Bolt
 

Can some tell me the name of the bolt that is used to hold the spindle on to the deck on the old decks. The head is flat and they have grooves around the bolt that will hold the bolt from turning as you put a nut on it. Thanks for any help in advance. Have a great night.
Bob

They are called carriage bolts

The flat head bolts with the square under the head are called “plow share bolts”.
Carriage bolts have a larger domed head.

Bolt
 

The bolts I am talking about have like splines under a slit curved head.
Bob

Cast iron spindles?

Ribbed Elevator Bolts

https://www.mcmaster.com/products/bo...ibbed/?s=bolts

If this is not what you want look around www.mcmaster.com they offer a huge selection of bolts and screws. Prime fasteners has a great selection as well.

I think you may have to post a photo
Oz

Sounds like he is describing a bolt like a lug stud on a hub or Axle that holds the wheel on.
Those a have spline sort of under the bolt head.
The head cam be somewhat flat like a plow bolt or domed like a carriage bolt.
Yes please post a pix so we can help further, as OZ suggested.:beerchug:

They are the square neck flat head plow bolts as athomas posted the link to up above.
You need the flat heads because the pulleys, specifically the center pulley, sit down so close to the top of the spindle housing that they would rub anything sticking up too high.