SharpProp

Hello,
This weekend I was trying to tune my OS 120 ax when it started making a weird noise and died. Further inspection showed that my prop came loose.
When I got it home I tried to take the prop off. But the main nut holding the prop on would not come loose. When I turn the nut, the crank turns with it. The prop can spin a little without turning the crank but the nut seems to be jammed up.
I'm really hoping that I didn't strip anything.
Any advice would be greatly appreciated.

big max 1935

My Feedback: (9)

One of my favorite tools is a piston stop that screws in the glow plug opening . The R/C car guys use them & one of their suppliers will have one . I rebuild a lot of engines & about 1/2 have plier marks on the drive washer . You can cut a piece of leather off a old belt that would go almost around the drive washer & then use a curved jaw pliers. Last would be to cut the prop off & then put a vice grip pliers on the crank shaft (TERRIBLE! I don't believe I said that !) could wrap some tape around it first I guess. DON'T stick any thing in the exhaust port !
Anyhow ,good luck. Max H. []

Gray Beard

My Feedback: (-1)

I just ran into this little problem last week with one of my four strokes. It was some bad threads in the nut, not the crank. I took and old glow plug and drilled and taped it for a 4-40. Installed the plug and screwed the screw in until it held the piston at TDC and removed the nut. I like the store bought piston stop but I saved myself 8 bucks by spending five minutes making my own. I know Hobby People stocks the piston stops.

ARUP

The plug is 1/4"- 28tpi. Buy a socket head bolt and screw into the glow plug hole to stop the piston at about mid travel. I have never needed to do this but would try it.

big max 1935

My Feedback: (9)

Glow plug threads are 1/4 x 32 . Max H.

Jetdesign

My Feedback: (8)

I don't like the idea of the end of a bolt coming in contact with the piston while trying to remove the prop nut. You are obviously going to scratch and dent the piston. I would remove the engine head and/or the back plate and work from there. The crank is a solid piece of steel and can handle a bit more abuse.

RCKen

My Feedback: (9)

Joe,
 Piston stops, and a bolt in the case of Gray Beard, are the best way to secure your piston in cases like this. It won't cause any damage to the piston at all and is a safe and secure way to keep the engine from turning while you remove/install a prop. It won't scratch or dent the piston as you should slowly turn the engine until the piston is in contact with the piston stop, and then remove or install the prop. Taking the head off of an engine for something like this could actually cause more problems than it solves, and removing the head will not do anything to secure the engine while removing the prop nut. Trying to remove the backplate and brace the connecting rod will turn into an ungracefull routine becuase there is no real way to do this that is simple. You are going to be facing weird angles and trying to insert something to stop the engine from turning, and then holding all of this while you try to remove the prop nut.  Using a piston stop is the best overall way of doing this as it is a hands free method of securing the engine. Simply screw in the piston stop, rotate the engine to contact the stop, and then you have both hands free to remove the prop nut.

Ken

Jonathan Ott

My Feedback: (25)

Take off the backplate and wedge a Bic cigarteet lighter in there to stop the crank from rotating (heli guys do this to put the clutch on). If you use a bolt or anything that contacts the piston to "lock" things up, you are risking screwing up the piston.

I'd rather take off the backplate than have to replace the piston and possibly a bent con-rod, or messed up wrist pin, or replace the crank pin or...

Lnewqban

http://www3.towerhobbies.com/cgi-bin...?&P=9&I=LXKTM7

http://www3.towerhobbies.com/cgi-bin...4p?&I=CEHE9955

SharpProp

Thanks for all the advice. The problem with taking off the back plate is that I can't get to it. I cannot remove the cowl without taking off the prop. My only option is the piston stop. I was worried about using one of these because some guys said it could damage the engine. I read another option that said to put two nuts on infront of the stuck nut and jam them together. Then put a wrench on the outside nut as leverage against the wrench on the stuck nut.
Any thoughts on this? It sounds like a good option but I don't want to do anything stupid and strip some threads. Thanks for the help this far guys!!!

landeck

My Feedback: (1)

I have been using a piston stop for years without any damage to my engines. I keep one in my field box and one on my engine work bench.

Bruce

irocbsa

My Feedback: (12)

Here's another idea to try that I use when changing valve springs on automotive engines while the head is still attached to the block.

Take some 1/8" diameter thread or twine and feed a couple of inches of it down through the glow plug hole. Turn the crank until the piston contacts the rope and feels snug. Remove the prop nut.

big max 1935

My Feedback: (9)

Try the double nuts if you have room.I doubt if you will hurt the crank threads as it is case hardened & the nut is not . They also make a nut splitter but they may be too large for our prop nuts. Could use a Dremel tool with a cutoff wheel & carefully cut the nut. You have a unique problem that don't happen too often . I would still buy a piston stop for a possible future problem. Good luck, Max H.

carrellh

DING! DING! DING!
Simple, effective, virtually free.

RCKen

My Feedback: (9)

Hmmm, interesting. Learn something new every day. I'll have to give this one a try!!

Ken

Crash Campbell

Hi,

There are many ways "to skin a cat" and all the methods mentioned here will work. When I'm faced with this situation I remove the plug, dribble some fuel into the chamber to ensure hydraulic lock, replace the plug and remove the nut. Check the nut and shaft for any thread damage and use a tap or die if either thread is damaged to repair it.

Cheers,

Colin

Gray Beard

My Feedback: (-1)

String or thin rope is one of the old school ways. Another was filling the head/cylinder with oil/fuel and when you bring it up to TDC it holds the piston. The piston stop has been around as long as these two old methods and so far I have never seen any rod/piston or crank damage at all. When the piston is brought up to the stop you do not slam the wrench back and forth to remove the nut, it is one steady smooth turn. It doesn't even remove the carbon off the piston. String in a four stroke can get hung up in the valves, the piston stop is the best way. Easy to make or just go buy one. Jambing the crank/rod from the back plate?? That's the long way around the block.

Jetdesign

My Feedback: (8)

I didn't realize there was a 'piston stop' specific tool. I was just thinking that a plain old bolt would be a bad idea. I was just machining one the other day and noticed the end was far from flat. That would have been a bad thing to use.

DGrant

My Feedback: (4)

If the cowling won't come off because of the prop... cut the prop. If you do end up using something for a "jam".. first thing I like is the piston-stop device.. but if you can get to the back of the engine.. take the back cover off and use a wooden dowel for a jam... I wouldn't use anything metal for any jam, anywhere.

DGrant

My Feedback: (4)

On second thought.. if there's a back-plate that wont' allow the cowl to come off.. I don't have any insight .. but I'm following along. Let us know how it goes.

Lnewqban

I believe there is metal debris preventing the nut to reach the end of the shaft (or the prop would have go flying after getting loose); hence, forcing the nut out will cause further damage to the shaft up to the end.
As recommended above, cut and remove the prop; then, try to screw-in the nut, cleaning the threads with wire brush and some light lubricant.

If still tight, I would cut the nut down to the shaft.
Sometimes, that, and the generated heat, are sufficient to loose the nut enough to liberate the interference.

Gray Beard

My Feedback: (-1)

A plain old bolt is just fine. When you noticed the far from flat end you could have given some thought to the grinder making it a little rounded. I have one I made with a piece of brass rod JB welded into the plug, this is great for my two strokes. When I tried it on my YS 1.20 the brass shaft was too short to hit the piston. I used a piece of coated wire forced into the cylinder until it held the piston in place. When I got home I mad up another stop but instead of a fixed brass stopper I threaded it and used a grade 10 4-40 hex head screw, one size fits all!! Home made like mine were around a very long time before someone decided to machine them and make some money with them. If you have a lathe it would take about 15 minutes to make one that looks better then mine. A Lathe, A Lathe, my kingdom for a Lathe!!!!!!!!!!!

SharpProp

Well the method using two nuts jammed together worked beautifully with no dammage. The prop nut was not severely stuck and a little nudge was all it took. Thanks to everyone for the great advice..... And I think I should go get one of those piston stops.

DGrant

My Feedback: (4)

Congrats. Glad it worked.

Lnewqban

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