The artificial spiders’ webs hanging in the windows of the Royal British Legion in East Kilbride, on the edge of Glasgow, are just part of the Halloween decorations. But they feel oddly appropriate on this bright, frosty morning in the company of men whose distant triumphs have recently had the cobwebs dusted off them. Sitting off to one side is the 41-year-old Chilean film-maker Felipe Bustos Sierra. Huddled around a table next to him are the former Rolls Royce plant workers whose bold statement of solidarity with the Chilean people in the mid-1970s is the subject of Nae Pasaran!, an inspirational documentary that proves principled acts can have positive consequences – even if they take decades to come to light.

Six months after the bloody coup of 11 September 1973, which began the brutal 17-year dictatorship of General Augusto Pinochet, these four Scotsmen – Bob Fulton, Robert Somerville, John Keenan, Stuart Barrie – downed tools and refused to service and repair engines for the Chilean air force’s Hawker Hunter planes. “Down tools?” says Bob, a former engine inspector and the instigator of the boycott. “We hadnae time to pick ’em up!”

Bob is 95 now, a gentle man with expansive hand gestures who sometimes holds on to the sides of the table while he talks, as if he’s planning to drive it away. He can still remember vividly the events of that March day in 1974: “I got to my desk in the morning and there was this compressor shaft up on the table ready for me to inspect. The first thing you do is check the card. Well, I turned the card round.” He acts out the scene, flipping a beermat over and staring disbelievingly at the underside. “And there it was: Chile.”

“We had already condemned the Chilean junta,” adds Stuart, who is 74. John, the 78-year-old former assembly unit worker and member of the works committee, leans in to clarify. “The people being tortured and murdered, many of them were just like us: trade unionists. At our monthly meeting, Robert had made a motion condemning the actions in Chile. And then when Bob recognised the engines – well, you tell him!”

Bob jabs a finger at me. “This is true,” he says, and I notice Felipe giving the fond smile of someone who has had that finger jabbed at him plenty of times. “I went to the foreman and said, ‘I cannae work on that.’ From there, I went to see Stuart, who was a shop steward, and told him there were bits and pieces of the Chilean engine possibly on the line already.” Stuart is chuckling: “I can hear him shouting, ‘There’s Chilean engines in here! The whole place is awash with ’em!’” Everyone falls about at his impersonation. “I would say you were somewhat volatile at times,” says Stuart.

“I might’ve been,” replies Bob, in a voice softer than falling snow.

“I told Bob, ‘Right. That’s it. We’ll black the fuckers.’” Blacking entailed attaching labels with the word “black” on them to each contested part, warning everyone in the plant to steer clear of them. The four engines – which had likely come from the Hawker Hunters involved in the attack on the presidential palace in Santiago – were eventually dumped outside in crates. Without protection from the elements, they were useless within a year.

In theory, the men could have been sacked for their protest, but the strength of the unions made that unlikely. “The only reason we could do what we did was because we were organised,” explains John. “We took strike action for the NHS, the Shrewsbury pickets, you name it.” But this was something different: a high-profile international case that brought hope to people 7,000 miles away.

Precisely how much hope, and to what end, had remained unclear until Felipe started researching his film six years ago. It was widely known that Hortensia Bussi de Allende – the widow of Salvador Allende, the democratically elected Marxist president killed in the coup – visited Glasgow in 1975 and expressed publicly her gratitude. Hundreds of letters of thanks poured in over the years, too, and Robert helped to resettle Chilean refugees in Lanarkshire as early as 1974.

“It was perfect, perfect,” he says now of the community’s reaction. “The amount of help we got – from the council, who gave them houses, and Rolls-Royce, who asked for people to help out by donating white goods.” It sounds a world away from the hostility that can accompany the arrival of refugees today. “Aye,” says Stuart sadly. “There was no trace of what you see now.”

The men always felt the story was unfinished, though, not least because of the way the boycott ended: after four years, the engines were mysteriously stolen in the middle of the night by vehicles with false licence plates. “It was as if the SAS had done it,” says Bob. “Obviously the government, no question,” says Stuart. “But personally, I’ve always seen it as a victory. They can only fly so many hours before they need to be overhauled – and we stopped that, didn’t we?”

Indeed, the film tracks down Fernando Rojas Vender, the unrepentant former commander of Chile’s air force, who makes clear the extent of the damage done. The entire squadron of 29 Hawker Hunters was close to being grounded by the boycott. Although India, Israel and South Africa came to Pinochet’s aid, probably with spare parts, East Kilbride was the only place in the world where those engines could be properly repaired.

The boycott was familiar to Felipe: as the son of a journalist exiled to Belgium after the coup, he remembered hearing East Kilbride mentioned at solidarity meetings. “I’d been told that many of the original guys had passed on, so I was intending the film to be fiction not documentary. Then, when I met them, I realised they’re natural born storytellers. That matches the way Chilean solidarity has been told over the years, as oral history.”

Robert was wary of revisiting the boycott, though, having been stung in the past by inaccurate reporting, while Stuart harboured little affection for those days. “I was about as interested in anything to do with Rolls-Royce as I was in the location of my last shite.”

When I ask Stuart what changed his mind, Felipe jogs his memory: it was the footage of Chilean political prisoners remembering how the protest saved their lives. Most government records were destroyed, but the film argues convincingly that members of the air force who had been sympathetic to Allende – and were imprisoned and tortured under Pinochet – may in fact have had their freedom granted in exchange for those Hawker Hunter engines.

“I remember watching that with you,” says Felipe, “and you kind of lost your sense of humour a bit.” Stuart agrees: “When those guys discussed the impact it had, I was impressed. Some people would be broken after what they’d been through. But they had got up again. I was touched by that. And I make an effort not to get touched too much.” Bob is more forthright: “It was a bit emotional watching all that. I think it’s a cracker of a film.”

Felipe says one of the first questions he was asked by the men was whether he could find out what happened to the engines. And one of the offending items does make a cameo appearance near the end of the film. How did it feel to see it again? “It was a good feeling,” says Stuart. “Ah, it was just an engine,” sighs Bob, to much laughter, though he brightens at the news that it will soon be on display permanently in East Kilbride. “I think it has affected Chileans more than these guys,” Felipe tells me, “because so much from Chilean history has been destroyed. Finding tangible evidence is quite a rare thing.”

The men are long retired and the sprawling plant has fallen silent: it closed in 2015 and was demolished the following year. But the question raised implicitly by the film is whether something like the East Kilbride boycott could ever happen again. “Anyone today who wants to mirror what these guys did is taking a much bigger personal risk,” says Felipe, citing the lack of trade unions. “But considering what’s happening now, some of us are going to have to take that risk.”

Bob gives it some thought. “I’m not sure. It would have to be very specific circumstances.” These men were responding, after all, to a coup that focused the world’s attention on Chile, whereas anyone wishing to obstruct, say, Britain’s sale of arms and torture equipment to countries on its own list of human rights abusers, faces a more diffuse task.

“It wouldn’t be so simple now,” says Stuart. “But never underestimate the power of someone standing up and speaking the truth.”

• Nae Pasaran! is released in the UK on 2 November.