If I get 170 marks in jee main April 2019 then in mechanical engineering can I get good Nits category - OBC

Hello Manisha,
You have a decent chance to get a seat in NITs.
There are many NITs where you can surely get admission.
I think your rank should be around to 40k and with this rank you can try in NITs like NIT Sikkim, NIT Manipur etc.
You can check here previous year cut off trends for mechanical in NITs
https://www.google.com/amp/s/engineering.careers360.com/articles/nits-cutoff-for-btech-mechanical-engineering
Hope this would be helpful.
Thank You.