I am having problems with my spinner and prop flying off the engine. I have flown a few airplanes before, but this problem is making me feel like a noob again. I have an OS .50SX engine, an aluminum spinner, and have tried wood zinger props as well as the APC prop shown in the photos. It has never flown off while I am in the air, but it does about half the time I try starting it (with an electric starter). I have on suspision, and have shown photos of it.

Here is the entire assembly:


I am thinking it is this piece that mates the spinner back plate with the engine shaft.


You can see there is a notch on it.


If you look at the engine shaft there is also a notch. You can even see some metal shavings inside it. Should there be a piece that holds these two notches together? This engine is brand new and I have always had this problem. I don't know what to do. I am absolutely positive that I have everything tight enough. I cannot get it any tighter without stepping on the wrench.

looks like its missing a woodruff key (it should be in that slot) if I'm not mistaken

[X(][X(][X(] Yes, first off there should be a keyway in there. It looks like a little orange slice. Except silver in color.That is a must have. Second , make sure that the engine is not set to rich or that you a have a fuel siphoning problem. Where fuel fills the carb after shut-off. What happens when you try to start it esp. with an electric starter is the engine hydro locks thus spinning the prop nut and prop loose. You can always check this by 1) looking in the carb or 2) give the engine one flip front with a chicken stick to make sure it turns with a pop and not resistance. When starting with an electric start remember to turn the spinner all the way clockwise to the compresssion stroke and then hit the starter ,that gives some speed build up and help to the starter.

Great, that means I am not as big a noob as it seemed I was. Any idea where I can get one of these "woodruff keys"? Maybe I should contact OS.

Your local hobby store might have one or You can order one here:
http://www2.towerhobbies.com/cgi-bin...0P?&I=OSMG9358

You can generally find woodruff keys in a variety of sizes from very tiny ones to pretty big ones at any good Hardware store. Any small hardware items like this I need or allen head bolts, set-screws, etc. I allways check the Ace Hardware store first before I go to the Hobby Shop. These type of small hardware items will usually be 3 to 4 times less at a Hardware store than at a Hobby Shop. They'll probably have a better selection too. Take the engine with you & find the one that fits best.

Don't cheap out here. Get the proper part from Tower or a LHS. It needs to fit properly or it will bang around in the slot and cause damage to the crank and the hub. This piece drops out rather easily as I have alost lost mine on several occasions now. Took about 2 hours hunting through my carpet one night to find it. [:@] Most keys come in the full crescent shape anyway so you would still have to cut and fit to size. Just buy the correct part, PLEASE.

I didn't realize it was a certain type of woodruff key, shaped differently then an ordinary key. If that's the case I would definitely do as Bruce says & order the right part. I stand corrected![sm=bananahead.gif]

Even the correct part is stuborn. Mine kept wanting to fall out.

Not sure but the manufactureres woodruff key may have a specific density or breaking strength as well.
Keep in mind that a woodruff key is not only to align the prop hub but also serves to protect the motor by breaking when you hit the ground with the prop turning. Better to break the key than the shaft.

I lost one in my lawn from a .50SX. The correct key is not a half moon. It is more pointed on one end and can be installed wrong.

Many of the woodruff keys are metric and most do have a flat at one end which normally goes against the engine...you should not be able to install it incorrectly and still get the hub on. A small amount of locktite will help to hold it in place, they are very easy to lose so a spare is a good idea. OS engines use this method also and I am sure there are others as well. Missileman is correct, they are normally made of an alloy which acts as a shear pin to protect the engine in case of a prop strike. Using the wrong material or size will casue you problems....

Thanks everyone,

I sent an email to OS and they replied already and are shipping me the part. Here is the email.

Dear Sir-

Thank you for your recent e-mail. We are always happy to hear from
fellow Modelers.

We are sorry to hear you are missing a part on your O.S. engine.

We will be glad to send you the part(s) you have requested at no charge
as a customer courtesy.

Please allow 5-7 business days for delivery.

I hope you find this information helpful. Should you have any further
questions or concerns, please do not hesitate to contact us at this
email address. (Please be sure to copy all previous emails into any
future questions.) You can also reach our product support technical
team at 217-398-8970, or via fax at 217-398-7721.

DID YOU KNOW that our web pages now all contain FAQs? (Frequently asked
questions) Please drop by and take a look! We hope you'll find the
information helpful and valuable to you.

Sincerely,
*** ***
Hobby Services

Well, that is certainly nice of them. Chances are YOU lost the part but they are willing to take the step to get their product back in the air and have a satisfied customer. As I stated before, it does fall out rather easily so be carefull in the future.

I did buy the engine from an individual, not a store. It was new and not used, but I am sure that it never had the key under my ownership. I just assumed it spun because it was a different engine than the ones I own.

What counts is that it will be OK soon.

I was also having problems with the engine not spinning the prop as fast as all the reviews said it should be able to. I cannot wait for the part to arrive. I am fairly certain that this is the cause of that problem as well.

That could also be due to the engine not being fully broken in. There could have been some prop slippage though.

The Woodruf Key fixed everything. The engine starts much easier now and the prop/spinner stay on. I found out the prop speed thing was caused by the thickness of the Zinger wood props. When replaced by the same sized APC prop the RPMs went up by 3k.

Zinger 12x5 = 10,600
APC 12x5 = 13,400

Who was Woodruff ? Just a little triva