Q: What thread size are the cooler ports on a TH350, TH400, 700R4 or 4L60?
A: The case ports are 1/4"-18 NPSM — a straight thread that seals against a washer, not a tapered NPT thread. This trips up a lot of builders because NPT is visually similar and will start to thread in, but it wedges as it tightens and can crack an aluminum case. NPSM torques to about 25 ft-lb against its sealing washer; if you ever have to use NPT, it goes to roughly 15 ft-lb with sealant and no more. Start every fitting by hand, and stop the moment it feels tight. If you need the correct converters, the 6AN male flare to 1/4"-18 NPSM adapters are made for exactly these ports. Note that some pre-1976 TH400s came with a 5/16" inverted flare adapter already threaded into the case — remove it and you will find the NPSM port underneath.
Q: Is 6AN large enough, or should I run 8AN transmission oil cooler lines?
A: For a street or strip TH350, TH400, 700R4 or 4L60, 6AN is the standard choice and what most transmission builders specify. The factory hard line on these transmissions is 3/8" OD, which corresponds directly to -6AN, and this hose carries a 0.31" inside diameter — equal to or slightly larger than the tube it replaces, with no restriction at the crimps.
Builders running very large stall converters, heavy sustained loads, or long line runs to a rear-mounted cooler sometimes step up to 8AN and report modest temperature gains. The trade-off is fluid volume and warm-up time: an oversized loop on a mild street car can keep fluid below its ideal operating range. Unless your build is unusual, 6AN is the size the rest of the system is designed around.
Q: Is 7FT of transmission oil cooler hose enough for my build?
A: For a typical front-mounted cooler on a full-size GM chassis, 7FT per run covers the trip from the case to a cooler ahead of the radiator with room to route around the crossmember and headers. Measure first: run a piece of string along the exact path you intend to follow, add roughly 10% for routing slack, and remember the 3.5" minimum bend radius means you cannot pull a tight corner — every direction change eats length.
Rear-mounted coolers, chassis with long wheelbases, or setups that route through the radiator first will need more. Full lengths and fitting combinations are available across the transmission oil cooler hose kits.
Q: Do I assemble the hose ends myself, and what tools do I need?
A: Yes. This is a cut-to-length kit, which is what makes it work across so many chassis and cooler positions. Wrap masking tape tightly around the cut point so the stainless braid cannot flare out, then cut square with PTFE hose shears or a fine-tooth blade rather than an abrasive wheel, which leaves grit inside the liner. Blow the hose clean, slide the socket on first, then seat the olive onto the PTFE liner without trapping any braid strands underneath. Use soft vice jaws and an aluminum AN wrench to avoid marking the fittings, and pressure-check each assembly before it goes on the car. Budget an hour for your first two ends and considerably less after that.
Q: Should I keep the radiator cooler in the loop or bypass it?
A: Keep it, in most cases. The radiator cooler is more effective than most people assume, and it also brings fluid up to temperature quickly on cold starts — something an air-to-oil cooler alone cannot do. The standard routing is transmission out, through the radiator cooler, then through the auxiliary cooler, then back to the transmission. Full bypass makes sense on a race car, a heavy tow rig, or anything living in extreme heat where you want maximum cooling and warm-up simply isn't a concern. If you do bypass, size the standalone core generously, since it is now handling the entire heat load on its own.
Q: How do I connect these lines to the radiator cooler or an auxiliary cooler?
A: The radiator side on most GM applications uses a 5/8"-18 inverted flare port, so you will need a 6AN to 5/8"-18 inverted flare adapter there. Auxiliary cores vary widely: stacked-plate coolers commonly use 1/2" NPT, 3/8" NPT, or come with -6AN ports built in, while some tube-and-fin units use barbed inlets that are not suited to AN plumbing at all. Identify the port style on your specific cooler before ordering adapters — the quickest tell is whether the threads are tapered (NPT), straight with a sealing surface (NPSM), or a flare cone. Matching cooler and fitting options in ransmission cooler.
Q: Why choose PTFE over rubber or push-lock transmission oil cooler hose?
A: Rubber and push-lock hose absorb ATF over time, then swell, harden, and eventually weep at the barb or clamp — the same aging that kills factory lines. PTFE is chemically inert: it does not absorb fluid, does not swell, and does not go brittle from years of heat cycling. It also does not expand under pressure, so the line stays dimensionally stable. In this hose the PTFE liner handles the fluid, the 304 stainless braid carries the pressure, and the braided nylon outer cover takes the abrasion where the line passes near headers, the crossmember, or the frame. Three layers, one job each.
Q: Will this transmission oil cooler hose handle track or towing heat?
A: Fluid leaving the transmission runs meaningfully hotter than what a pan-mounted gauge reports, and generic hose rated only to around 250°F is marginal in that environment — it hardens first, then weeps at the fitting. This hose is rated from -76°F to 446°F, with a PTFE liner that neither softens at temperature nor cracks in winter storage. The stainless braid holds its shape when hot rather than ballooning, and the nylon cover shields the line from radiant heat where it passes near headers. For any build where fluid temperature is the limiting factor, pairing the lines with a properly sized core from the cooler kit matters as much as the plumbing itself.
Q: Can this hose be used for engine oil, power steering, or fuel?
A: The PTFE liner is compatible with transmission fluid, engine oil, and hydraulic power steering fluid, and the material also resists E85, race fuel, and diesel. That said, this kit is configured and sold for transmission cooler line work.
Fuel systems carry additional requirements around static dissipation, filtration, and routing that fall outside the scope of a cooler line kit, so use a purpose-built fuel line kit with a conductive-liner hose for that job.
For engine oil coolers and power steering coolers, the same 6AN hose and fitting approach transfers directly.
