The Valtra 6850, also known as the Valmet 6850, has its own page here with more than 130 parts for steering and front axle, trailer couplings and electrics.
If the bonnet of your 6850 still says Valmet rather than Valtra, you are in the right place all the same. It is the same tractor from the Hitech range with a different name on the bonnet. This page holds more than 130 replacement parts that fit this model, from GRANIT, DANA Spicer, Scharmüller, Walterscheid and MAHLE.
For the engine there are seal kits, valve cover gaskets and piston ring sets from MAHLE, plus an engine speed sensor. Before new piston rings go on, check the end gap of each ring inside its own cylinder. On the exhaust side you will find seals for the exhaust manifold at the turbocharger and silencers.
A tip on that manifold seal: clean both faces back to bare metal with no trace of the old gasket, then tighten the nuts in several stages working crosswise. Let the engine warm up and check that they are still tight. A manifold with a small leak gives itself away as a ticking noise under throttle, and it blows hot gas past the turbo.
The front axle is covered by ball joints and a bolt set for the steering valve. At the rear there are ball drawbars and separate balls, from Scharmüller and Walterscheid among others. For the driveline there are O-rings, for instance for the differential and the feed pump.
Work lamps, combined indicator and position lights and a flasher unit keep the tractor visible on the road. For the cab there are door locks. A stiff lock is worth a little graphite or light oil before you replace it. If you convert the lights to LED, bear in mind that a conventional flasher unit will blink too fast with the lower load, so choose a unit designed for it.
The rating plate shows the model whether the tractor left the factory as a Valtra or a Valmet. Go by that, since each model in the Hitech range has a page of its own. If the old part carries a number, use it to compare with the parts on this page. With door locks and lamps, check the side as well.
These models from the same range have their own pages, useful when more than one Valtra works on the farm:
The Valtra 8950 belongs to this range too. For that model and every other one, start at Valtra parts.