STATS: The entire Route 27, from Zernez (Graubunden , CH) to Mals (Italy) is about 52km and 1250m elevation (1,700m in opposite direction) https://www.schweizmobil.ch/en/cycling-in-switzerland/routes/route-027.html
Route 27 leaves right from Zernez (1,470), ascends the Ofenpass (2149m) through the scenic Swiss National Park and then descends to Tschierv and Val Müstair ( St. Johann Monastery), finally crossing the border into Italy.
However, having only 1 day for the roundtrip tour (and no railway to come back), I opted to go as far as the summit at the Ofenpass and then bike back.
Thus, the roundtrip was “only” about 45km and 1,100m elevation.
Leaving Zernez (located between Upper and Lower Engadine), the route ascends through the Swiss National Park surrounded by a peaceful and immense pine forest. This Park is the oldest national park in the Alps (founded in 1914) and a UNESCO biosphere reserve comprising a vast surface up to the border with Italy.
Further up, trees give way to a spectacular landscape of streams, grasslands and mountain peaks, until the summit at Ofenpass.
In this cold May, there were still piles of snow all around the Pass
At this point I turned back and retraced the road down to Zernez. However, another option would have been to continue through the old villages of Fuldera, Valchava and Sta. Maria down to Müstair where one can visit the St. Johann Monastery dating from Carolingian times (UNESCO World Heritage site). Then, shortly after, the Section crosses into Italy, ending at the medieval town of Mals.
Link to Introduction and other Routes from Zernez: