# Yamaha RX-1 Snowmobile Engine Conversion

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#### Marc W

##### Well-Known Member
I first saw or heard of the Yamaha sled engines being converted for aircraft at the Copperstate Flyin last year. Teal Jenkins, the man behind Skytrax, had an Apex engine with a gearbox of his own design on it displayed. The sign said 160 HP at 165 lbs. I was smitten!

Recently a RX-1 engine, Skytrax adapter and Rotax "C" gearbox came up for sale. The seller started with a high price and I passed on it, as did everyone else. Weeks passed and the price dropped. A couple more weeks passed and I made a low offer and accepted a counteroffer. It was to good a price to pass up so I bought it. Here it is sitting in front of the airplane as I figure out the engine mount and where to hang accessories. Pictured is the "C" box, flywheel, Hardy disc and the Skytrax adapter.

This is a picture of a RX-1 engine in a sled. The RX-1 is carbureted and you can see the tops of the four carbs. The other necessary parts are the thermostat housing, which is the aluminum domed object at the near end of the cylinder head. The radiator header tank is left and a little above the thermostat housing. The engine has a dry sump so you need the oil tank which is below the carbs in the picture with all the hoses going to it. The carbs are electrically heated and also heated by coolant.

The RX-1 was built 2003 to 2005 and was rated at 140 HP at 10,200 RPM. It was replaced by the fuel injected Apex from 2006 to 2008. The Apex gained 10 HP or so over the RX-1. They are basically the same engine. They both have COP electronic ignition. They also have 1.19:1 internal gear reduction with a rubber damper in the countershaft.

There are other companies that make adapters to run the Rotax gearboxes on the RX-1. I have the Skytrax adapter. There are quite a few of these engines on PPC's, autogyros and the like. There are a few on airplanes. You can run a low inertia prop with the rubber Hardy disc. People running heavier props can use the RK400 clutch. The RX-1 prop turns left.

Teal Jenkins makes a gearbox that bolts directly to the Apex engine. His gearbox uses three gears and so it raises the thrust line a little and it makes a little shorter engine. Since it uses three gears the prop will rotate to the right.

So there it is, my next project!

#### TarDevil

##### Well-Known Member
I've been impressed with Steve Henry's installations. Looks like a winner.

#### cohocarl

##### Member
The conversion of the Yamaha Genesis (YG4) engines have caught my attention also...I picked up an unmolested '06 Apex with <2000 miles last year with the possibility of picking up a Skytrax adapter. Until then, it's a blast to ride.
There's a good Facebook Group on the Yamaha conversions.

#### Marc W

##### Well-Known Member
I finally started making a little progress on my conversion. Things were on hold until I got the exterior of the engine cleaned up some. It was ugly! It had areas of corrosion and a white coat of something that I finally realized was the remains of a clear coat. I worked on it with small stainless steel rotary brushes from Harbor Freight that work fairly well to clean up aluminum. There were to many areas I couldn't get with the brushes and just to much area. I ordered a soda blaster on Ebay. First, the seller couldn't ship because he was out of town. He had a death in the family and then he couldn't find it! It turned into a soap opera! I found an open box blaster at Harbor Freight for a good price. Bought it and discovered the tube was missing that carries the soda up out of the tank. I did some measuring and found a piece of 1/4" tube in the scrap pile. Guessed at hole sizes based on the diagram in the operators manual and made the part. It sorta worked! Adjusted the hole sizes and it worked like a champ. I braved the 22 degree temps and put up with the fogging eyeglasses and blasted it. I couldn't see and I missed a few areas but it is good enough.

I finished the cleanup yesterday and today I installed the Skytrax adapter and the Rotax flywheel. The Rotax flywheel has the same taper as the PTO shaft on the RX-1 engine. The diameter is a little different so you have to cut the end of the PTO shaft off like so.

The shaft needs to be recessed in the flywheel like so.

This is a shot of the internal reduction gears in the engine. 31 teeth on the crank gear and 37 teeth on the PTO gear. You can also see the timing chain. It is a sophisticated little engine with dual overhead camshafts and five tiny valves per cylinder.

Here is the adapter and flywheel mounted and bolts torqued.

#### cheapracer

##### Well-Known Member
Log Member
Noice!

Great to see a real genuine alternative cost effective engine coming through the cracks, this is the real deal.

#### Marc W

##### Well-Known Member
Didn't get much done today except measuring and figuring. The engine originally came with 4 carbs. They have electric heat and coolant heat so more wiring and plumbing plus rebuild and rejet the carbs. To much complexity for my simple mind! I am going to use the SDS EFI instead. SDS needs a trigger wheel driven at crankshaft speeds. The RX-1 engine does not have any external crank driven shaft to mount a trigger wheel on. The PTO shaft is driven at a reduced speed so it will not work for SDS EFI.

Todays project was to design an adapter to drive an external trigger wheel. I took inspiration from "Irene the Gyro" on Facebook. Irene's builder made an adapter to drive a pre rotator off the end of the crankshaft. Below you can see the alternator housing and through the hole in the center of the housing you can see the bolt that holds the alternator rotor on the end of the crank. Next picture shows the housing and bolt removed. Also visible is the starter drive gears, but I digress.

Irene's builder bored out the center hole in the housing and installed a 35mm x 50mm oil seal there. He didn't give details on the extension, but I assume he bolted it to the end of the crank shaft possibly with a longer bolt. I will do the same. My extension will be lighter since I am not driving a pre rotator and I am going to use a 49mm seal to leave more meat in the housing. I finished my design and found a suitable piece of bar stock in the scrap pile today. It is supposed to be warmer tomorrow so I will be in my unheated "machine shop" making chips tomorrow.

#### Marc W

##### Well-Known Member
I got the crankshaft extension done. It took way to long. I didn't have suitable toolbits so I had to grind some new ones. Had trouble with the lathe. It wouldn't shut off so I would have to unplug it to stop it. I tried a bunch of different things and finally discovered one of the magnetic switches was sticking when it got hot. Went in the house and found one on Ebay. Went back out in the shop and discovered it works fine if I leave the switch box open. It just needs a little cooling air and there is no shortage of that today!

A pic of the almost finished part in the lathe and a pic of where it will mount. More parts to make but the rest won't generate as many chips!

#### Marc W

##### Well-Known Member
I made more progress on the trigger wheel today. It looks like I got it a little close to the housing. I want to mount the Hall sensor between the wheel and the housing. It looks like it will be tight. I may have to modify it later. We will see.

Next step is to bore the housing out for the oil seal. I have been pondering how to keep the bore concentric to the shaft. Maybe I will get lucky!

#### rv7charlie

##### Well-Known Member
[snipped]

There are other companies that make adapters to run the Rotax gearboxes on the RX-1. I have the Skytrax adapter. There are quite a few of these engines on PPC's, autogyros and the like. There are a few on airplanes. You can run a low inertia prop with the rubber Hardy disc. People running heavier props can use the RK400 clutch. The RX-1 prop turns left.

Teal Jenkins makes a gearbox that bolts directly to the Apex engine. His gearbox uses three gears and so it raises the thrust line a little and it makes a little shorter engine. Since it uses three gears the prop will rotate to the right.

So there it is, my next project!
Hi Marc,

Can you point us to a web site detailing Jenkins' 3-gear reduction? I"m not having any luck finding info on it.

Thanks,

Charlie

#### Marc W

##### Well-Known Member
Jenkins doesn't have a website. He does frequent the Facebook group. He will also return phone calls so your best bet is to call him. His number is (623)734-0185.

#### rv7charlie

##### Well-Known Member
Thanks, Mark. I'll hang onto the number, but I hate to take up his time for what amounts to 'window shopping' at this point. Are there any specifics about the drive in the Facebook page? I spent almost an hour in the publicly accessible section last night looking through the photos section, but didn't see any info on the 3 gear setup. I also tried looking through some of the messages, but couldn't figure out any way to search for specific terms. No idea of weight, price, power handling, which engines it will fit, etc etc.

Thanks,

Charlie
(not a fan of FB)

#### Marc W

It is machined to only fit the Yamaha Apex engine. There is a company in Norway that is testing it on a turboed engine running up to 300 HP. Steve Henry is flying a turboed engine with it. I don't remember the numbers off hand but over 200 HP. Last years price was $3500 but I don't know if it still is. Just remembered there is some info here: http://avidfoxflyers.com/index.php?/topic/5824-yamaha-apex-skytrax-adapters/ #### Marc W ##### Well-Known Member Finally made some progress. I have been tearing my hair out trying to accurately locate the oil seal bore for my crankshaft extension. I started with the large threaded hole in the housing. The extension was to large to fit through the hole so I first set the cover up on the mill and bored the threads out. Next I had to locate the center of the bore. It took a while but I came up with the idea of assembling the housing and extension and filling the hole around the extension with Bondo. Then I could locate off the hole in the Bondo. Worked good except I screwed up and had to repeat the process. I set it up on the mill and did a preliminary cut and then put it back on the engine to check the runout. The runout was excessive so I figured I made a mistake and adjusted the setup on the mill and made another cut. Rechecked it on the engine and still no good. I bought my lathe and mill/drill from an estate. The lathe was filthy but had been lightly used and it does good work. The mill/drill was also filthy and had clearly been hard used in a production shop. It does fine for most of what I do but it didn't work so well to accurately locate this hole. The spindle finds a different center when it is under power than it has when rotating the spindle by hand. The lead screws have a yard of back lash and things are just a little sloppy. After 3 days I gave it up and decided I was going to have to use the lathe. It is to hard to get the chuck off and put the faceplate on so I made a face plate to clamp in the lathe chuck. That also allowed me to dial in the bore by adjusting the chuck jaws. Success! Here is the finished bore. I ended up with less than .001" TIR(total indicated runout) on the oil seal bore and I have under .002" TIR on the extension. That is within the max runout usually allowed on oil seals so I am well satisfied. #### rv7charlie ##### Well-Known Member It is machined to only fit the Yamaha Apex engine. There is a company in Norway that is testing it on a turboed engine running up to 300 HP. Steve Henry is flying a turboed engine with it. I don't remember the numbers off hand but over 200 HP. Last years price was$3500 but I don't know if it still is.

Just remembered there is some info here: http://avidfoxflyers.com/index.php?/topic/5824-yamaha-apex-skytrax-adapters/

#### Marc W

##### Well-Known Member
House projects are really interfering with my airplane habit!

I needed a way to index my crankshaft extension. The alternator rotor is keyed to the shaft with a woodruff key so the keyway is not cut all the way to the end of the shaft. The keyway in the rotor goes to the end of the shaft and the end of the shaft is recessed inside the rotor. I made a little L-shaped key to slip into the keyway and milled a slot in the base of the extension to match.

Key installed.

I have been puzzling over how to fit everything inside the cowl because that will effect the engine mount design. I have a things pretty well layed out except the oil tank. I have only seen examples of this engine installed in 2 seat side by side aircraft. My narrow little single seater just doesn't have room to install the stock oil tank. I will either have to bulge the cowl to fit it in or make a custom tank. My next step is to get engine and airplane lined up and build the engine mount. Maybe I can get the oil tank in too.

#### AIRCAB

##### Well-Known Member
I see no harmonic isolator between engine and PSRU. What is the theory behind not using one. I am new to this engine application.

#### Marc W

##### Well-Known Member
Look again at the first picture in the first post. The part to the right, in front of the flywheel is the rubber Hardy disc that bolts to the flywheel. The pinion gear is driven by the Hardy disc. The engine also has a rubber damper built into the internal gear reduction.