SAMMY HAGAR has been through two pretty gnarly splits from VAN HALEN, and yet he'd still be up for another run... under one condition:

And that is: EDDIE VAN HALEN cleans up his act... meaning:  He gets sober.  That's because Sammy claims Eddie's alcoholism led to their most recent split, after Van Halen's 2004 reunion tour.

Of course, a reunion would also have to mean that Eddie would want Sammy back.  And at least for now, that doesn't seem anywhere CLOSE to happening. (The band is working on new music with original singer DAVID LEE ROTH.)

Regardless, Sammy is almost GUARANTEEING it'll happen before he dies.

"Rolling Stone" asked him what the chances of that happening were:

"I'd say it's up there around 90%.  I would love to make another record with Van Halen.  If Eddie was totally cool, and was back to the guy I used to know, or a new guy, not the guy I knew.

"He can't be that guy.  I wouldn't do it if he was like that... But if Eddie really got his life together, which it seems he has judging by the pictures I've seen, then definitely."

But he does admit that NOTHING is imminent:

"Right now, zero chance... but someday, before we all die, (eff) yeah.  We might be in our 90s though."

(Eddie has reunited with David Lee Roth twice, and has made up with Sammy before so I suppose anything is possible . . . but judging by how things have gone over the past few years, I definitely can't see this happening soon.)