I also found a video on the EAA site that shows how to jig the engine mount. The attached photo shows the jig. The threaded rod set-up allows very minute adjustments to the mount, and is strong enough for a one-off welding job.
I had an 80 horse Rotax 912UL engine in a Zenith 701, and I grew quite fond of that engine. It gave me zero problems, just flew and flew.