Ancient wisdom suggests putting two nuts back to back on the prop shaft to avoid the prop from getting loose. In my limited RC experience, I haven't seen an engine that already comes with two nuts, so I usually add a lock nut to be safe. My new ASP .91 FS, however, already comes with two nuts. The second nut has a part that protrudes out and gets lodged in the first nut (see picture.) My questions are:

- What is the purpose of the piece that protrudes out on the second nut?
- I take it with this arrangement I am OK and don't need to add/change anything else for added safety?

Thanks.

Most 4 strokes come with the second nut. If you run one lean, there's a really good chance it will spit the prop off. The second nut helps a lot. The protrusion extends into the first nut and gets squeezed tight onto the prop shaft, hopefully locking it in place. They still spit the prop occasionally. Don't stand in front of it while running.

If these are tightened properly, you do not need an additional nut.

If you run this engine correctly, it will never backfire anyway.

well all my 4 strokes O.S. engines has it, but the 2 strokes doesnt, just tight everything right you will not need it.

I don't think people are looking at your photo. That looks as though that may be a stud for attaching a spinner.

That is what Saito hardware looks like.

No, That is the lock-nut systen that most 4-strokes use.

Maybe if not bolted together. But what was he talking about when he said something protudes from the second nut. I assumed it was the stud, but now I see it may be the same stud, though a bit longer than some. Not mine, LOL.

In the picture, nothing is tight. The washer is not even making full contact with the prop. The nuts in the picture are the standard nuts that come with Magnum/ASP four strokes.

The crankshaft does look pretty long. My brother and I have several Magnum 91s and the crankshaft length varies quite a bit. I am guessing they have multiple vendors for crankshafts but that is only a guess.

The Picture is of a ASP 91 Four stroke engine as they come from the factory new in the box. The nuts are standard issue

The outer Jam nut is started on the shaft but nut snugged into the main nut as it would be for running.

Edit:

Carell and I had the same idea but trying to get a response out of the editor to post, he beat me to the post. -Jim

The skirt in the forward nut helps with more surface area and a bit of wedge effect to help the prop nut stay locked. It works well.

Note that the double jam nut is also now standard on the OS 75 AX two stroke engine.

Bruce

From the pix the first nut is put on backwards. It has a recess on one side to accept the one side of second nut which fits whithin it. This when tightened will not come loose. However if put together the way pic shows hhas no advantage.

The main nut is on correct for it has a shoulder that goes inside the front prop washer. You can see the shoulder not yet inside the washer

i think he was just doing that so we could see it. the main bolt isnt tight either.
once you tighten everything down that setup works very well my magnum .91fs hasnt spit a prop yet

I had one for about 6 years and it never backfired

mine backfires all the time but never spits the prop. mainly when i kill it, it sputters to a stop then backfires, its never done it in flight only on startup (when hand starting) and killing it

Sounds as though you are killing it by shutting the fuel off. You should kill it by completely closing the throttle

Thanks for the replies everyone. Two comments:

1. I learned the correct terminology for the "protrusion" is "skirt". Thanks TFF!
2. As some pointed out, I had loosened the set up to ensure the picture and my question were easier to decipher.

nope throttle, i think i have a air leak (where carb connects to intake),

An air leak can occur in several places, you need to check all of them. That would include the carb throttle barrel, carb to case, front bearing, shaft to inner case clearance, backplate; and for those with removable cylinder's, the cylinder to case. Some engines may also have screws stopping up unused pressure ports or openings to allow removal of piston wrist pin's, etc.