Evinrude 7516 7.5 hp Fleetwin Flywheel nut size?

[Buccaneer]

The Evinrude, Model 7516, 7.5 hp Fleetwin I’m working on was
missing the flywheel nut. I measured the crankshaft threads
the other day and thought I needed a 1/2" x 20 tpi nut.
I bought said nut, and sure enough, it doesn’t fit 🙄
Anyone know right off what size nut I need?
Thanks!

Prepare to be boarded!

[Mumbles]

Yup, it should be 1/2" – 20 thread on your motor. I don’t think the ’54 crank is any different than the later ones.

When buying fasteners at the big orange store up the road, I like to check them before leaving as they tend to get thrown back in the wrong bins causing all kinds of excitement when I go to use them.

[fleetwin]

I know all about that type of "excitement", and the foul language that goes with it….
I HATE buying hardware at the big orange store, you never find quite the right piece. Either a little too long, a little too short, or perhaps it is not threaded up far enough. Usually it takes me about three trips to the !@##$ orange store before I get something that works….

[frankr]

Perhaps the crankshaft has been hammered on in an effort to remove the flywheel without a puller, which battered it so the nut won’t go on?

[Buccaneer]

The crankshaft threads seemed okay. I did find an old castle nut that
screwed on okay, but a little loose I thought. Will re-measure tomorrow.
Chart shows that the Minor diameter of a 1/2", 20 tpi thread bolt is .4387,
so I guess the inside of the nut should be close to that on the veneer
caliper. I went to a small town True Value Hardware and the helpful
lady pulled it out of the box for me, so it could be metric for all I know.

Prepare to be boarded!

[fisherman6]

The tap drill size for a 1/2-20 is 0.453" for standard 70% depth of thread. The ID of the nut may actually be a little larger due to the use of rolled or formed threads in mass produced fasteners, but that will get you close. For peace of mind I doubt it’s metric. The closest size would be M12 and that is quite a bit small so it won’t go far if it will even start. The next normal size up is M16 and that’s WAY too big. I’m not sure, but the OEM flywheel nut may be a different thread class for a more snug fit. They don’t typically feel as sloppy as a hardware store fastener to me.
-Ben

OldJohnnyRude on YouTube

[Buccaneer]

Ben, thanks for the enlightenment on metric nuts.
Heading to the garage soon, and at least I have
one less thing to rule out, lol.

Prepare to be boarded!

[fisherman6]

Good luck. Hope it’s back up and running soon.
-Ben

OldJohnnyRude on YouTube

[Buccaneer]

Well, back from the garage and I’m still "nutless"
A new 1/2" x 20 bolt I had was .493 OD.
The smallest measurement on the crank threads were
.501.
Evidently OMC’s parts are made to different tolerances / size.
The end of the crank thread was slightly mushroom .005 over.
I had no good 1/2" x 20 die (which was probably a good thing)
so I split a used 1/2" x 20 nut, got it threaded on, then filed down
the .005 mushroom, and cleaned it up with a thread file.
New hardware store nut still wont go on, so I guess I’ll look for
a OMC nut.
Anyone else run into this situation?

Prepare to be boarded!

[Mumbles]

Something’s not right. Would you have a pitch gauge in your box to check the threads? The threads could be tight from previous pounding on the crank, the opposite of how a bolt will stretch before breaking. 0.501" is way too large for a 1/2" thread so I’ll bet the threads are not only tight, but the shaft has expanded in size from pounding on the end of it. Probably best to find the proper size die and chase them.

About your 1/2" bolt measuring 0.593, wouldn’t that be 0.493 which is closer to what your crank should measure? A thread will always measure several thou under its nominal size for clearance purposes.

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