Replaced the radiator in my truck everything was going smoothly

Welcome to 2CarPros.

I have to be honest, you have a good question. Most times, it is simply that they are not aligned with the threads in the radiator. However, I have seen aftermarket radiators that came with a different size thread. What I always do is this. The threaded coupling that is around the transmission line, pull the line back as far as you can so that it isn't pushing through the connector and causing problems. Then try to thread it into the radiator.

I attached two pics. The first simply shows the lines you are dealing with and the second is a close up of the problem area. Push the threaded part (circled in pic) forward until the flared portion of the steel line is flush with it and then try. If that doesn't work, remove the retaining clips in picture three to see if that helps.

These can be a pain. Let me know if you get it.

Take care,
Joe

I attached the procedure for replacing the radiator below with some pictures for you.

https://www.2carpros.com/articles/how-to-replace-a-car-radiator

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

Let us know if you have any other questions.

Roy