I'm replacing the head gaskets in 1994 mitsubishi Diamante ES.But before I do (ALREADY THE HEADS OFF THE MOTOR) how do I know if I need to replace pistons or just pistons O rings.:confused:

Should have done a compression test before disassembly. Closely examine the cylinder bores. Look for any scratch marks. Should be clean and smooth.
Drop the pan and mark all the piston/rod caps orientation. Pop those pistons out and re ring them. Pay close attention to where the ring gaps line up. Clean all the ring grooves thoroughly.
Be sure to replace the valve stem seals when doing a head job.
DO IT ONCE. DO IT RIGHT!!

Should have done a compression test before disassembly. Closely examine the cylinder bores. Look for any scratch marks. Should be clean and smooth.
Drop the pan and mark all the piston/rod caps orientation. Pop those pistons out and re ring them. Pay close attention to where the ring gaps line up. Clean all the ring grooves thoroughly.
Be sure to replace the valve stem seals when doing a head job.
DO IT ONCE. DO IT RIGHT!!!

Thanks for the advise, but do I need to take the center block off the car before starting this process? ( the center block is still in the car) Thank You.

Thanks for the advise, but do I need to take the center block off the car before starting this process? ( the center block is still in the car) Thank You.

"Center" block?

First, Why are you replacing the head gasket?
Without knowing more of the vehicle history, proper advise will be sketchy.
Is the vehicle using oil or coolant excessively? Loss of power, etc..

The pistons themselves are probably OK, unless there is some physical damage. The piston's compression/oil rings may need replace due to mileage/wear (there are no "O-rings" on the pistons themselves).

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html

"Center" block?

First, Why are you replacing the head gasket?
Without knowing more of the vehicle history, proper advise will be sketchy.
Is the vehicle using oil or coolant excessively? Loss of power, etc....?

The pistons themselves are probably OK, unless there is some physical damage. The piston's compression/oil rings may need replace due to mileage/wear (there are no "O-rings" on the pistons themselves).

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html

I bought the car then 2 months later it started smoking a lot.. and overheating. ( O ring in water pump needed to be replaced.) Then I found the head gaskets were in bad shape while I was working on the car.