School me on trans basics

JoeSBP said
What is the suggested inner diameter for cooler lines? Plan on using push locks fittings and flexible hose.

The factory lines were 5/16" tube which has an I.D. of about 1/4" but the adapter fittings that screw into the trans are 1/8" pipe so unless you change them also there's really no need to go larger than 1/4" i.d.