Guys,

Just got my first four stroke flyer. Its a used Sig Hog Bipe. It has an OS.91 with an aluminum prop nut. My questio is this, what keeps the prop nut from coming off when I use my electric starter? Is it just a matter of tightening it down until it screams or what. Any help is greatly appreciated.

Airroy

I know your problem!
i had the same with my irvine40.
If you have a spinner, the threads on the backplate has probably
worn out. Change the spinner and it should work.
if you dont use a spinner, i dont no idea!

The best way to secure the prop nut is to use a lock nut ahead of it..... Let me see if I can explain that a better way.

After you put the prop on you add a thin standard nut like the one in this link...

http://www2.towerhobbies.com/cgi-bin...?&I=LXL742&P=0

Then you put you prop nut on over that and the two work against each other to stay tight. Four stroke motors usually are more likely to throw props than two strokes. I think it has to do with backfiring in the cylinder. Hope this helps some.[8D]

I've had one of my 4 strokers throw a prop once due to being too lean causing a back fire. Crank down on the prop nut enough to get it good and tight, then start with a richer mixture than you have now and reset the engine. I also recommend using a lock nut. It's much safer.

Basically, yes. And then tighten it more. And once you're done tightening the first nut, do the same to a lock nut.

I was having a bit of that with my new Saito 100. The cure was to make sure the idle was as low as possible, for me. Higher throttle settings just made it cough and pop back and loosen the prop nut.

Hasn't bothered, now, since I have a few flights on it. Took considerable adjusting, also.

Good luck,
Dave Olson

My experience is that most 4 strokes come with two nuts that are machined, one has a recess, the other a protrusion (male and female) so that one fits inside the other and locks. I also use a nylock nut as extra security.

Airroy-

Tightening it down until it screams for sure isn't the answer. You have steel threads and an aluminum nut. If you tighten the nut too much all you are going to do is "pull" or enlarge the threads in the nut because aluminum is a relatively soft metal. This makes it that much easier to loosen up under the vibration of the engine since there is now additional "slop" in the thread. This ain't a good thing.

The other fellas are correct in telling you to use a double nut with four strokes. This works because the friction on the faces of both nuts takes much of the strain off the threads if tightened correctly. Tighten the inner nut securely to hold the prop and on aluminum spinner nuts I've learned a little past "snug' is usually enough to lock the spinner nut on.

Also as you have learned by now 4 strokes act a little different than 2 strokes so be extra sure the line of the prop arc is clear and unnecessary people aren't standing in front of the engine when you start it up. I've seen 4 strokes kick nuts and props a good way.

Before installing the prop nut, add a drop of locktite thread lock to the threads. Use the blue one as it is not as aggressive as the red and will still come off when you need it to. Then tighten the nut securely. This should cure the problem.

get a steel nut and THEN tighten it down until it screams.

Maybe I'm being too silly here but are you using the washer? You need to use the washer with a prop nut or else you won't be able to keep it tight. I suggest for fourstrokes 40 or below you can use a washer but anything larger, use a lock nut. If you are using a Y.S. PLEASE USE A LOCK NUT. I've had my Y.S. throw props and it's scary as hell. So for your 91, I suppose a lock nut is good. I will sometimes use one of the original nuts behind a prop nut to ensure the prop is secure.

if you are goint to use a spinner, truturn makes a great jam nut set that has yet to fail me, and i have been using them for two years on my ys 110. no locknut or locktite involved. if the jam nut is any good, you wont need either a locknut or locktite

Is this a hub? If it's just a nut and it's aluminum, get rid of it and replace it with a steel nut. You could flip start the engine to avoid spinning the hub loose.

Of course the real logical answer to this problem would be for the manufacures to put reverse threads on the shaft so when your starting your engine you would be tightening the nut instead of pulling it loose.

Nope. The prop shaft has right hand threads for good reason. The force of a counter clockwise running engine would spin the prop nut off if the threads were left hand.

Try using a thin nut, as suggested, then your aluminum spinner nut- the aluminum nut doesn't have to be too overly tight.

Also, if you are using a WOOD PROP then after just a little while no matter how tight you make it, the wood compresses, and the nut/locknut combo will be loose.

The answer is before each day of flying, and maybe once during the day, tighten the flat nut, then the spinner, or locknut.

As mentioned, try not to run the bottom end adjustment too lean.

The problem with the aluminum hub on a 4-stroke is it's gonna spin loose as soon as you hit it with an electric starter unless it's very tight. I would do away with it and use a standard prop nut/jam nut with a cone spinner or flip start the engine instead.

I guess I am gonna be the minority here. I assume you are using a spinner nut. I have used them almost exclusivly. Never had a problem. Just tighten it up as you would a prop nut. Be sure you are not too lean on the mixture. You should be fine. If you are afraid then a drop or two of blue loctite should do the trick.

Mark Shuman

All,

Thanks for all of the quick replies. I very much appreciate you guys taking your time to help me. I have settled on a configuration that I think will be just fine. First off I am running a 14x6 K series Master Airscrew. Next I have a Higley brass prop lock up against it and then a hex nut as a binding nut. After this combo I finally have a GP Brass Heavy prop nut. I needed the front weight for a proper balance. I have started it several times with no loosening. It tachs at a peak rpm of about 10,200. I have the top end adjusted to peak at about 9800 or so. I have not flown it yet. Hoping to do so soon.

Thanks again for all of your input.

Airroy