Fuel Cap 431-9R ? Again!

[PatrickM]

Hi Everyone, First post.

I've gone through the fuel cap threads, and they were very helpful.  Looks like sourcing a replacement cap is my best choice, but this seems unique.
The cap has the R added post casting.  Does this mean the cap was modified post installation? Post production? On initial assembly?
Just noticed @OSUAV8TER has one in his pic....

 

 

Reason being is the center post has pitted to the point that it lets water by the O ring (don't look at the one in the pic, it was new a year ago)

The question being, can I grab an 431-9 and combine the caps to make a 431-9R ?
Does anyone have a clean post? (just checking)

My buddy is a metal whisperer, and i'd love to hand him the $$ to make a new post, but airplane.

Our 1977 M20C has the stainless mating surface for the cap, that looks clean.  

Anyone have an interesting 3D print file for covers?

Thanks,

Pat

 

[N201MKTurbo]

R is for Rust. It was a factory option...

[OSUAV8TER]

I think the R has to do with the way the locking mechanism rotates. It just rotates the other way.

[N201MKTurbo]

I’m amazed by my junk collection sometimes.

It is from a different model fuel cap, but it looks about right.

[N201MKTurbo]

[PatrickM]

30 minutes ago, N201MKTurbo said:

 

let me see if I can get a set of calipers on mine!

[DCarlton]

I'm installing fuel bladders now that use a different fuel cap.  I'll have 3-4 fuel caps to sell cheap on Mooneyspace soon.  A couple have the marsh washers and fluorosilicon o-rings.  I just need time to pull them all out on the work bench.  I have a few extra fluorosilicon o-ring kits too.  

[EricJ]

3 hours ago, PatrickM said:

My buddy is a metal whisperer, and i'd love to hand him the $$ to make a new post, but airplane.

You can do exactly that via Owner Produced Parts.   Coordinate with your A&P, but that's totally doable if you want to do that.

[PatrickM]

4 minutes ago, EricJ said:

You can do exactly that via Owner Produced Parts.   Coordinate with your A&P, but that's totally doable if you want to do that.

wow - I thought that was only for experimental!

 

1 hour ago, DCarlton said:

I'm installing fuel bladders now that use a different fuel cap.  I'll have 3-4 fuel caps to sell cheap on Mooneyspace soon.  A couple have the marsh washers and fluorosilicon o-rings.  I just need time to pull them all out on the work bench.  I have a few extra fluorosilicon o-ring kits too.  

 

Great - let me know! think that would be the route of least resistance.

[EricJ]

11 minutes ago, PatrickM said:

wow - I thought that was only for experimental!

For "vintage" airplanes like ours it is a handy process recognized by the FAA to produce a part for a specific airplane.   Can't make more and sell them, but you can make stuff for your own airplane.   It gets used here and elsewhere quite a bit for parts that are otherwise difficult to obtain.   There is a fair amount to know, but you can poke around online for details.   Here is a reasonable overview:

https://www.cessnaflyer.org/magazine/article-archives/maintenance-technical/owner-produced-parts-regulations-interpretations-and-applications.html

[PatrickM]

On 1/9/2025 at 8:55 AM, N201MKTurbo said:

 

The plane is going to be at the shop for another few weeks - and the shop is 60 miles away...
We had a lower seal blow on a newer mag, and the supplier is rebuilding it - was a bit out of warranty,
so I should find out who it is, and give them a shout out.

Does anyone have a 431-9 disassembled that can mic the length, diameter, and location of the o-ring groove?