synchromesh
Concise Oxford English Dictionary © 2008 Oxford University Press:
synchromesh/ˈsɪŋkrəmɛʃ/
▶noun a system of gear changing in which the driving and driven gearwheels are made to revolve at the same speed during engagement.
– origin 1920s: contr. of synchronized mesh.
'synchromesh' also found in these Oxford entries:

