A "bolt head" refers to the rounded or flat part at the end of a bolt, which is typically used to secure objects together by being fastened with a tool like a wrench or a socket. Full definition
As you can see, there is very little bolt head showing. (humblemechanic.com)
At the top and bottom are two rubber bumpers, with cool metal bolt heads at each corner. (cnet.com)
The event doesn't offer Bolt a head - to - head with world No. 1 Justin Gatlin, however, with the American not invited by organisers British Athletics because of the two drugs bans in his past. (athleticsweekly.com)