I am removing parts to replace my 4.0 with another '99 engine. Does anyone know the sizes for the flare nuts for the transmission cooler lines into the radiator? Or at least if standard or metric? And if fuel lines are flare fittings I need sizes for those. I don't have any flare nut wrenches so if I can buy just either standard or metric for now, I'll get the other later. And are crowfoot sets better since they usually have a larger range of sizes?

Also, I have what looks like another much smaller cooler mounted in front of my radiator behind the grill. Hard to see in the photo but it is on the left of the emblem facing the truck. Is this a secondary transmission cooler? My truck does have AC so that is another possibility.

 

Well my brother finally stopped by and confirmed the second photo is an external tranny cooler. And he was able to loosen the tranny lines on the radiator so I could remove it.

 

Pictures are worth a thousand words.
Your 1999 Radiator appears simular to my 2001 Radiator (original 2RowPlasticTanked since upgraded to 4RowAllAluminum),
excepting yours is 1"Top+BotChannels instead of my 2"Top+BotChannels.
The TransHeatExchanger connections in the left PlasticTank are each 2piece:
the Adaptor is 15mmDiam-1.00mmMetricThreadMale to 16mmDiam-1.50mmMetricThreadFemale (same as 5/8"Diam-18NPTpiFemale),
the TubingFlareFitting is 16mmDiam-1.50mmMetricThreadMale (same as 5/8"Diam~18NPTpiMale).
Considering the age\rust, I'd soak these with PenetratingOil before attempting to loosen.

These external Transmission ATF Tubes run
from the Transmission outlet to the RadiatorHeatExchanger inlet,
from the RadiatorHeatExchanger outlet to the TransCooler inlet in the front grille,
from the TransCooler outlet back to the Transmission inlet.
Assuming you've got a 4r44\5r55 series Transmission,
Believe there's an internal Tranny ThermaStat that opens this external loop when the Tranny reaches a certain Temp (150f?).
I added a 2nd TransCooler, identical to 1st, mirror image mounted behind grille, & piped\hosed up parallel with the 1st.

Click here for details on my Radiator swap\upgrade.

 

Would you recommend trying to find the same Trans cooler in a JY? And what did you use for the rubber hoses? Mine are very stiff and cracked at the end coming from factory trans cooler so I want to replace those to be safe.

 

Trans cooler lines have no "pressure"(under 12psi) but do have FLOW, so pressure rated rubber hose would be good to use, the heat and exposure to air is what kills rubber hose
You can buy "transmission cooler" rated rubber hose in various diameters

Any trans cooler with similar dimensions that can be mounted in same location will be fine to use
Most vehicles in JY were in front end accidents so......................
But certainly worth a look, similar trans cooler would be found on any Ford truck or van, its not Ranger specific
If you do get a used one flush it out well, don't want debris from previous transmission sent in to YOUR transmission

 

Used 3/8"ID Fuel Hose to replace the old ATF Hoses, then 2 WormClamps on all connections.
Used two 3/8"ID Barbed Stainless Tees to parallel hose up 1st & 2nd TransCoolers.
TransCooler is FordMotorCraft# 5L2z-7a095-a or Dorman# 918-220; find these in JYs\Ebay\Amazon from $30 to $100+.
Bought 2nd one $35 used off Ebay, then internal BrakeCleaner flush & external pick clean,
pressured up to 40psi & let set overnight for leak test, then lite coat of HiTempBlack on both TransCoolers.
Actually removed 1st original bracket that had partially broken loose,
rotated 90degrees then bolted edge to right cross frame; same for 2nd other side.
Hope this helps.