It’s the shaved head. It has already given Bruce Willis 10 extra years in the heavy metal movie business and I’d be willing to wager on 10 more. He’s only 57 after all.

While the Steroid Boys of 1980s and ’90s action thunder (Sly and Ah-nuld) have come up against a maze of brick walls lately, a trim, slim Willis can clearly keep the “Die Hard” franchise going for a long time. All those steroids won’t help you if your hair is now being dyed a color completely unrecognizable in nature. But an eternally bad attitude might.

In fact, the opening scene in Willis’ new “A Good Day to Die Hard” shows our boy at the gun range preparing to fly to Moscow to liberate his n’er-do-well son, who’s been jailed for trying to assassinate a Putin-type Russian strongman. Willis’ shaven pate is as bald as ever, but it’s surrounded by a fringe of gray-white hair befitting an actor who is now 25 years and five films into the hugely successful “Die Hard” franchise. Pay heed. You never see that fringe again.

What has aged so well in Willis – and will keep on doing so – is his squinting, smirky, Jersey-boy wiseacre presence. Once you’ve located American tenacity and resourcefulness in impudent smugness, you’re good to go for decades. Muscleboys need not apply.

Unvarnished truth: “A Good Day to Die Hard” is the worst film of the five in the “Die Hard” franchise. But a weak “Die Hard” movie is still more fun than 80 percent of all the other action films out there – especially of the heavy metal variety, full of big explosions, constant automatic weapons fire and monstrous armored trucks bullying rush hour highways and turning everyone else on the road into bumper cars ripe for annihilation.

It’s a movie that puts all the “guilt” back into the phrase “guilty pleasure.” All through the movie, I kept thinking “this really isn’t so hot. So why am I having such a good time?”

Because it’s a “Die Hard” movie, that’s why. I was there when the first “Die Hard” movie was unveiled to film critics in Manhattan in 1988. I was pretty sure at the time that 20th Century Fox had salted the audience with screamers for the final 10 minutes of the film but it never bothered me. I felt a bit like screaming with pleasure myself. I was having a terrific time at the movies. So were most of the critics around me, with or without prompting from whatever shills were there.

John McClane now has to come clean on screen about the quarter century that has passed since the first film. In the last “Die Hard” movie – a much better one called “Live Free or Die Hard” – Willis was paired off with the good young comic actor Justin Long, playing a stammering, nerdly computer hacker.

This time, McClane’s son – the McClane family’s “problem child” Dad calls him – is played by Jai Courtney who, frankly, doesn’t look anything at all like the offspring of Willis and Bonnie Bedelia. When Dad decides to fly to Moscow on vacation to liberate the kid from a Moscow jail, he discovers quickly that the son he’d assumed had finally bottomed out in the world’s esteem had, in fact, become a CIA agent entrusted with a Top Secret mission to get a hugely important Russian dissident out of prison.

The dissident has proof hidden somewhere that the next Putin-style Russian big shot is a crook and a potential international nuclear menace.

Easier said than done, it seems. All that rescuing involves roaring vehicular mayhem on Moscow highways, more bullets than are stockpiled at Fort Ord, and enough artfully placed explosives to remove everyone who can’t be mowed down by mere bulletry. Sometimes, in a “Die Hard” movie, even big stunts and extremely loud automatic weapons fire just aren’t enough.

Let’s all cede that it’s a bit of an outdated stretch to ask us all to be this paranoid about the New Russia and its potential for misusing nukes and uranium stockpiles. But then the movie knows that, too. Willis and his CIA son are smugly told by the lead Russian bad guy – a frustrated dancer (remember that ballet star Alexander Godunov played a bad guy in the original “Die Hard”) – “it’s not 1986, you know. Reagan is dead.”

McClane is also told by the guy “you know what I hate about the Americans? Everything. Especially cowboys.”

But then we know there are some things you just don’t say to our infinitely resourceful Jersey-boy wiseacre, even in the worst of his films.

In the audience for a “Die Hard” movie, we are all modern cowboys. And having a ripping good time of it.

A good day to die hard

Rating: 3 Stars

Starring: Bruce Willis, Jai Courtney, Sebastian Koch, Mary Elizabeth Winstead, Yukuva Snigir

Director: John Moore

Running time: 97 minutes

Rating: R for violence and language.

The Lowdown: John McClane goes to Moscow to rescue his n’er-do-well son from prison and discovers he’s a CIA agent.