I replaced 2 times the flywheel after 6.000 miles I want to now the reason
 

I replaced the flywheel 2 times I want to now what is reason this broken please help

What is your vehicle/engine/transmission? What exactly breaks on the flywheel? Pictures?

It crack the flex plate of Mitsubishi Montero limited 4x4 2002
 

it cracked the flywheel the flex plate and started to make a noise a tic tac but when given a gas stop the noise and the mechanic replaced it was cracked he replaced 2 times a ready after 6 .000 millage
The mechanic toll me the reason can be is because the tires are bigger it's ring 20 the regular size are 16 I don't know if that's the problem ,my brother has a Toyota lancruser 4x4 and has big tires and doesn't have that problem help please

Bigger tires will not (or at least should not) cause flex plate to crack. That end of transmission get input that is only what engine can handle. The back end of transmission will output higher torque required by larger tires. The issue with failed flex plate will be due to engine and transmission not being aligned properly. That's why I'm asking what engine you have and what transmission. Is it OEM setup or modified? Does flywheel (flex plate) mounted correctly to the engine? Does transmission case mate properly to the engine or is it just a smidge off? Was flex plate dropped prior to install? Is it a Mitsubishi part or made in China aftermarket "universal fit"? Are there several indexing hoes on the flex plate? We're to right ones used?
Again - better description of your vehicle and photos of the failure would be helpful.