The one off my 350 crank doesn't fit. It's too big. I am going to get one tomorrow. I need to know length and threads per inch and width. Thanks in advance.

 

Wow. I would have guessed a 7/16-20 x 2", just like most other small blocks. I don't get close to the unbalanced cranks too often, so I'm not sure...

 

I thought that the 350 one would work, also. I took it out and tried to put it in. No luck. I'd say that 20 threads would be the same and the length but I put a 3/8 bolt up there and it slid right in without screwing it in. So does this mean it could be 13/32"? This seems crazy. Do I have some oddball crank?

 

If you have a 7/16-20 tap, you might try cleaning up the threads in the 400 crank snout. If you encounter excessive resistance, you may have an odd thread, but the next nominal size over 3/8 is 7/16. A 10mm would be slightly larger than 3/8", but shouldn't exist in the nose of an SBC crank.

 

It is 7/16"-20, no question about it. Same as any other SBC that has a bolt.

Sounds to me like your crank threads need some work.

 

or maybe they already have had some work done to them?

 

Possibly... in which case it needs to be undone, and re-done correctly.

 

Good point. I have had to re-tap the threads to 1/2-20 to save a crank after some careless attempts at hub installation. However, that would make the standard bolt just slide right in. If the 7/16 won't start, chances are the threads are bunged a bit.

If you do get a replacement bolt, make certain it is at least a grade 5, and preferably a grade 8.

 

I got a 7/16-20 tap and ran it through. It barely went in for all the rust. I had to keep going back and forth to get the rust out of the snout. I took a 7/16-20 bolt and put it in no problem. This engine had been sitting outside upside down for several years. It was really rusted up. Thanks for the help guys. Before the tap there would have been no way the bolt would go in.

 

i ended up having to re-tap the crank in my 305 to a 1/2 inch bolt with 20 thread.

tried to turn the engine over by the snout bolt. stripped the threads right out.