McLaren technical boss, James Key has praised "fantastic" Mercedes for its assistance in helping the Woking team adapt its cars for this year's engine switch.

With the previous rekindling of its marriage to Honda having proved an almighty failure, McLaren will be hoping for an entirely different ending as it joins forces with another former partner, Mercedes.

After two seasons with Renault, the Woking squad joins Aston Martin and Williams in becoming a customer of the German manufacturer whose engines have dominated the hybrid formula, having won 103 of the 138 (74.6%) Grand Prix held since the formula was introduced in 2014.

Though the teams are using essentially the same cars they ran in 2020, McLaren has had to make major changes to its car in order to accommodate the new power unit, and technical boss, James Key admits that Mercedes has been "fantastic" in aiding the Woking outfit.

"We're in reasonable shape, I think,” he told the F1 Nation podcast. "We're where we would expect to be at this time of year, despite the obvious delays we had earlier in the year.

"Normally we'd want to start earlier than we did," he admitted. "We obviously didn't start as early as we'd liked to have, but I think we caught up well.

"The interaction with Mercedes has been fantastic," he continued, "they've been extremely supportive, recognised the fact that we had short timescales. And we've worked with them very effectively over the past few months.

"I think we're where we'd expect to be with the maturity of the engine installation and the parts that we're already making," he added.

Along with the changes required to accommodate the new power unit, like its rivals McLaren has had to deal with the downforce changes that encompass the cars' floors, diffusers and rear brake ducts, and all whilst keeping within the $145m budget cap that comes into force this year.

"That's still a work in progress,” he admits. "We've had this project going for a while, although the total regulations for 2021 were quite late.

"We're clawing it back," he says of the downforce, "it was a reasonably big hit to begin with. Floor changes from the side profile of the floor which will be quite obvious on the 2021 car; small diffuser, rear brake ducts - they all influence a complex area of aerodynamics around the rear tyre and really it's a case of trying to pull that back as best we can, and we are making progress every week at the moment, but there's still some work to do."