Some senior citizens with too much time on their hands pick up bocce or dominoes as a hobby. And then there are the “Bad Grandpas”: Eight old men who pulled off one of Britain’s most audacious jewel heists, capturing some $20 million in precious goods.

The plan began coming together over fish and chips at the Castle, a North London pub, in 2012. Ranging in age from 52 to 73 at the time, the friends were some of the UK’s most notorious break-in artists and had served prison time for armed robbery, jumping bail, fencing stolen goods and other offenses. At this point, however, they were generally out of the game — although they still enjoyed hanging out and reminiscing about their bad old days.

Brian “The Guv’nor” Reader, 73, started complaining about being tight on cash (despite living in a home valued at nearly $1 million) and wondered what it would take to pull off one last job. His white whale? Hatton Garden Safe Deposit in London’s diamond district.

It housed nearly 1,000 security boxes where local jewelers stowed a fortune in gems, gold and cash. Although a fat target, the place had not been successfully fleeced in decades.

Somehow, the fanciful musing metastasized into a bona fide plan, largely masterminded by Reader. As chronicled in the new book “The Last Job: The ‘Bad Grandpas’ and the Hatton Garden Heist” (Norton), the friends — who dubbed themselves The Firm — spent three years plotting their ­career-topping caper.

“This represented a final hurrah,” author Dan Bilefsky told The Post. “They were motivated by cash, but, at a time in life when many of their contemporaries lived in nursing homes, the excitement of a final heist got their hearts going.”

The FIRM started preparing.

They invested in a copy of “Forensics for Dummies” to learn about DNA detection at crime scenes. (One tip they used: scrub down the area with bleach before fleeing.) Member Danny Jones, 60, researched the best diamond-tipped drill for breaking through the vault’s reinforced concrete wall: a $5,200 Hilti DD350 with the capacity of 667 rotations per minute.

The gang watched YouTube videos for pointers on how to use the tool, and planned to bring along a battering ram for the final push. They allegedly practiced at the plumbing shop of plumber Hugh Doyle, 47, whose neighbors heard The Firm trying out their drill.

It was the Thursday start of the long Easter weekend in 2015 when Reader embarked on a 20-mile bus ride from his Kent home to the Hatton Garden Safety Deposit building, not far from the Castle pub.

There he convened with Jones and four others: lock and alarm specialist Michael “Basil” Seed, then 54; getaway driver John “Kenny” Collins, 74; and “extra pairs of hands” Carl Wood, 58, and Terry Perkins, 66. (After the fact, they would enlist the help of William “Billy the Fish” Lincoln, 59, who would provide transportation and storage of the stolen goods.)

At around 8:30 p.m., the men smashed security cameras, disabled alarms and lugged their equipment — including an assortment of tools and rolling trash cans to be loaded with jewels — inside.

They spent hours drilling into the wall. Perkins had to take a break for an insulin shot. Getaway driver Collins dozed while serving as lookout in a building across the street.

Finally they broke through — only to come up against the steel backs of safety deposit boxes. Jones punched at them with the ram until the tool shattered into pieces. On Friday morning, frustrated by their failure, the old-timers decided to head home for a nap.

They planned to meet up again in the wee hours of Easter, enough time for them to get their hands on a more powerful hydraulic ram. But by the time Sunday rolled around, Reader had lost faith and backed out. Wood showed up, changed his mind and left.

Undaunted, the remaining Firm members were pleased that there would be two fewer people sharing in the take.

“They felt that they had nothing to lose,” said Bilefsky. “There was a brazenness that was born of age.”

Finally, with the cheerleading Jones urging, “Smash that up!” they penetrated the steel backings. Climbing through the narrow hole, the robbers emptied some 70 boxes, stopping only when they’d gathered more loot than they could roll out in the trash cans.

On Tuesday morning, a jeweler with offices on the same floor as the vault encountered the trashed premises. “The Last Job” quotes a security guard: “It was like a bomb hit the place.”

Outraged customers tried to assess the financial damage. The London Metropolitan Police’s elite law-enforcement division, the Flying Squad, swooped in to investigate. But the “Forensics for Dummies” bleach tip had worked: Not a single fingerprint remained.

Still, the crime was far from perfect. As Bilefsky put it, the old men had failed in one big way: “By not understanding technology.”

Ignoring the fact that they lived in what is said to be the world’s most surveilled nation, the Bad Grandpas showed no respect for England’s CCTV system, with its extensive network of cameras on streets and in public places. Detectives made their first ID by discerning ownership of Collins’ Mercedes-Benz, which he foolishly drove to the crime scene. Soon after, investigators managed to bug some of the gang members’ automobiles. They surreptitiously heard The Firm’s members crowing about their crime and bad-mouthing each other.

Reader, once nicknamed “The Master,” was mercilessly derided behind his back. His former partners trashed him as “an old ponce” who “talked about all our yesterdays” and “bottled out at the last ­minute.”

Immediately after robbing Hatton Garden, the crew stashed most of their haul in the garage of Lincoln’s unknowing nephew, but took some of it home with them — storing the diamonds, gold and cash in kitchen pots and pans and, in the case of Jones, under the tombstone of his girlfriend’s late father. Then they went to the pub to celebrate.

Some six weeks later, The Firm started talking about splitting up the rest of the loot. They agreed to a meet-up at the home of Perkins’ daughter to do the chop. Just as the men laid out the goods, along with a smelter for melting down precious metals, police busted in with a battering ram of their own.

Taken by total surprise, the men mostly accepted their impending fate. Jones, however, bolted out a back door, gaining only a few yards before being tackled and handcuffed by officers.

Reader was apprehended at home. Wood was arrested getting out of his car, and promptly wet himself.

Despite the cold feet of Reader and Wood, Britain’s legal system made no accommodations for second thoughts. All six of the main crew pleaded guilty, ­receiving prison sentences of around seven years each. (Lincoln also got seven years and Doyle received a ­suspended sentence.)

Perkins died behind bars in 2015 after a heart attack. The others continue to serve terms at Belmarsh prison, a maximum-security facility in southeast London that now also houses Julian Assange.

Through it all, public sympathy sided with the Bad Grandpas.

“[The gang members] didn’t hurt anyone and there was a feeling of ‘good on them,’ ” said Bilefsky, who believes the surviving Firm members are finally done with crime. “If they bore a hole through Belmarsh, that would be impressive. But I don’t think they will be escaping anytime soon.”