An Adelaide manufacturer of Harley-Davidson wheel rims will close, impacting about 120 local workers.

New Castalloy at North Plympton — which in an unrelated matter is at the centre of a renewed search for the Beaumont children — will shut down in just over a year.

The Australian Workers Union said 120 jobs would be lost.

"Apparently this has to do with sales of large motorcycles and ongoing demand for the product that is produced at the North Plympton site," union official Peter Lamps told the ABC.

"This is not about competitiveness or cost issues. Harley-Davidson has made it abundantly clear to us that this has been more of a product mix and product demand issue.

"Certainly the workers and the way they operate down there has nothing to do with the decision that has been made."

Harley-Davidson is closing two operations — the other in Kansas — after its motorcycle shipments to dealers fell to their lowest level in six years.

It is making plans for new models in an effort to reverse the decline.

Workers told at end of shifts

Mr Lamps said workers at the Adelaide site received the news as they ended their shifts late yesterday, and he was making arrangements to visit the factory on Thursday, because the workers have today off.

"Part of my visit on-site tomorrow will be to make sure all of those workers' entitlements, when redundancies do occur, will be met in full," he said.

He said there might be some flow-on impact to materials suppliers to the Australian operation, but the bigger concern was the looming loss of more Australian manufacturing skills.

"I think the broader aspect is these highly-skilled jobs, making these niche products [will go]," he said.

"[It is concerning] if we pride ourselves on being a country that can make things rather than just produce a raw product and have that finished elsewhere."

Castalloy was facing closure back in 2011 until Harley-Davidson came to its financial rescue, then a rent-free deal for the foundry was negotiated two years later.

Minister for Manufacturing and Innovation Kyam Maher said

"The State Government had provided rent relief to New Castalloy in the past and had made New Castalloy last year formally aware that we were committed to continuing that so there's nothing the State Government could have done further, the company's told us, to help New Castalloy stay open," he said.

"We're going to make sure we stand shoulder to shoulder with these workers.

"We're going to have government officials down with the union, the Australian Workers Union, at the site tomorrow, offering the same level of support we provided to workers in the auto industry to make sure workers have the best possible chance to retrain, to get career advice and to access employment services.”

The factory site will be searched in the coming weeks for the remains of the Beaumont children, who disappeared from an Adelaide beach on Australia Day 1966.