which year are u in? it seems that u must be some 1st or 2nd year student to be posting this....
well, what do u mean by true core? if core, according to u is managing people and processes in a production company, then most of the "core" pvt firms visiting fall in that criteria. But, if u are actually looking forward to be learning and using mechanical engineering concepts and design things, then some of the good companies having visited us are Alstom, Siemens Power, Fluor Daniel.. "may be" TCE and GS engineering... and if u feel that u are the field work kinda guy, then prolly tata power and punj lloyd... not sure if tata has visited by now... and many more wud be coming i believe...
so, before posing such question, do elaborate on what u consider a core company..
as far as what to prepare is concerned, there is not much actually... when u'd be having ur placement season, u'd knw what to study, and also, whether to study or not... but then, wrk a bit on ur soft skills also... sometimes, these soft skills help u more than the tech skills to get a job...
_________________
Abby
"Adversity is another way to measure the greatness of individuals. I never had a crisis that didn't make me stronger."
- Lou Holtz