The main pulley has this inside it and I don't know the purpose of it. 1993 mazda rx7

 

Sorry. Picture here

 

it keeps the front pulley bolt from falling out

 

Yes your right. Now that I think about it the pulley doesn't have a shape of the bolt head. Thank you.

 

Its supposed to keep the eshaft bolt from coming lose. Most people neglect to put it back after rebuilding the motor or replacing the front pulley. I use red Loctite and torque to 200 ft/lbs during my builds. I've never had a bolt come loose on me.

 

Mazda didn't use it on every engine either

 

Japanese Bottle opener..
Start car..rev to 6k..place bottle near opener...

 

^Yep. My car's OEM engine didn't have it. But when removing I found out it was in absolutely no danger of coming loose.

 

it used to happen to the FC engines here and there, and well, they fixed that problem!