I was going through a box of old Mopar parts I have and came across a couple of throwout bearings. I am an automatic guy so I don't know much about 4-speed stuff. What I would like to know is are throwout bearings small and big block specific? If so, how can I tell the difference in order to figure out what I have?

Small and big block TO bearings are the same, but there are 2 types. One for a 23 spline and one for an 18 spline transmission.

Here is a picture of both bearings. They both have 1 5/16" holes in them. They are the same height but notice the difference in the shape on the bottoms ends.

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Not so on all Mopar applications, if it is a passenger car then that may apply If thier from a pickup, van or a larger turck maybe not I've seen some that are a lot different in the length of the mounting collars between the clucth fork and bearing mounts positions

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Sorry, MY BAD. I thought we were talking about cars.

Is the difference in the shape of the two bearings in the picture I posted of any significance?

No. The difference in the thickness of the bearing its self is due to the manufacturer. The mounting body looks identical. The difference between a 23 spline and an 18 spline is the inside diameter of the mounting body/bearing. The bearing you have pictured look like car bearings. Admittedly, I know very little about truck bearings. They could be the same or different. I you couldn't prove it either way by me.

Interesting....I would like to know the difference also....

Here is a supposed Hemi release bearing that doesn't look anything like the pictured ones.

http://www.ebay.com/itm/OEM-Mopar-HEMI-4...tr#ht_932wt_788

The Hemi T.O. bearing has a larger center hole.