I have been doing some looking into doing a conversion on a yamaha YG4 apex FI snowmobile engine. I have found a lot of older discussion on some other forums on using them in Highlanders but am wondering if there are any zenith builders who have successfully done this conversion? On a CH750 STOL in particular. I believe there is a integrated gearbox available that is out there for there engines from a company called skytrax that is supposed to handle around 200hp. Is this worth looking into further or am I wasting my time? It seems like it would be a pretty good fit to me but I would like some opinions and information from a few folks who have been at the experimental aviation game longer than my 2 months. Thanks in advance for your replies.



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It could be done for shure

I beleive the adapter uses a rotax geaebox

Price should be reasonable

Performance shoild be great

The question is how muxh time is it going to add to your build

You will have to customize the firewall forward,

Engine mount, cowling , radiator,  fuel system.

Firsr step would be to figure  out weight and balance.

Then you can determine how far from the firewall the enginw should sit.

Look at hiw long the ul motor mounts are.

Thanks Fred. It sounds like the Highlander installs are saying around that 180# mark for the engine, gearbox and accessories. I think the UL350 is around that 180# neck of the woods. Do most builders do their own mounts or 3rd party them out for design and construction? How does a person test them for load if you do it yourself?

I’m slowly putting an Apex into my 601xlb. I have the engine and half the mount that bolts to the engine. Still need to order the gearbox and do the math to weld up a mount. After seeing one of these engines in a flying plane you will be sold. They are a beast. But no doubt will add a lot more work and time to the project.

Hi Mitchell,

I would love to see some photos. I have just been reading about the YG4 engines and you may be onto something. Great power to weight ratios. Please tell us more! Cheers!


There is a a youtuber doing an APEX build in his kitfox, he is documenting his whole build.



I have been following the project kitfox channel for a while now. It is slow progress but he has some good information. I now have another complicating factor. Although I am quite a ways out from needing an engine I was more or less settled on the apex conversion despite the added headaches of doing most of my own work for the conversion and firewall forward. Now I have discovered that my mom's uncle had bought a partially finished project which included a freshly zero timed O235 Lycoming. He has agreed to sell me the entire project, tools, materials and engine with a new prop included for a more than reasonable price. I had not considered the O235 because of its weight but I know there are a few 750s flying with them. Might be too good of a deal to pass up. I could always swap engines later if I find the performance or weight penalty unacceptable. Still deciding. 

Thanks to whomever asked to have the Yamaha engine added to this forum. I’m putting a Yamaha Apex into my CH750 Cruzer build. The gearbox is from Teal Jenkins at Skytraxusa.com.  $3500 US.   Lots have used a Rotax C or an RK400 gearboxes in past but everyone I know now is using one of Teal’s gearboxes.  He has them for the Apex (150 to 300+hp with added turbo), Phaser (80 hp) and soon one for the Sidewinder  (200+ Hp factory turbo) that is in production and crate new engines are available.  Working on the electrical at the moment for my Apex and plan to be flying late spring.  Check out Facebook group Yamaha Aircraft Conversions as everything you may want to know is there.  I designed and built my own engine mount if anyone is interested.  Also will be using a carbon fiber E-Prop from France which I can get for forum members if interested as its an amazing prop that only weighs 2.6 kg / 6 lbs. !  I’’ll post pictures and updates on my build here as well for those that are interested.   Cheers. Jim


I'm building a ch 650 and am thinking of putting a non turbo yamaha nytro 3 cylinder engine on it.  Would like to hear about how you fabricated your engine mount.




I based my engine mount design on the bed mount Rotax 912 engine mount and the Yamaha engine mounts which can be seen in the Yamaha Aircraft Conversions Facebook site.   The Yamaha and Rotax are very similar weights so dimensions from firewall etc will also be similar.  I designed it using Solidworks.  Made templates from the drawings to cut and cope the tubes.  I made a weld jig but others have just used the firewall and tacked it together but you’ll have to hold the engine in place using a lift or something. 


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