Hello , 

Azure query is you have completed your Diploma and you have a caste certificate of OBC further you want to take admission as a lateral entry in mechanical engineering. 

since you haven't mentioned the college or the university you are looking forward to take admission therefore I can't help you in that Depth but I can tell you that you all that you need is to go to the college or the university where you want to apply for the lateral entry provide your all details about your diploma degree and your certificates and according to them if you will be eligible they will provide you admission. 

apart from that if you want to take admission in good colleges or top colleges you must have at least at the rate of more than 80 to 85% for the safe end.

Thank you.