If you score 240 marks in JEE main your approx. rank will be around 3000.

According, to 2017 data if your rank is around 3000 you can easily get top NITs in Mechanical branch.

You can also check the cut off rank from the given below link

https://engineering.careers360.com/articles/nits-cutoff-for-btech-mechanical-engineering

Good Luck!!