What size cooler lines for 4L80E

[ZEKE68]

Have found that GM used 5/16" cooler lines on the 4L80E until 1998. They must have gone to 3/8" when they moved the return line to the middle of the transmission case. All the radiators I have found for my 67 to 72 truck have 1.2" - 20 threads for 5/16" cooler lines so there will be some restriction at the radiator even if I run 3/8" lines to it. Thinking about splitting at the radiator and adding an aftermarket cooler in parallel to keep the flow up. Since GM ran 5/16" lines on the 4L80E in the early years I assume that unless I work the transmission hard I would be ok even if I stayed with the 5/16" lines.

[FL71C10]

Originally Posted by ZEKE68

Have found that GM used 5/16" cooler lines on the 4L80E until 1998. They must have gone to 3/8" when they moved the return line to the middle of the transmission case. All the radiators I have found for my 67 to 72 truck have 1.2" - 20 threads for 5/16" cooler lines so there will be some restriction at the radiator even if I run 3/8" lines to it. Thinking about splitting at the radiator and adding an aftermarket cooler in parallel to keep the flow up. Since GM ran 5/16" lines on the 4L80E in the early years I assume that unless I work the transmission hard I would be ok even if I stayed with the 5/16" lines.

The Roadmaster I took my 4L60E out of had a separate external cooler. I can look at the size of the lines on that.

[Robznob11]

Run an aftermarket with 1/2 lines....Its cheap insurance. I run a derale with fan more than acquit bout 250 bucks for the whole kit. But they offer smaller ones a little cheaper. I went big for the 3800 stall 😜