I lost a crank nut on my 26inch Huffy Cranbrook cruiser. Can anybody tell me what size nut I need to replace it?

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There are only two types. 28 threads per inch and 24 threads per inch. You'll need a ruler to measure and count your crank threads. Or, buy one of each and use the one that works.

One-piece ("Ashtabula") Cranks

Your local bike shop probably has a box of old crank parts stashed under their work bench and you could grab one for next to nothing. If not, you can buy one here for $1.99.

Bicycle American bottom bracket one piece crank lock nut 24T | Porkchop BMX

Local bike shop should have one, they may even give it to you. Local swap meet with used bike parts would probably have one as well. It's a lock nut and if you're unfamiliar with it, it takes a bit to set it just right.