Mr Stanley says he and his partner, Sue Lyons, took pity on Shirtcliff and visited him in prison.
Two weeks into his 20-month sentence, they pleaded with a judge to let him serve the rest of his term on home detention at their New Brighton house.
"We looked after him like he was one of the family," Mr Stanley said.
During that time, Mr Stanley was working on the engine project, and says he often discussed it with Shirtcliff.
Mr Stanley believes that once Shirtcliff's home-detention bracelet was removed, he disappeared to Australia with comprehensive notes and drawings on the project.
He spoke out after Shirtcliff's appearance before the royal commission of inquiry into the collapse of the CTV building, killing 115 people, in the February 2011 Christchurch earthquake.
Shirtcliff was construction manager on the project.
Mr Stanley, 62, alleges that when Shirtcliff left him in 2007, he moved to Brisbane, patented the engine idea and sold the technology.
Investor Wayne Smith said he paid Shirtcliff a $1000-a-week retainer to bring it to market.
The contract was severed after "about a year", according to Mr Smith, who says he has since spent $1 million employing German experts to work on the design.
In the meantime, Shirtcliff was contracting for global engineering consultancy WorleyParsons and chipping away at the American market, getting his patent on July 16 through DGC Industries, a company linked to the Fisher alias.
Shirtcliff, 67, could not be traced at the home outside Brisbane he shares with his wife, Julie Rook.
But his lawyer, David Tucker, said Shirtcliff strenuously denied Mr Stanley's claim.
"No allegation has ever been made of that nature by the Stanleys to our client, no demand from lawyers of the Stanleys has ever been received, nor demand by any other person.
"We are instructed that it is a complete fabrication."
Shirtcliff initially refused to give evidence to the royal commission.
When he did, he was accused of "distancing" himself from responsibility for the CTV building collapse after claiming he had limited input into the construction process.
Shirtcliff told the hearing he was a graduate civil engineer. Asked why he lived in Australia under the Fisher identity, he said it was because of "family issues".
Since giving evidence, he has been accused of stealing the identity of William Fisher, with whom he worked in South Africa, and faking an engineering degree to get jobs in New Zealand and Australia.
It is alleged the fake degree from Sheffield University gave him entry to a masters programme at the University of New South Wales in 1971, which led to his getting a master of engineering science in highway engineering.
The university is investigating Shirtcliff's background and says that if the claims are proved, it will cancel his degree.
Australia's professional engineering body has written to Shirtcliff asking him to respond to allegations he faked his degree and lied about his qualifications.
The allegations may be referred to the Australian Federal Police.
Shirtcliff has denied the claims, but yesterday it emerged they were being investigated by a senior New Zealand police officer.
Building and Construction Minister Maurice Williamson said Shirtcliff could face extradition to New Zealand and criminal charges.
"One hundred and 15 people lost their lives," he told Radio NZ. "If a person was using a fake ID and was the engineer supervising the building ... I think there could be some pretty serious charges to be laid."
Shirtcliff's contract with WorleyParsons ended last month. The firm said a review of projects he worked on had found no irregularities.
Mr Stanley is now living at Mapua near Nelson, "on the breadline".
He would like to challenge the US patent but said he couldn't afford the $100,000-plus he estimates it would cost in legal bills.
He said Mr Smith had made him a "generous offer" of a financial return if the technology became profitable.
"I'm happy to leave it at that. But I'm certainly happy for Gerry [Shirtcliff]] to face what he deserves, and I'll assist in seeing that happen."
