I am swapping a 518 OD in to replace a manual, and bought a 14,400 BTU B&M SuperCooler, but it uses 5/16" hose barbs. Is this going to be a problem? I have read 3/8" lines are important on the 518.

If the 518 needs the flow of the 3/8" lines, then this could be a problem. But if the 3/8" line is more to deal with the inefficiency of the stock, radiator based cooler, will the more efficient B&M cooler be fine with the 5/16" inlets? I can even run 3/8" to the cooler, bought a 20' roll of tube already.

For what it is worth, I have already added a deeper aluminum pan and a Autometer trans temp gauge sender into the pan. So I will know if there is a heat problem, but i would rather head it off, than fix it later.

I've read the problem is due to the return oil from the cooler is what supplies lubrication to the O/D section & so the volume is needed...I've done two 518 conversions & had Fine Lines bend me up cooler lines to stock shape out of 3/8 tubing, modified the stock radiator mounted cooler by drilling out the fittings while blowing air into the cooler from the opposite fitting & modifying every fitting in the flow path by drilling it to the ID of the 3/8 tubing...Neither vehicle has had any problems so apparently I haven't compromised their needed flow to badly...

I could see drilling out flare fittings, but this cooler uses barb connections. There isn't anything to drill.

I wonder if the pressure from the pump is sufficient to increase the velocity of the fluid through the 5/16" barb connection to maintain the volume on return to the transmission?

The only point the system would be necked down from 3/8" to 5/16" would be the 2 one inch long barb connections.

It's probably fine, I just tend to be overly cautious..

You can buy some of the B&M or Long coolers with NPT screw-in fittings, just get one of these and buy the 3/8 barb to NPT fittings to fit it from a hardware store.

I had the same concerned when I swapped a 518 into my 87 Ram. It was replacing a 904 which, just like your car, had 5/16" lines and fittings into the cooler.

Raised the concern to my transmission guy. He couldn't advise me either way so he wanted to put a number on it.

Pulled the truck into his shop and disconnected my transmission lines. He then connected them up to his machine which measured the flow. Today I do not recall the number but he was more than satified with the Gallons/minute his machine was flowing through my cooler set up so he gave it his passing grade.

It was steel 5/16" lines into the factory radiator cooler. Rubber lines into a B&M stacked plate cooler. Back to steel lines, return to transmission.

I am using a stock converter, not sure where it stalls, but I am sure it is under 2000 rpm.

Last night I dug out another cooler I have. I picked it up at the U-Pick It from a mid 90's F150. It is larger than the B&M SuperCooler, but is a plate and fin design, verses the B&M stacked plate. It used 5/16 flare fittings, and had a 3/4" straight thread on the outside of the fitting. So I drilled out the inside of the fitting to 7/16" and screwed on a 3/4" pipe to 3/8" flare adapter. The fitting seated well on the straight threads, but I will braze around the base just for extra insurance.

I realize a sendor in the line would be more accurate, but in the pan will still allow a base line and see if the temp is off from there. If I don't like how it reads I can always move it to the line. I did this on my Fury.

The Magnum PCM has a wire for a trans temp sensor light that is normally installed in the line, but was optional equipment. I am considering adding that sensor and a light on the dash. That would get my attention a lot quicker. I believe the PCM will also lock out overdrive at that point also.

I run 2 20k coolers by-passed from the radiator in my 95 Ram. no problems at all.

you ask why? truck has 40" tires on it, and i use it for offroad and i do tow with it. trans has 155k on it.