SpudFiles

Is the inside diameter of a SCH40 4" male adapter opposite of the treaded part is the same of a SCH40 4" pipe ?

because in my coaxial piston, the piston wont go back if the inside diameter isnt the same

And if my piston dont fit in the male adaptor, can I grind it inside to make it thiner or it will make my adaptor to much weak to put pressurized it ?

male adaptors are usually the same diam, as the pipe.

If you are useing a 3'' coupling it will be fine.

For this you'll want to take your piston with you to Home Depot and find a 4" male that it'll slide through. While the IDs should be identical, the actual parts are manufactured to loose enough tolerances that you need to pick em out by hand. Its kinda like how you can try sleeving 1.5" SDR in 2" sch80 and it can be either an ungodly tight fit or really really loose. Its simply not that precise.