Running a RK400 clutch on a Rotax C (RXC) gearbox requires inspection and/or wear maintenance such that every 75 hours (like about once a year for most weekend warriors) the mechanic must disassemble the RXC by first draining the oil then splitting the case halves, and then pulling the back half of the PSRU off of the engine mount or adapter. There are other solutions to this type of power plant conversion than this time-consuming, vexing and messy adaptation.
First: Why use a clutch at all? Easy starting. Prevent cracked welds. Some YG3 and YG4 conversions with certain prop combos will absolutely not start w/o one.
What else is there? Mohawk's GT4, a maintenance-free roller ramp clutch that is a little bit smaller and exactly 1/2 the weight of an RK400. (RK400 weighs in at 7 pounds, the GT4 clutches weigh 3.5 pounds.)
Next: Why use an RXC gearbox? I've worked with them. We all know of Steve Henry, and he uses it - right? Well, here's the thing - He uses them for one reason, and one reason only: It is shorter than other mass-produced gearboxes that are readily available, and thus allows it to fit under the cowl of his Highlander w/o mods.
That's it. There is no other reason.
The first time I split one open and looked inside I walked away and looked to other solutions w/o a second thought. That was 8 years ago. Since then I have read on other forums and heard first-person accounts of RXC bearings failing after as little as 12 hours, and others at 300 to 400 hours on high HP applications. These horror stories are kept under tight wraps for the most part, no one wants you to know how fragile the RXC can be.
Guys, I don't know what you "know" or don't, I don't know what you've been told, but there are other options available. First in 2012 came the universal Mohawk YG4 adapter which fits any PSRU ever built. That super light-weight adapter has been flawless from day one - tested and flown for at least 500 hrs with many different types of PSRUs, including an amazingly smooth, robust, versatile and light weight Hy-Vo chain drive, the Mohawk Silent Drive which includes a prop-strike protection clutch.
Next, a couple of years later, Teal called and inquired about my adapter, but decided he didn't want a "stacked" adapter, instead he wanted something to replace the crankcase cover and went on to fabricate and sell (much to my chagrine) his own adapter, limited to the RXC and YG4 (RX1). I ahve no idea why he required an engine cover replacement. If we were dealing with my old 500 HP 500 Ft Lbs torque Porsche GT1 race car I guess I could see the point, but in my extensive motorcraft experience and knowledge of engineering practices I decided - quite rightly - that adapter design was more than adequate. The proof is in the pudding: the design has been a perfect, a flawless performer right out of the gate and from then on for well over 500 hours to date and counting. Both types of adapters use exactly the same mounting points, removing the engine cover is pointless with only 150 HP and 90 FPT involved. The way my adapter is built, fitted and attached it simply cannot move under these forces, and any difference in performance of the two designs for this application has been well proven to be a moot point.
Regardless, there is now a third option which, like Teal's YG4-to-RXC adapter, replaces the crankcase cover. Unlike Teal's model which is designed around the diminutive RXC, this one fits a heavy-duty SPG or Mohawk SP gearbox, rated at 9000 kgm (compare to RXC at just 2/3 that, 6000 kgm). I do not fabricate these adapters. I do, however, sell all three - Mohawk universal GTA, The latest uni-cover adapter for the Mohawk SP and the Air Trikes SPG, and the Sky Trax uni-cover adapter for the RXC. They all sell for $1500.00. But, tell you what: I am in a generous mood, and will sell you one of these new uni-cover adapters for just $1195 for a limited time. Again, that is for a limited time, only, so if you see this post next month and want this deal, I doubt I will still be feeling so generous.
This third adapter is for use with a special Mohawk SP gearbox (MSP, which is the same as the Air Trikes' SPG version but built to different specs to accommodate a shorter installation and clutch of your choice). Comparing to an RXC: The shafts are much larger diameter, the gears are much wider, and the bearings are 50% larger. All of this means more weight, of course, and the proof is in that the MSP comes in around 18 pounds compared to the little RXC which weighs around 14.5 lbs.
But connect the RXC to a RK400 clutch and you get 14.5 = 7 = 21.5 LBs, whereas the Mohawk SP with GT4 weighs 18 + 3.5 = 21.5 LBs. It's a wash.
You don't have to "settle" for the high-maintenance RK400 nor do you have to "settle" for the RXC which was spec designed to a 90 HP Rotax 582 or 617 engine. Want numbers? (Again) The RXC is rated at 6000 KGM, the MSP (and SPG) is rated at 9000 KGM.
I have been building YG4 and YG4i (Apex) adapters for many PSRUs, including RXC, Arrow, SPG, Mohawk SP, and my own Hy-Vo Silent Drive since 2011 - several years longer than anyone else - and you will not find anyone, anywhere, who has built YG4 adapters for more than one, single type of PSRU - let alone five very different types, for both the 140 HP YG4 and the 150 HP YG4i (Apex).
Did you catch that last line? You don't have to "settle" for a carb 140 HP YG4. The 150 HP YG4i Apex gearbox and adapter conversion kit is here - and has been here since January 2017.
Mohawk Aero is the only source for the YG4i - Yamaha Genesis 4 (Apex) 150 HP fuel injected motor aircraft conversion kit. This isn't some dream, this isn't some prototype to be built somewhere down the road in the future. The future is now, and it is here.
Personally, I have found the Hy-Vo Silent chain drive to be far smoother, lighter, quieter, and efficient than any gearbox I have ever tried regardless of brand, and I've run a bunch of them. They don't compare to the Hy-Vo silent chain, and the big difference is most notable in flight with this type PSRU being far smoother than any gearbox could ever hope to be. As goes efficiency, bear in mind that with every helical gear & shaft you lose 1-2% power through heat generated by the axial loads produced against the bearings and housing by the angular-cut gears pushing them sideways. For a two-gear set that is 3-6 HP on a 140-150 HP engine. For a three-gear set, that works out to 4.5 - 7.5 HP lost (this can be determined with an infra red heat sensor, converting the PSRU heat measured to HP converted/lost), not to mention added, weight, noise and vibration transferred throughout the power plant and airframe.
For more info and help with YG4 and YG4i conversions, using RXC, SPG, MSP, Arrow, & Silent Drive PSRU applications, or any of another type you come up with on your own, email Greg at