The Cessna 120-140 Association Forum

The threads are stripped for my wing sump. A&P is telling me I need to cut the wing, remove the tank for repair, then replace and patch the fabric. For some reason, the sump is smaller than the SAF-AIR called out (CAV-160). Can the tank drain be tapped to fit the CAV-160? Can Loctite Aviation Gasket Sealant be used?
Thanks in advance.

As I understand, some tanks outlets had coarse threads, some had fine threads. My plane had SafeAir valves when I got it and one side leaked because the threads were damaged. I cleaned the threads with a tap and changed over to Curtis valves which have a longer thread area and therefore more sealing area. I also used some fuel lube on the threads. It's been dry ever since.

It's my recollection some tanks have pipe (tapered) threads and others have straight threads. the difference was early or late manufacture. I forget which was which, but I'm thinking early was tapered. My plane has one of each...

I just finished this exact repair on my friends 140 last week. Used a 7/16 HELI-COIL & the Curtis straight thread. Stopped the insert when 1/32 recessed. Also take a file to the insert to remove the coils sharp trailing end before screwing into place. The straight Curtis valve uses an O ring to affect a seal. Go slow with the drilling (work your way up in drill size). I put grease on the drill bits & ran a vacuum sweeper when re-sizing the hole, then used a Q-tip to carefully clean out any debris. Worked great, just take your time. A specific drill size, tap & breakoff tool are needed and should be included in the Heli-coil "kit". Your mechanic probably has on of these kits laying around his shop, if not they easy to find. good luck Jim S

Thanks for all the advice!
I might be able to get away with cleaning the threads with a tap and using a Curtis valve, but I will need to figure out how long a valve to use. The SAF-AIR CAV-170 stem with threads look to be about 5/16" long and there is definitely a metal stop preventing the sump from going too far in the tank. The SAF-AIR CAV-160's stem is about a 1/16" longer, but is also about 1/16" thicker in diameter and a tapered thread.
The CAV-160 is what Spruce calls out for the C140 and the smaller CAV-170 just spins. But itlooks like there is enough metal to drill and Tap the sump hole with a 1/4" NPT Tap. Maybe I can get away without using a HELI-COIL. Any thoughts?
I'd also like to use Loctite 567 on the threads.

Thanks for the grease on the drill, go slow, and Q-tip tips!

mmoyer777 wrote: ↑Thu Jul 14, 2022 5:38 pmI'd also like to use Loctite 567 on the threads.

Why? Use "Fuel-lub" on the tapered threads. Tapered threads don't require any locking device. If you're using a straight thread, the o-ring provides the seal and it should be safetied to a hole in the aluminum skin under the fabric surrounding the hole for the drain.

Drill size for a 1/4" NTP tap is 7/16", so a stripped out 7/16" thread should be just about perfect.

Thank you again for the reply!
I know SAF-AIR recommends using Sealube and I have Parker - OLUBE884-2 - 884 O-Lube O-Ring Lubricant. But there is a study by Oasis; "Best Practice for NPT Thread Sealing" (https://www.oasisngv.com/vdb/document/1179) that recommends Loctite 567.
Also, there were no safeties on either wing nor are there holes in the CAV-160 (w/NPT) for safeties. I didn't think about running safeties to the aluminum skin. But I still have the issue of no hole in the CAV-160 for a safetie.

I'm new to aircraft ownership and comments are greatly appreciated!

You only have to safetie the ones with straight threads.