C5 Transmission Cooler-Line fittings

[55blacktie]

FYI: Contrary to what you might find on the net, C5 cooler-line fittings at the transmission are not 1/4 NPSM (straight thread). The correct fitting is 1/4 NPT (tapered thread).

[paul2748]

I thought most trans cooler line were inverted flare?

Originally Posted by 55blacktie

FYI: Contrary to what you might find on the net, C5 cooler-line fittings at the transmission are not 1/4 NPSM (straight thread). The correct fitting is 1/4 NPT (tapered thread).

[55blacktie]

I'm using -6AN to 1/4 NPT fittings for braided-steel PTFE hose, not hard lines. The transmission is replacing the original Fordomatic. Whether inverted flair/not, requires 18 tapered threads per inch, not straight threads.

[5851a]

I believe the C5 and AOD is different than C4/C6. Why the earlier transmissions with the NPSM fitting had the place for Oring and a tapered seat in the case, and the fitting was inverted flare. The 90-degree fittings had the lock nuts. Glad you have it sorted out.

[55blacktie]

Thanks, 5851a. You are right, as far as C4/C6 not being the same as C5/AOD. At this point, I don't even know if C5/AOD are the same. I thought I had it licked, with the NPT catching more threads, but not bottoming out. Although there's not much info. out there, more people are saying that straight thread, not tapered thread, is correct. Considering I'm trying to install aluminum fittings into an aluminum case,I haven't attempted to use any tools to tighten, only my hand, and careful at that. I considered putting a socket on the fitting and then tightening by hand, but fear that the socket could damage the aluminum fitting. I guess I should consider adding a -6an aluminum wrench to my toolbox.

[5851a]

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NPSM I thought was used originally because NPT could split the boss on the case. What did your original case fittings look like?

[55blacktie]

The transmission is a rebuildable core that did not include the fittings.

[paul2748]

The C5 and AOD are completely different tran's. C5 is an updated C4 and no O/D


Are you using AN fittings? Those are the only ones I know that are aluminum. You should be able to use regular wrenches on them. I haven't use a lot of AN fittings, but the ones I did I used regular wrenches on them.


As I can recall, the 1980 C4 that I used had brass fittings that screwed into the trans. Same on the C4 I used in my other car. Possibly, the fittings are straight thread with an inverted flare for the lines.

Originally Posted by 55blacktie

Thanks, 5851a. You are right, as far as C4/C6 not being the same as C5/AOD. At this point, I don't even know if C5/AOD are the same. I thought I had it licked, with the NPT catching more threads, but not bottoming out. Although there's not much info. out there, more people are saying that straight thread, not tapered thread, is correct. Considering I'm trying to install aluminum fittings into an aluminum case,I haven't attempted to use any tools to tighten, only my hand, and careful at that. I considered putting a socket on the fitting and then tightening by hand, but fear that the socket could damage the aluminum fitting. I guess I should consider adding a -6an aluminum wrench to my toolbox.

[55blacktie]

The C5 transmission-cooler ports definitely are 1/4 NPT. The port threads are shallow, so that the fitting does not extend far enough into the port to block the fluid passage in the port wall of the forward port. The fluid passage in the rear port is located in the center of the bottom of the port.

I had a spare 1/4 NPT brass fitting that screwed in effortlessly. I also looked up an aftermarket fitting specific to 1985 Thunderbird/w 3.8 V6 and C5 transmission; it's 1/4 NPT.

Although the C5, which is just an updated C4/w lockup converter, has little in common with the AOD, it's possible that they do use the same fitting, which is larger than the fittings used in C4s.

By the way, someone I know just bought a 1975 C4 transmission. He measured the transmission-cooler fittings, and they were 10mm-1.0.