The International Cessna^® 170 Association

Hello Everyone,

What is the correct fuel drain for the '48 original? I ordered the F391-72 according to the spruce catalog info, but it slightly too big.

Can someone clarify this? The drains in it are corroded and can't be read, but are Safe-Air flush drains.

From what I read, I needed the 7/16" like the SafeAir or the Curtis CAV-170. I guess what is needed is the 3/8", but why

would this be? Did the tank get changed out to something smaller at some point or was it just smaller in the '48 model?

My '48 has 1/8 NPT for all fuel drain valves, and I know of at least 3 other '48's that also have this thread.
This conflicts with the charts that some fuel drain valve suppliers use, which claim that the fuel tanks have 7/16-20 threads (which is true for the 170A and 170B I believe).

Also, on my airplane, I need to use the flush valves for the wing tanks, rather than the CAV-110, due to clearance with the door as it swings open. This may not be necessary on all '48's.

Howdy!

1/8" or 3/8" NPT?

sfarringer wrote: ↑Sun Feb 23, 2025 10:44 pm My '48 has 1/8 NPT for all fuel drain valves, and I know of at least 3 other '48's that also have this thread.
This conflicts with the charts that some fuel drain valve suppliers use, which claim that the fuel tanks have 7/16-20 threads (which is true for the 170A and 170B I believe).

Also, on my airplane, I need to use the flush valves for the wing tanks, rather than the CAV-110, due to clearance with the door as it swings open. This may not be necessary on all '48's.

1/8 NPT
The outside diameter of 1/8 inch pipe is about .405 inch.

1/8 NPT, like he said. Do you have a mechanic helping you?

No help, but I was able to get the number on the one that was in it.

F-391-32 is what came out of it. That's a 3/8"x24.

This is the spruce link to this particular brand of drain: https://www.aircraftspruce.com/catalog/appages/f391.php

Now I am wondering why mine is 3/8' and you all have 1/8"?

Any ideas?

cessnut wrote: ↑Tue Feb 25, 2025 2:51 am 1/8 NPT, like he said. Do you have a mechanic helping you?

howelldw wrote: ↑Tue Feb 25, 2025 3:00 am No help, but I was able to get the number on the one that was in it.

F-391-32 is what came out of it. That's a 3/8"x24.

This is the spruce link to this particular brand of drain: https://www.aircraftspruce.com/catalog/appages/f391.php

Now I am wondering why mine is 3/8' and you all have 1/8"?

Any ideas?

cessnut wrote: ↑Tue Feb 25, 2025 2:51 am 1/8 NPT, like he said. Do you have a mechanic helping you?

I’m not particularly knowledgeable about the ragwing fuel drains…but in answer to your question “why mine…(is different)..” The answer is:
…
These are 80-year old airplanes which have been maintained and altered by numerous people of unknown qualifications/capabilities using parts of equally-unkown pedigree…. and (hopefully only in times-past)…it was often considered by private owners that if they didn’t “Log” the alteration they might avoid legal troubles if they pretended ignorance.

There exists in the minds of some owners that “hot-rodding” their airplanes is no different than hot-rodding their grand-daddy’s ‘32 Chevy Roadster with parts from the Whitney’s catalog.

When some subsequent owner or technician or lawyer runs into this situation… it becomes “interesting.”

Having an “interesting” version of a certificated airplane … is Not necessarily like Martha Stewart's “Good Thing”.

(If having a unique fuel drain is your only problem you’re a very lucky person…. almost as lucky as I was when my airplane’s brass engine-mount bolts were discovered and replaced by a competent restoration.)

I speculate that some prior person didn't know better and ran a 3/8-24 tap in the hole to "clean-up" the threads.
But on a 77 year old airplane it's anyone's guess.

Thank you. That is a very good answer and likely.

sfarringer wrote: ↑Tue Feb 25, 2025 12:23 pm I speculate that some prior person didn't know better and ran a 3/8-24 tap in the hole to "clean-up" the threads.
But on a 77 year old airplane it's anyone's guess.

sfarringer wrote: ↑Tue Feb 25, 2025 12:23 pm I speculate that some prior person didn't know better and ran a 3/8-24 tap in the hole to "clean-up" the threads.
But on a 77 year old airplane it's anyone's guess.

Which means they likely did not follow “approved procedure” and also did not “record” or “log” it.

If you find this is the case, to dot the “i” and cross the “T”… a logbook entry taking note of it and recording it as a “minor alteration”.

Or don’t.

Yes sir! We'll get it right.

GAHorn wrote: ↑Tue Feb 25, 2025 10:12 pm

sfarringer wrote: ↑Tue Feb 25, 2025 12:23 pm I speculate that some prior person didn't know better and ran a 3/8-24 tap in the hole to "clean-up" the threads.
But on a 77 year old airplane it's anyone's guess.

Which means they likely did not follow “approved procedure” and also did not “record” or “log” it.

If you find this is the case, to dot the “i” and cross the “T”… a logbook entry taking note of it and recording it as a “minor alteration”.

Or don’t.