diesels run colder than petrols, and when under no load they run a LOT colder than petrols.
Mates farm has a large perkins v8 derv powered generator, and you can start it up without bringing the generator online for testing purposes, it runs so cool in that mode you can actually touch the exhaust manifolds with the motor running, and yes they're hot, but not hot enough to burn you.
Clearly if the generator was coupled in and it was producing the 200kw its designed to with the engine running full bore they'd melt your hand off, but its still weird to see.