700R4 Transmission Cooler 1981-87

Pink fluid on the driveway under a square-body half ton, a cooler line that crumbled when you touched it with a wrench, or a hose-and-clamp patch someone spliced in years ago? This is a two-piece set of preformed transmission cooler lines for 1981-1987 Chevrolet C10 and GMC C1500 two-wheel-drive pickups with a gas engine and the 700R4. The set is made by Inline Tube from 5/16 in. OE-style steel tubing, bent on CNC benders to the factory pattern, with the correct coated fittings and the spring wrap where the original lines had it. It goes in like the lines that left the factory instead of a roll of tube you have to bend and flare yourself.

What the cooler lines do on a 700R4

The 700R4 builds a lot of heat in the torque converter, especially in a truck that tows, idles in traffic or runs an unlocked converter for long stretches. One line carries hot converter oil from the case up to the cooler inside the radiator tank; the other brings the cooled oil back, and that return flow feeds the lubrication circuit that oils the bushings, thrust washers and planetary gears. So the lines are part of the lube system, not just plumbing. A seeping fitting slowly drops the level until the pump starts sucking air, and a pinched or dented line cuts both cooling and lube flow at the same time. On a two-wheel-drive C-series truck the lines run back along the frame rail, right where road spray and salt sit, which is why 35 to 40 year old lines rust through.

Symptoms of failing cooler lines

• Fluid dripping from a line, a fitting at the case or the radiator, or a crusty wet spot along the frame
• Fluid level that drops between checks with no leak at the pan gasket or seals
• Slow or delayed engagement into Drive or Reverse once the level gets low
• Slipping and flare on the 3-4 shift when hot, a common 700R4 complaint that low fluid and overheating make worse
• Dark, burnt-smelling fluid from running hot after a line was kinked or partly blocked

How to confirm the lines are the problem

Clean the case fittings, the radiator fittings and the full length of both lines, then idle the truck in Park and look again with a light. Run a hand along the frame: deep pitting or scale that flakes off means the tube wall is thin even if it is not leaking yet. Check the level with the engine running and the fluid at operating temperature. If the level has been low long enough to cause slow engagement, read our 700R4 delayed engagement guide before assuming new lines will fix everything, because low fluid can also cost the pump its prime and glaze the clutches. Our 700R4 common problems guide covers the 3-4 clutch and TV cable issues that heat makes worse.

Fitment

This set is listed for 1981-1987 Chevrolet and GMC 2WD half-ton pickups (C10 / C1500) with a gas engine and the 700R4. It is gas engine only; diesel trucks use different routing. It is also a 2WD set, so it will not route correctly on a K10 or K1500 4x4. GM introduced the 700R4 for the 1982 model year, so on a 1981 truck this set only applies if the truck has been converted from a TH350 or TH400 to a 700R4. Aftermarket radiators, add-on coolers and engine swaps can all change where the lines end, so look at the fittings on your truck before you install. If you are not sure, send us the year, model and VIN and we will check it. You can find the rest of our parts for this transmission on the 700R4 parts page.

Install notes

• Set a drain pan under the case fittings and loosen the old nuts with a flare-nut wrench. Rusted nuts round off quickly with an open-end wrench.
• Lay the new lines next to the old ones before removing anything so you can see which piece is the feed and which is the return.
• Save the frame clips and move them to the new lines; a line left loose will vibrate against the frame and wear through.
• Start each fitting by hand before using a wrench, then snug it. Cross-threading the case connector or radiator fitting is the easiest way to turn a one-hour job into a bigger one.
• If the old fluid was dark or had debris in it, flush the radiator cooler so the junk does not go straight back into the transmission.
• Refill with Dexron ATF (a current Dexron-VI fluid covers the older Dexron specs), check the level hot in Park, and look at every fitting again after a drive.

Our 700R4 cooler line replacement guide walks through the job in more detail. If the truck shifts soft or late after running low, a 700R4 shift kit or a 1982-1986 700R4 valve body may be the next fix, and an 700R4 overhaul kit covers the seals and gaskets if the unit is coming out anyway.

Will these cooler lines fit my 4x4 K10 or K1500?

No. This set is the two-wheel-drive routing for C10 and C1500 trucks. 4x4 trucks route the lines differently around the transfer case and front axle, so they need a 4WD-specific set.

Do these fit a diesel C10?

No. The set is listed for gas engine trucks only. Diesel trucks use a different line routing to the radiator.

My truck is a 1981. Will these lines fit?

The 700R4 started in the 1982 model year, so a 1981 truck came with a TH350 or TH400. These lines apply to a 1981 only if the truck has been converted to a 700R4, and you should compare the fittings on the radiator end before installing.

What size and material are the lines?

They are 5/16 in. OE-style steel tubing in a two-piece set, preformed to the factory shape with the correct fittings and spring wrap where the original had it.

What fluid should I refill with after replacing the lines?

Use Dexron ATF. A current Dexron-VI fluid is backward compatible with the older Dexron specifications the 700R4 was filled with. Check the level hot, engine running, in Park.

Should I flush the radiator cooler when I replace the lines?

Yes, if the old fluid was dark, burnt or had debris in it. Otherwise the old contamination in the cooler washes straight back into the transmission through the new return line.

• 700R4 Cooler Line Replacement Guide
• 700R4 Delayed Engagement: Causes and Fix
• 700R4 Common Problems and Fixes
• 700R4 Pump Failure Symptoms and Replacement