stihl 25cc conversion.
 

I have managed to snag a 25 CC STIHL engine from a weedwacker. I've stripped of the plastic, but not real sure where to go from here, and would like some help. (This will end up on an airboat, so min weight is not as big an issue). I know that i need to take off the flywheel, but is it a right or left handed nut?

Also, Are there simple cheep things that I can do to up the ponies on this? Preferably thigns I can do myself? I have a really tight budget, so going all out is not an option. What one item woudl you upgrade? Carb? Muffler?

Also if you have any web-links to help with this that would be great.

Thanks!

RE: stihl 25cc conversion.
 

howdy, I'm pretty sure its a regular nut and not left handed, as for increasing horsepower the first place to go is the muffler IMO. The stock mufflers are somewhat restricted. Your 25cc has more hp than the 27cc bc yours is off of the "pro series" equipment. Are you running an adjustable Walbro carb or the Zama junk? Walbro wt227-1 or wt264-1 would be a possible upgrade if its not adjustable that way you can really dial in your prop;) I've never seen one converted for a boat but sounds cool. Oh almost forgot if its a used trimmer engine ck the exhaust port when u get the muffler off and clean out the built up carbon. What size "prop" will this thing use?

RE: stihl 25cc conversion.
 

I saw the carbon, and was thinking the same thing.

I just checked the carb, and it's a zama. Thanks for the heads up.

I'm not real sure what size prop I'll be using, most of teh airboaters are using a 14 to 16" prop, haven'r really decided on teh pitch, but it will be lower than most planes, so the RPM's will get up there

This is my first attempt at this. Most of what I know is nitro, so all help is appriciated.

RE: stihl 25cc conversion.
 

PS how do I get that stupid flywheel off?!?!?!?!?

RE: stihl 25cc conversion.
 

take clutch off then thread it back on just a few threads. Now hold the flywheel in one hand and hit the clutch with a hammer and she'll break loose. This way you wont damage the crank threads;)

RE: stihl 25cc conversion.
 

Be careful with the hammer. You can repositions the crankshaft in the crankcase and cause trouble?

Enjoy,

Jim

RE: stihl 25cc conversion.
 

Now that I got the fly wheel off, what's next? (are these balanced like a prop?) I know some lighten the flywheel, do i need to, how much of the fins do i take off?

Do I need to take apart the case for any reason?

Do I need to open up the exhaust port?

Sorry for all the dumb ?'s, but there is no one around here that converts engines are far as I know anyway.

thanks for all the help!!

RE: stihl 25cc conversion.
 

The flywheel fins are usually removed down even with the height of the counter weight. It is normally balanced. I use the High point balancer from B&B but the Dubro works fine.

Enjoy,

Jim

RE: stihl 25cc conversion.
 

no dont take the case apart or change anything about the exhaust port. Do a search on "25cc Stihl" in this forum and you'll find some pics. Ken Lambert, Lambertrc.com converted my Stihl for aircraft use. Ck out his website! Ck out his CNC machines! I dont know how busy he is now but he could machine a hub for your boat prop. I guess you will leave the recoil starter and rail mount this bad boy?[sm=surprised.gif]

RE: stihl 25cc conversion.
 

never mind my dumb ? I just read the post over and realized your building an "air boat", but you can rail mount if you want to. Dont necessarily need the starter though:eek:

RE: stihl 25cc conversion.
 

Do I balance the flywheel like a prop? Should one side be heavier than the other? Don't really need to have this thing self-destruct in my face.

I'll post some pic's once I get some of the side work/trimming done.

RE: stihl 25cc conversion.
 

what is the easiest way to rail mount this anyway? That's my preference w/the design that I am putting together.

Also, anyone have a guess what RPM this should turn?

Thanks all

RE: stihl 25cc conversion.
 

should turn 10,000 rpm, and yes definately BALANCE the flywheel or dont touch it at all. If you dont have a lathe better let someone else do it. These flywheels can explode (literally) in your face if out of balance or cracked.

RE: stihl 25cc conversion.
 

I balanced my flywheels just like balancing a prop. I've done a half dozen. I've never had any trouble with them.

Enjoy,

Jim

RE: stihl 25cc conversion.
 

One more stupid question.

What size of nose cone will I need? is 3" big enough or do I need something bigger?

RE: stihl 25cc conversion.
 

there is a pic f 25cc on my website if you would like to see one www.lambertsrc.com and I also have mufflers made for them

RE: stihl 25cc conversion.
 

thanks,
I looked for this site earlier as it was mentioned, but they spelled it wrong and couldn't find it.


I book marked it, so I'll be able to reference it as needed.

RE: stihl 25cc conversion.
 

Why don't you take off all the fins on the flywheel? It's not needed for cooling, I assume then it's for balance?
I know it's a dumb question, but like a said before, I'm just begining to learn all this stuff.

RE: stihl 25cc conversion.
 

if you take all the fins off it wil lesen the drag but not enough you would ever notice .and it makes for a cleaner look but no wieght savings that is noticable either.

RE: stihl 25cc conversion.
 

Plus, with our airplanes we replace the flywheels cooling effect with a "prop" in the front. You will be using your prop in the back so I assume you will need to leave the flywheel and the shroud (to direct the airflow) unless I'm missing something.

RE: stihl 25cc conversion.
 

I gotta go to Sioux Falls tomorrow, and plan on picking up some parts while there. I know I need a resistor plug to stop interference w/Rx. It was mentioned by a buddy that I may need a way to insulate the plug wire and coil. Do I? If so, what do I need?

Thanks
Chad

RE: stihl 25cc conversion.
 

The fins should create turbulence in front of the engine and hamper clean air moving across the fins.
Plus the fins need a shroud to direct the air a cross the cylinder and head.

As long as the plane is flying there should be an ample supply of air across the engine any way.

Or am I missing something?

RE: stihl 25cc conversion.
 

its a boat

RE: stihl 25cc conversion.
 

Hi,
Is it a pusher or tractor? Is the prop in front or back? If the prop is in back, you can use aluminum flashing from the hardware store to make a shroud to duct the air past the fins. If the shroud extends to a 1/2" of the prop, the prop should suck air through the shroud cooling the engine. It would be formed in the shape of a squared off inverted "U". Trackers always cool better.

Dave

RE: stihl 25cc conversion.
 

ok, got ahold of stihl, and they said engine turns clockwise.

this is a probLEM ISN'T IT? DON'T I WANT iT TO turn counter clockwise?

i hant not decided weather this will be pusher or puller set-up.

sorry abou caps, baby is on lap, helping:D

RE: stihl 25cc conversion.
 

that would depend on which way you mount the engine I would think

RE: stihl 25cc conversion.
 

don't normal nitro's go counter clock wise?
so wouldn't i need a special prop?
or am i just really messed up and need to go to bed?

RE: stihl 25cc conversion.
 

they normally run clock wise , standing behind the engine is the way you look at it .

RE: stihl 25cc conversion.
 

actually, i think if it runs clockwise, that would be great. you can mount the engine like how a pusher should be mount, yet you will have the luxury of choosing all the puller props!! that would be great i think. isnt it?:eek:

RE: stihl 25cc conversion.
 

here are some pics of what I have accomplished so far.

Please let me know if there is more metal that I should remove.

http://img.photobucket.com/albums/v2...r/100_1760.jpg

http://img.photobucket.com/albums/v2...r/100_1759.jpg

http://img.photobucket.com/albums/v2...r/100_1757.jpg

http://img.photobucket.com/albums/v2...r/100_1756.jpg

RE: stihl 25cc conversion.
 

I make cnc mounting plates and prophubs and mufflers ( tig welded aluminum ) for these engines the is one on my website you can look at.

RE: stihl 25cc conversion.
 

As soon as I get some cash flow freed up, I'll be hitting you up for those Kenneth

RE: stihl 25cc conversion.
 

I'm pretty sure it turns counterclockwise like all their trimmers. The recoil starter mounts with the rope handle coming out the left side right? the stub shaft the starter bolts to should turn clockwise and the pto side will be counterclockwise. Good for puller not pusher. Thats why its such a good airplane conversion.

RE: stihl 25cc conversion.
 

Tripp , I have the tooling ordered to make your parts as soon as they arive I will get on that engine .



One other thing which way does a clock turn mine all seem to be digital !!! [sm=biggrin.gif]

RE: stihl 25cc conversion.
 

clockwise duh! :D

RE: stihl 25cc conversion.
 

Hi,
Clockwise isn't a problem if you use a pusher setup. You will be able to use standard props with the flat side if the blade facing to the rear. Use the metal shroud that I mentioned earlier to help draw air over the fins. It will make a cool setup. LOL:)

Dave

RE: stihl 25cc conversion.
 

I emailed Sthil, and they said it was made to go clockwise. So I'll plan accordingly.

I don't really like the idea of a pusher for 2 reasons.

1 poor prop selection

2 moves the CG way back, makes it less stable.

RE: stihl 25cc conversion.
 

You can have a pusher with out affecting the CG.

But you are dead right about prop selection.

RE: stihl 25cc conversion.
 

Hello!!
Pusher props are for engines that turn counter clockwise, not clockwise. The beauty of the engine is that it will push with standard props. With this setup you have the same selection as a tracker. You can position the engine for the best C/G either way. The only concern is cooling and a aluminum shroud will fix that.

Dave