Fiat CEO Sergio Marchionne is staking a fair amount on the Mirafiori situation insisting the unions must buy into new working arrangements.

And, after interminable negotiations with the plethora of unions, namely FIM-CISL, UILM, UGL, FISMIC and FIOM, all the former and of course not the latter, have accepted a deal that now goes to a general vote.

Precise details of what Marchionne is requesting remain a mystery. But this week FIOM shed some light on the new requirements that it says appear to involve a small reduction in daily work breaks - from 40min to 30min, sickness change benefits and what it vaguely refers to as "strike rights."

None or all of this may be true. But it's certainly likely Marchionne will extract some fairly hefty productivity improvements in exchange for the EUR1bn (US$1.31bn) proposed investment into Mirafiori that will see Chrysler enter into a joint venture. There are also further, unspecified carrots being dangled of salary and bonus.

But Marchionne has at least secured the backing of the vast majority of the unions - with the obvious exception of FIOM or "our old friends" as a source close to Fiat in Turin wearily told just-auto this week.

And even those other unions are starting to close ranks, with the Italian Metalworkers Federation (FIM) urging its fellow-labour body to sign up.

FIOM continues its stubborn path however, insisting the deal is "awful" and has resorted to its traditional tactic of calling a strike for 28 January. "The question is a simple one - the answer will not be as simple," was cryptically, how FIOM put the situation to just-auto this week.

Marchionne however, recently struck a note of caution surrounding industrial relations at Mirafiori and FIOM's reluctance to jump on board.

"I am pleased that, in the end, the sense of responsibility has prevailed, even if we would have preferred that this project had the support of all trade union organisations," he said, adding tellingly: "We now need to work on defining a collective agreement specific to the joint venture that will enable workers to transfer to the new Fiat-Chrysler enterprise."

Is that yet a further deal that needs to be thrashed out following the vote at Mirafiori next week or will it be included in the referendum? This again from Marchionne: "For our part, we are ready to launch the investment programme as rapidly as possible."

But the Fiat boss has also threatened to take his money elsewhere and seems utterly determined to force a showdown with the company's organised labour.

Do the other four unions have enough clout to force FIOM back to the negotiating table. Or will FIOM throw the baby out with the bath water in the name of principle?