Unite said the 30 engineers, who service generators around the world, voted overwhelmingly in favour of striking and will stage industrial action every day from 25 May to 16 August.
The proposed contracts include reductions to overtime rates, allowances, holidays and other terms and conditions that would result in a pay cut of between £10,000 and £15,000 a year.
The engineers’ jobs have been threatened if they do not sign the new contracts, which will leave them on pay rates "well below the industry standard".
The union said the strikes will ‘cause havoc to the firm’s worldwide servicing schedule and serious inconvenience to its international clients’. It added that parent company Melrose has a "reputation for targeting workers and viable operations to boost short-term profits".
Melrose is currently also in dispute with Unite over its plans to close its ‘extremely viable’ GKN automotive driveline factory in Birmingham - which it said threatens 500 jobs.
Unite regional officer Lakhy Mahal, said: “Throughout the pandemic, these engineers have continued to travel wherever Brush Electrical needed them to be, even though they had to spend weeks quarantining in solitary confinement to do so.
“Now their bosses, who have most likely been sat comfortably working from home during lockdown, are rewarding our members for their loyalty and service with fire and rehire pay cuts of up to £15,000.
“Threatening our members with the sack unless they sign contracts that will see them being paid well below the industry standard has only strengthened their resolve to win this dispute.
“Unless Brush Electrical and Melrose walk back on their fire and rehire plans, these strikes, which will cause havoc to the firm’s worldwide servicing schedule and serious inconvenience to its international clients, will go ahead.”
Unite general secretary Len McCluskey said: “Fire and rehire' is ripping through our workplaces like a disease.
“Weak law lets bad bosses force through brutal changes to contracts, sometimes taking thousands of pounds off wages that families need to get by.
"It's a disgraceful practice that's outlawed in much of Europe and should be here.
“Unite is fighting for UK workers to be treated with the same decency. We won't stop until the law is changed to protect working people from attack."
