What do you use for trans cooler lines? Thinking of changing mine. What size?

hi
5/16 line
mancinni has nice pre formed ones !

That would depend on the trans... the 904 and 727 uses 5/16" and the 500 type use 3/8"

Race car? Street car? OEM? What cooler? Where is it mounted? What trans?

727 trans race car only

I used the stainless braided line kit from CRT. Worked really well.

https://www.coperacingtrans.com/?product=cooler-install-kit

Stainless braided, usually make my own

-6 teflon lined hose - like Aeroquip's TFE, .320"ID - with appropriate fittings and adapters. Use a cooler with threaded connections. The old rubber hose and hose clamps on barbed fittings are no good and create a huge mess when they blow apart.

I've mounted this stuff under the car, sometimes with a cooler with a small dedicated fan for the really abusive combinations, with good results. Saves all the plumbing and crap at the front of the car.

I've used -5 stuff when using it as connections for 5/16 tubing, but that stuff is oddball. -6 is much easier.

3/8" Bundyweld brake line with compression fittings and very short pieces of braided line where needed for small bends.
Using 3,000 psi stainle$$ braid for a cooler line is a bit overkill....

That will work IF the steel line is properly flared and/or adapted to AN or SAE and the hose installed using the correct threaded fittings. Hose clamps on braided hose is a mess looking for a place to happen.

I'm OK with overkill if it means I don't have to worry about it failing.

i guess looking at the Cope site, the fittings for the lines at the trans should be 1/4" npt to whatever AN size you use.
I just ordered a derale stacked plate cooler from Mancini to hook up on my car since the rubber hose and clamps on the radiator trans cooler won't stop leaking...plus the existing steel lines rub on the everything...

probably just going to use some kind of flexible line or steel braided since moving steel tubing lines out of the way to pull the motor/trans seems annoying.

You can order 1/4 NPT to #6 AN male adapter fittings and use #6 AN hose ends on #6 AN PTFE lined (Teflon) hose to make your own flexible tranny cooler lines
I do on my race cars

the fittings on a 727 are 1/8NTP, not 1/4, correct ?

Your probably correct I'm not sure Maybe
When it gets down to using them I take them off the shelf and use them without looking at the numbers on the carton

well, just to be sure my feeble old mind wasn't spouting BS information, i went out into the shop and checked a 727 that was on the cart.
it is indeed 1/8" NPT thread for the cooler lines.
hopefully, there was no confusion caused.

How high is the pressure in the cooler lines ?

Maybe 5-100PSI depending on the trans and how it's set up. Not much, but the chemicals in the oil and the heat need to be taken into consideration when choosing hose material and construction. Even the rubber lined AQP style hose is questionable.

There are -6 x 1/8" NPSM fittings available which rely on a gasket to seal vs. pipe thread. Helps eliminate the possibility of cracking a case from over tightening NPT fittings.

somewhere around 15-30 i think.
hopefully, someone can confirm this. i looked in the FSM and the ATSG books i have, and couldn't find it.
i just did a quick look, so i may have missed it.

I'm sure the pressure and volume are different at idle RPM than at 5500 RPM

I know some will think I'm , and it is a racecar, but my bracket car has rubber-lined -4 ss braid and its been on there for at least 20 years.