Hi all. I'm getting ready to put new tranny lines on from the transmission to the radiator. Do the fitting need any kind of sealant to ensure no leaks. If so what do you recommend for sealant Thanks.

Do not use anything on the flare nut fittings but for the NPT threads I use teflon tape or Thread sealant.

I’ve never needed anything.

No, transmission lines use flare fittings, as pointed out above. The threads do NOT do any of the sealing. So thread sealant is not needed.

Thanks guys .I appreciate that info.

One last thing since it may be your first time at transmission lines.

It is better to use 2 line wrenches (if possible) but at the very least use 2 wrenches when you tighten them. One on the fitting and one on the flare nut.

Great advice, I will do that. Thanks.

Everyone else's reply is technically correct here and is in total agreement with what I learned in trade school and in all my refrigeration textbooks.

I put it on all flare threads, its not really for the purpose of a sealant, but it stops the flare itself from coming loose due to temperature change and vibration. Not really sure of the physics behind it, but it makes the flare fitting retain its original tightness while in service. We tried loctite and leak lock on machinery back in the day, but it didn't work to stop the joints from loosening off and leaking. Telfon tape and nylog seem to be the only stuff that stops the self loosening of flare joints on refrigerant, oil, glycol etc.