De-Thawing Your Hot Rod

Here in the midwest, we are having our first appetizer of spring, which means it cracked 30 degrees and the sun was out for over an hour without interruption.
If you’re anything like me, you’re probably now thinking about the projects you wanted to do over the winter on your hotrod. Only life got in the way and you never got to a lot of them. So now the full court press is on to finish in a few weeks what we should have spent a few months doing…
Today I wanted to share something that is the most frequent subject of our tech calls. In fact, if you’ve spoken with Frank, you might have asked him this. Can anyone guess what it is?
– What type of coolant should I use? Nope, that’s not it. But the answer is a major brand (Delphi, Prestone, etc). 50/50 premix.
– “Frank, have you ever driven a Rolls Royce?” Nope, that’s not it either. Though he has and we recently built a custom radiator for a 64 Rolls Royce limo on an Art Morrison chassis with a LS3 motor in it.
– Why do women scrutinize hotrod budgets so closely? That’s a mystery of the universe, similar to if there was ever life on Mars.
The answer is: What type of transmission cooler fittings does my radiator need???
It’s a great question and today I want to cover the major types of transmission cooler fittings used in GM, Ford and Chrysler from 1940 – the mid 2000s.
The Curse Of Pipe Threads


Up until the mid 1960s, the most common type of thread used for transmission cooler lines was pipe threads. Pipe threads should be sealed with teflon tape and have a taper to them. They seal by distorting the thread and creating a seal. With aluminum, once you tighten a pipe thread to the point that it seals, it’s pretty much so done. Meaning if you remove the pipe fitting and try to get it to seal again, it’s going to be difficult it not impossible to seal again.
I remember when I was in my last year of engineering school and I was in the hydraulics lab working on a project. The professor, half joking, said “After you leave here and get jobs as engineers, I will have your degrees rescinded if I ever find any of you designing anything with pipe threads.” After 25 years of experience  later, I agree with him and I’m not half joking… Pipe threads are the nagging wide incarnated into a thread.
There is no reason to be using pipe threads if they can be avoided, they are problematic and the source of a lot of grief in the hot rodding world. That grief mainly manifests itself in 1) stripped threads and 2) fittings they are impossible to seal. Plus, in my opinion, it looks quite sloppy on an otherwise pristine build to find teflon tape on every fitting under the hood – it screams amateur hour.
The most commonly used pipe thread is the 1/4″ NPT in the automotive world for transmission cooler fittings. Occasionally on some Ford products like the early 64-66 Mustangs, you’ll find 1/8″ NPT fittings but this is quite rare.
Where are pipe threads used? They were used on most Ford and Chrysler vehicles well into the 1990s. GM got the wake up call much earlier and in the mid 1960s, started implementing an alternative to the pipe thread, the inverted flare.
Peace Out Pipe Thread, Meet The Inverted Flare
Just as hippies started singing about love and Timothy Leary was telling people to tune in and drop out, GM must have heard the part about love and decided to show hot rodders a lot more compassion. Around 1965, GM started rolling out the inverted flare fitting in brute force to replace the pipe thread. By 1968, essentially every GM vehicle that rolled off the assembly line had inverted flare fittings instead of pipe thread fittings. This was a good thing, a very good thing.
There are many advantages of inverted flare fittings over pipe threads. Inverted flare fittings use straight threads instead of tapered threads. They don’t seal on the threads, instead they seal on a flared seat. They are reusable and don’t require any teflon tape. This is because inverted flare fittings seal on the flare of the fitting and the seat inside the transmission cooler.
For transmission cooler lines, the common line size is 5/16″. Inverted flare fittings are sized based on the line size, not the actual thread size of the fitting. A 5/16″ inverted flare fitting is designed to be used with 5/16″ line, yet the thread size is 1/2-20 for 5/16″ inverted flare fittings.  This can be really confusing to people, and I get it. The important thing to remember here is when dealing with inverted flare fittings, you buy fittings based on line size.
For example, let’s say you have a 69 Chevelle and want to replace the hard steel transmission cooler lines with braided -6 lines. On the transmission cooler end, you would need a 5/16″ to AN-6 adapter fitting to go into the cooler to mate with your -6 AN hose end.
Where Problems Arise
The most common problem we find the customers have is not understanding which type of thread their transmission cooler has: pipe thread or inverted flare. This gets especially confusing for guys since 1/4″ pipe thread and 5/16″ inverted flare are almost the same size. The result: guys will try to ram a pipe thread fitting into an inverted flare transmission cooler and destroy the transmission cooler. In most cases, once this is done, there is no saving it and they’ll need to send the radiator back to us to have the cooler replaced. You don’t want to do this.
Avoiding Mixing Pipe Thread With Inverted Flares
The easiest way to tell the difference between an inverted flare transmission cooler and a pipe thread cooler is the seat. From the photo above, you can see that a pipe thread cooler has no seat the bottom and the port flows directly into the transmission cooler. From the further up photo of the inverted flare cooler, you can see it has a seat in the bottom where the fitting seals.
Bottom line:
Inverted flare trans cooler fittings have a seat at the bottom, pipe threads don’t. If you’re not sure, just shoot us an e-mail or give us a call.
What I’ve shared is a lot and in the next newsletter, I’ll dive into GM quick disconnect fittings and the difference between oil cooler fittings and transcooler fittings. Until then, enjoy finishing up your winter projects and reach out to us if you need any help figuring out how to cool your hot rod. You can reach Frank directly at (773)303-6291 or via e-mail at