In going through of inventory of ancient engines, I came up short several 1/4-28 prop nuts.  Most (maybe all) of the manufacturers are out of business.

I got some grade 8 nuts from the hardware store. 

Does anybody have a better idea, or see a problem, or have experience with them?

I may be wrong but I thought grade 8 was a metric rating.  Anyway, I have used hardware 1/4-28 nuts a bunch.  As long as they are a little softer than the crankshaft threads they should work fine.  If I remember right those old Spinner nuts were aluminum.  After all, it is just keeping a prop turning 8-10,000 rpm driven by explosions from coming off...what could go wrong?

This reminds me of one of the most exciting parts of flying stunt with an IC engine.  The adrenaline rush you get when your motor backfires knocking your three bladed prop lose and the judges stand there watching you run for the allen wrench to take off the spinner you left in your tool back in the pits.  Now tell me you have never had that happen, even before you learned to take a tool box with you?

In going through of inventory of ancient engines, I came up short several 1/4-28 prop nuts.  Most (maybe all) of the manufacturers are out of business.

I got some grade 8 nuts from the hardware store. 

Does anybody have a better idea, or see a problem, or have experience with them?

     A nut is a nut, the manufacturers just buy them wholesale, you can buy them retail, its no difference. For a prop nut, anything will work - Grade 8 is great overkill but will be fine. I think those that come with RO-Jetts are Grade 8.

      BTW, the bolt and nut grades are SAE standards, not metric.

     Brett

Prop nuts are subject to low stress.  As long as 3 or 4 threads are engaged, any grade steel prop nut will do the job.

     People have been seriously injured or killed by following this advice. A standard that is usually followed is that the nut or female threaded surface should be at least as thick as the diameter of the bolt or stud. There is no way I would stand in front of or near any prop on a running engine that only has a few threads engaged. 

     Most good hardware store fasteners are in the grade 5 area and more than adequate for the job. The only reason for a grade 8 nut would be just for durability due to frequent removal and application of torque. I don't recall ever round off or stripping a stock prop nut of any brand. If you change props a lot, I would think nuts that come with most engines are hardened just for this reason and if you change props a lot, I would use stock nuts or the grade 8 nuts. I have worked in manufacturing and industrial maintenance my whole life and have built a lot of stuff and followed behind a lot of other mechanics. One thing I have seen a lot if worn hardware from improper fitting tools and just plain being ham handed working on things. You would be amazed at how many supposedly intelligent people would put a worn or strip bolt or screw back in a machine , guard, or some other critical part. Then I had to deal with the consequences!
  Type at you later,
  Dan McEntee

Grade 8 would be a non-metric/SAE rating.
Equivalent DIN/metric rating would be 8.8
Stronger would be 10.9 or 12.9. I have no idea why the DIN standard has the decimal points

  I think SAE Grade 8 is closer to DIN 10.9, rather than 8.8, but they don't have 1:1 correspondence. Decimal points because of the ever-whimsical German mind.

    It actually describes the tensile strength - 10  = 1000 megapascals tensile strength, .9 = yield strength percentage of tensile strength (when it strips).

     Plenty of people use dead soft aluminum prop nuts that come with spinners, they are strong enough but tend to gall and strip if they get "dry". Any grade of steel nut is acceptable.

    The only oddball prop nuts I have seen are 1/4-28 nuts with 10mm hex outsides instead of the standard 7/16" - OS and maybe Enya, too.

    Brett

I tend to use the aluminum or brass Harry Higley "safety" nuts.  What I like about them is that they are easy to service, being a single part in front of the prop instead of the several parts that spinners entail.  Also, they come in different weights, which can make trimming easier.

But, glad to hear the "go with grade five" comment.  While grade 8 would work, the compliant grade 5 sounds better for a crank.

Standard SAE hex nuts are .8D thick. If the nut and the bolt (male threaded fastener) are the same grade the nut is strong enough to pull the bolt apart. I do not know how hard a typical crankshaft is, I have never done a Rockwell hardness test on one. I no longer have access to a Rockwell hardness tester, so I can't perform the test. I would prefer that the threads in the nut fail before the crankshaft is stripped or elongated. I think that a standard hardware store Grade 2 nut is more than sufficient to retain the prop. I know they are strong enough to split an Aluminum tapered ID prop drive washer if they are over torqued. Prop thrust loads are small compared to the strength of the snout of the crank. I think crank threads are sized more to resist bending loads from starting and ground strikes than prop thrust loads. If a #5 socket Head Cap Screw is used for prop retention on a Cox 1/2A engine it has a minimum tensile strength of 1430 pounds force. A 1/4-28 Socket Head Cap Screw at 39-45 Rockwell C has a minimum tensile strength of 6500 pounds force. It is strong enough to lift most small cars. A Grade 2 SAE nut is the same strength as a Grade 8.8 ISO metric nut.
Pat

Grade 8 hardware is some of the strongest made .  I use grade 8 on car engines and race engines in rods and head bolts/nuts !
So it’s not necessary to use grade 8 hardware on a prop nut . A standard nut will work just fine but it you are worried then use grade 5 . I have use standard bolts and washers and they hold up just fine . If you do use the grade 5 or 8 hardware just remember not to overtighten because the crank threads will give away first before the better grade hardware used .
Grade 5 & 8 come in both SAE and Metric . The grade is only the hardness of the steel !

From what I could see, the grades were "generic no grade", grade 5 and grade 8.  These grades had nothing to do with English or French threads.  I was just wondering if the engine-makers used some special trick nuts that were not the same as off-the shelf hardware fasteners.

I'll have bench runs pretty soon to find out for myself.

The only trick most manufacturers use is a good method to put a fairly wear resistant black finish on the nuts and washers.  Helps camouflage them in the grass.