I’m going to look at two used Rotax 912ULS engines next week. I’m bringing along two friends who know the engine very well and both happen to know the seller as well.
If everything looks ok with the logs and everything, what parts do I need to mount the engine to my Sonex B kit?
The kit came with a Jabiru tri-gear engine mount, but I’m not a fan of how the nose gear is offset to one side (which is part of the reason I’m considering the Rotax).
I’ll take a crack at it since I’m building a trigear legacy Sonex with a Rotax 912is. Most of it should be the similar but I’ll note if I’m not sure. The Rotax install costs quite a bit more than the other engine installs so make sure to budget for that in addition to the cost of the engine. As I’m transitioning to prepare for flying my Sonex, I just flew an RV12 with the 912ULS and I’m really happy I’m going with the Rotax. For the below I’m also assuming using the new ring style mount, not the bed mount. To my knowledge below gets you as close as possible to a FWF kit but there are other ways to install as well.
Rotax parts
-If the used engine doesn’t have a Engine Suspension Frame (AKA ring mount) you’ll need one of them PN 886563. You don’t need the bushings as you can use the Sonex ones: https://www.advancedpowerplant.com/rotax-engines-parts/genuine-rotax-parts/4-stroke-parts-912-914-series/912-914-series-parts/engine-suspension-frame/912-914-engine-suspension-frame.html
-If the used engine wasn’t used with a different style of mount you might also need different water pump outlets if they interfere with the suspension frame
Sonex Parts for B model: Sonex Web Store - B-Model Rotax Ring Mounts B-Model Rotax Ring Mounts
-Engine mount
-Bushings
-Prop extension
On the legacy, the hole for the nose wheel steer rod is the same as the more centered one as the Aerovee and there wasn’t any changes to the nose wheel parts . You may want to verify with Sonex for the B-model.
Other Parts
-Propeller: Sonex sells a 60" Sensenich. There are a few other 60" options out there but I didn’t see any tested on a Sonex. Some people use larger ones with a tailwheel but with the trigear ground clearance is a bigger concern.
-Spinner: The above Sonex prop uses the Aerovee spinner. Different propellers may need a different spinner solution
-Cooling system: Casey Cooper makes a kit that he’s modifying to work with the new ring mount: https://www.cooperaircraftcorporation.com/release-of-sonex-a-and-b-rotax-installation-kits
-Oil tank mount: Oil tank has to be mounted vertical. Casey also makes a bracket with his kit for the firewall. I’ve also seen it mounted to the engine frame
-Exhaust: Toucan makes as system http://www.toucanexhaust.com/ I believe that the stock Rotax exhaust also fits but it can’t be used with the Cooper cooling system. Check with him if you want to go that route
-Throttle: The McFarlane MCTV throttles seems to be the most recommended and has a duel cable one. I just flew an RV with one and really liked it.
-Choke cable: I’m not too sure on this.
-Fuel Pump: I believe a low pressure electric fuel pump is required in addition to the mechanical one but I’m not familiar with them
-Fuel Filter: Also not familiar but I’m sure the installation manual has specs
-Fuel Pressure transducer/gauge: Check the installation manual for specs
-Check Valve & office for fuel bypass: Andair make nice valves that the 912is system uses. Check the installation manual for specs
-Ignition and starter switches: I believe they’re different than the typical magneto ignition switch. Check the installation manual for details
Rotax has their installation manuals online which helps to spec out all the parts: Technical documentation | Manuals | Rotax Aircraft Engines
Also check out this video about the Rotax install: https://www.youtube.com/watch?v=0tCOz3SsSsw
Wow, this is a great reply. Thanks for taking the time - I’m sure others will look back on this post going forward. I contacted Sonex as well and got a pretty good answer from Mark about it, but not as detailed as what you posted.
Is there a reason to go with the ring mount instead of the bed mount? Is the bed mount not as good for some reason? It’s definitely less expensive, especially when you factor in the cost of the engine-side ring that you still have to buy for another $1400 + parts (see: Lockwood Aero | Rotax Aircraft Engines).
Unless there’s a good reason to avoid the bed mount, I think I’ll probably just go that direction. If my engine comes with the suspension frame already attached, I might go with the ring mount because the cost is not far off at that point.
Anyway, thanks again for the reply!
I have the bed mount and wish I didn’t!
It lets the engine move quite a bit and under power it pulls the engine down and to the left enough to give me cramps holding enough rudder in to keep balanced flight. I also was running out of rudder on climb out with left x winds. I eventually made new bed rails and gave it 2.5 degrees right thrust and 1 degree up which now has the Waiex in cruise with almost neutral rudder to keep balanced. I have only done 10 knot x winds but it now handles that quite well. I also put harder mount rubbers on at the same time as I did this modification.
The ring mount should support the engine much firmer and also allows for these kinds of adjustments without having to make new parts. I guess you will have to weigh up the cost difference but definitely the ring mount is a better option if budget allows.
I forgot to mention, the bed mount isn’t rated at 6 G. Can’t remember what it is rated for but perhaps a question to Sonex could clarify this. Something to consider if you want to do some Loops etc.
Thanks for the additional info. I’ll need to reach out to Sonex for more info. I don’t plan to do any aerobatics, but I would still want an engine mount that meets the max G rating of the rest of the airframe.
Out of curiosity, did you use the newer bed mount system? Sounds like Casey Cooper helped design it and it’s better than the one Sonex originally sold.
Yes, I used the newer version of the bed mounts that Casey helped design.
Some other parts Rotax you may need with a used engine:
-Oil tank: They’re not cheap if they don’t come with one but may be able to find a used one
-Starter relay: I believe Rotax specs a specific one
Reduced engine movement and stronger mount are why I went with the ring mount and considered making an adapter before Sonex came out with their own mount. It may be worth trying to find the engine side ring mount used.
ampzapper has supplied a lot of good information BUT his lack of Rotax experince has let him (you) down in a number of relativly minor points. In reverse order;
The ring mount system is the best, however I have the Mk 2 bed mount with slightly stiffer rubbers (than Sonex supply) and home made cup washers - it does the job just fine.
Any 12 Volt starter realay will do the job. See Rotax Installation Manual
Oil tank can be mounted on the engine frame or firewall. The important thing here is that its mounted within Rotax height specifications, relative to the oil pump. See Rotax Installation Manual.
The Mk 2 Sonex /Rotax engine bed mount is significantly better than the Mk1 (which I would not use in a pink fit).
Ignition and starter switch. Your choice however Rotax advise against the 3 position rotating (Cessna style) ignition switch.
Check Valve & office (?) for fuel bypass: If you have a single (Sonex standard) tank any single fuel ON/OFF valve will be suitable. Can only guess what “office for fuel bypass” is about.
Fuel Pressure transducer/gauge: Not required but a handy safety feature. Depending on what sort of instruments you have/are installing this may be stand alone or integrated with an EFB eg Dyon Skyview.
Fuel Filter: I favour in line , transparent gauze filters that can be sourced from Hengst - H102WK, H103WK or Baldwin - BF17850, BF7863 (& others). DO NOT USE pleated paper media style. Gascolator’s are another but significantly costlier/heavier option. Note: There are a lot of “fake” imitation filters on the market, be sure to purchase the genuine item.
Fuel Pump: The engine has a mechanical pump, however I recomend an additional in line back up/auxiliary pump - “Facet” make a wide range /very poplar with home builders. Be sure to select a pump that is within Rotax specifications.
Choke cable; Rotax 912’s have two carburettors so the “choke” cable (technically an enrichment circuit )will need to be a Siamese (two wire) style. Any descent motorcycle cable will do the job nicely.
Throttle: Again a Siamese cable is required, as above. The cockpit end will depend on throttle style - panel Vernier knob or quadrant style.
Exhaust: Rotax OM is the best (matched to engine & likly more durable)
Cooling system: Casey Cooper kit looks the goods however I have a fairly standard arrangement that works well - ie don’t think that there are no other viable alternatives.
Spinner; Should be matched to your propeller and engine cowling arrangements. Can be metal or composite,
Propeller: There are a staggering number of suitable props. Ground adjustable types probably the best bang for the buck, as can be “tuned” for your aircraft/mission objectives. Murray Parr has fitted an e-Prop which sounds promising. I have an Airmaster CS/ Whirlwind 3 blade, Blade type: WWR64W - considerable overkill.*
Not mentioned - I strongly advise the installation of a fuel return/bleed to tank system, to reduce the chances of fuel vaporisation impacting negatively on your hot day engine performance.
Join the Rotax Owners Forum for really good answers & resources (Installation /Operators /Service Manuals/ etc)t o all Rotax questions. 
Thanks for the follow-up! Not sure how I missed this, but I appreciate that you took the time to write it.