I've been having a problem with the prop nut coming loose while starting.I have 4 running planes with engines from .25 to .61.It makes no difference which one I try to start,with or w/o spinners,with starter or by hand,about 25% of the time the nut comes loose! Is this in the normal range or am I doing something wrong? I've never had one come loose once running,but I've certainly developed a healthy fear of the prop!How about the nuts with the nylon compression lock?Thanks in advance for your thoughts in this matter!

Maybe you're not getting the prop nut torqued down enough. I always make sure that mine are as tight as I can get them and have never had a problem with them coming loose.

Good luck,

What type of motor and prop? Some motors have a thrust washer with a better "bite" than others. Some props have more surface area in contact with the thrust washer than others too.

Well,I'm a pretty big boy, and I could tighten the nut till I snap the shaft clean off!I Know it's a hands on kind of thing, but I think I'm going as tight as anyone would dare.All the props are Master Airscrew plastic in various sizes,and I've tried T/F Powerpoint wood.Some spinners come with sandpaper type discs...tried em'...comes loose!Looks to me like the spinner backplate could be a weak point, so I take the spinner assy. off completely for a good bite into the engines' toothed plate.Guess what...comes loose! I Dunno,maybe it's my curse?

For a spinner, use a good quality one with a metal backplate rather than one of those plastic ones (the plastic "gives" too much, allowing the nut to loosen).

When you are not using a spinner, the depth of the prop on the shaft may mean that you are running out of thread before you are really snugged up against the prop properly, so check for that.

Other than that - snug it up good & tight, and make sure your mixture is set correctly. Backfiring is a common cause of prop-nuts coming loose on 4-strokes, but can also happen on 2-strokes as well.

Good luck
Gordon

Not sure but a couple of things come to mind

I have had the same problem with old K&B engines with the removable shaft. If that is your case obtain a hardened stud from an auto supply and increase the torque.

I have seen problems caused by the starting method. Many will "spin up" the starter before engaging the spinner to compensate for a too weak starter/battery or by habit that will act as an impact driver eventually loosening the prop.

Last but not least the torque of the nut may not be enough. I know you think it is being tightened sufficiently but those little boogers have to be "oilfield tight"(as tight as you can get it and then one more round)LOL just kidding but really, increase the torque a little bit and see what happens.

Just put two nuts on. The second one will help to lock the first one in place. I had the same problem with a four stroke but it hasn't happened since using two nuts. You can also buy special locking nuts designed for four strokes (o.s. make some)

I dunno, but for me I wasn't using a glow plug wrench. I was using a screwdriver with a hex attachment. Once it was secured with a glow plug wrench, mine was fine.

Are you sure? The steel is pretty hard, and I think the prop or the threads would give before you snapped the shaft. Whatever. If you're using one of those 4 way glow plug wrenches, they aren't long enough to tighten the nut securely. Get a box end wrench, which for a 10mm nut will be about 4 inches long. For a wood prop, you should see the washer sink into the wood visibly. For plastic props, you can't compress them too much.

Has anybody out there ever damaged a prop shaft by tightening a prop nut too tight? Just curious.

Bluebrew,
Sounds like you are simply not tightening the nuts enough. They need to be TIGHT. Use a 6" wrench and don't be afraid to put some pressure on it. Nylon lock nuts won't work for the prop as there is no way to hold the prop shaft to spin the nut down against the washer.

I agree with Stick and others, you aren't tightening the nut enough. I CAN and do get them tight wnough with a 4 way glow plug wrench by the way. I have 5 planes flying currently, three with four strokes, and have had hundreds of flights over the past eight years. I've had props come off twice. (Not counting your's David. )

Four strokes that throw props are usually set too lean.
Dennis-

Ok guys, it seems I'll have to grunt a little more and see what happens! After reading Gordon Mc's post I went out to check on whether there was any thread left for a good bite,but that was ok on all of them( Good thought though,I hadn't considered that possibility).At this point I was convinced the problem must be a combination of, a)not tight enough and,b) my starting technigue needs work.Besides cracking me up,The Trolls' post got me thinking about the spin-up factor and the way a starter can impact if not used right(I think to myself,"I don't do the spin-up thing",but on further reflection...hmm)The fact is this happens all the time with any engine(all 2cycle) and prop, and it only happens when they kickback(backfire).I do use a 3/8" ratchet and socket so there's plenty of leverage.I have a 3/8" click type torque wrench that may be the ticket to setting the "oil field tight" just so on them little boogers!I work on automatic trannys for a living and they're not very forgiving if you get crazy on the torque. Maybe I've become conditioned to take it easy.Anyway thanks to all for your thoughts and ideas, I think this mystery may be solved! Paul

what engine are you using?if super tigre then your prop driver has a crack in it,you cant see the crack until you start tightening the nut.