Need to replace the prop nut on my OS 61 FX. Parts list says it is 5/16 - 24 thread.
Any reason I couldn't just go to local hardware store and get one for 5 cents? What makes the ones sold specially for RC, necessary?
Also, why is it not common to use a lock washer behind the prop nut?
Thanks

The nuts provided with your engine are hardened to a very high level. Much harder than typical hardware nuts. Lockwashers should only be used once and then discarded. Get the proper part, it's cheap enough.

There are different types of lockwashers. Some are split ring. Some are star washers. Those introduce uneven stress into the prop hubs. That is something that, when allowed, is very controlled. For example, drive washers often have shallow radial serrations, but drive washer tolerances are tightly controlled by the manufacturer. And those same manufacturers have chosen over the years to use lock nuts whenever they wished to strengthen that area. Look at 4strokes. Almost all of them come with a nut and lock nut.

You could use a lock washer between the prop washer and the prop nut, but a lock washer has little effect on the hardness of the washers used and the nuts used. And to use one against the front of a prop would not be a good idea.

Also, 5/16-24 is a less common thread, and may not be available at your hardware store. 5/16-18 is the common thread.

I highly recommend the Dubro prop nuts. They work well with electric starters, last forever, are easy to install just using the shaft of a screwdriver, and you can avoid all the headaches that go along with spinners.

I consider myself to be a special prop nut

IF you can get a Grade 8 nut, go for it. Good luck!

And just why do you need to replace the prop nut?

Dr.1

Actually, 5/16-24 is not that uncommon

5/16-18 is NC (National Course) and 5/16-24 is NF (National Fine)

Check at an auto parts store and ask for a 5/16-24 grade 5 or grade 8 nut