When his abusive stepfather tortures him for wrecking his Harley Davidson, a boy enlists in a underground fight club to toughen up and confront him.

The logline describes a story with no end, he enlists in the fight club but how long before he will actually do something to the stepfather?

Perhaps define his intentions in more detail. What if the boy's nose was broken by?the stepfather, and he wants to break the stepfather's nose? Other wise what does confront the stepfather actually mean? What exactly is the boy's goal?

"Actually,?I think?the Harley is an important detail. ?Harleys have a cult status; it's a motorcycle that many bikers don't just own, they fiercely identify with. ? I've never seen a man with a tattoo for a Suzuki or a Honda or a BMW, ?but if I've seen one, I've seen hundreds of men proudly branded with Harley-Davidson logo tattoos. ?Women, too."
That is true, but I don't think that was really FFF's point. I think he was really saying that what the boy does isn't what should be in the logline, but remove it and use what the father does as the inciting incident. So, to me it's good in the story but it could be removed from the logline.

For example: When his abusive stepfather hammers a nail into his hand, a boy joins an underground fight club to toughen up and confront him.?(~23 words)

But I suppose it's actually fine either way. :

When a boy's abusive stepfather hammers a nail into his hand when he wrecks his beloved Harley-Davidson, the boy joins an underground fight club to toughen up and confront him.?(~32 words)

Actually,?I think?the Harley is an important detail. ?Harleys have a cult status; it's a motorcycle that many bikers don't just own, they fiercely identify with. ? I've never seen a man with a tattoo for a Suzuki or a Honda or a BMW, ?but if I've seen one, I've seen hundreds of men proudly branded with Harley-Davidson logo tattoos. ?Women, too.

I suggest inserting the word beloved as in "wrecking his beloved Harley-Davidson motorcycle".