Mercedes' customer teams will not be given the manufacturer's latest specification power unit when they will be expected to make their fourth and final change at this weekend's Russian Grand Prix.
According to Autosport, Williams, Lotus and Force India's Sergio Perez (Nico Hulkenberg is already using his fourth power unit) will not receive the latest version of Mercedes' all conquering PU106B power unit but championship leader Lewis Hamilton will.
The new engine comes after Mercedes opted to use their final 2015 development tokens prior to last month's Italian Grand Prix to make improvements to the power unit's combustion element.
Although leading customer Williams had hoped to tie up a deal to run the latest specification power units, it is believed that Mercedes were unable to produce enough parts to supple the upgraded engines to all of their customers.
Williams and Force India will now have to wait until 2016 to get their hands on an upgraded version of Mercedes' power unit. Lotus meanwhile are expected to tie up a deal with Renault to once again become the French marque's factory team.
The resurrected Manor team will replace them on Mercedes customer roster and are expected to be supplied with current-generation engines next season.
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