It's Nuts!

[n2582d]

Not to beat a dead horse here, but according to my understanding an MS21044N nut is not a replacement for an AN 365-720C. It is a replacement for an AN 365-720. The bottom line is that the NTSB missed the fact that Cessna calls for an all metal locknut. Any Nylon nut, corrosion resistant or otherwise, would not be an approved substitute. Textron also gets this wrong since they superseded both AN 365-720 and AN 365-720C to an MS21044N.

This post regards the nut at the inboard gear leg attachment as shown here.

Gear Attach Nut.png

Here is a table outlining the bolts and nuts used on various Cessna taildraggers:

Screen Shot 2025-02-13 at 11.25.23 AM.png

Here is a screenshot from the Military Standard Spec sheet for the MS20365 nut which is identical to, and interchangeable with, the AN365 nut.

Screen Shot 2025-02-13 at 11.15.00 AM.png

I've always equated the ubiquitous AN365 nut to be one with the nylon insert as shown here. This is not necessarily true; if the AN365 nut has a "C" suffix it doesn't denote that it is a stainless nut with a fiber or nylon insert. It means that it is all metal, i.e. there is no nylon or fiber insert.

an365.jpg

But, if the spec sheet is correct, without the "A" or the "C" suffix it could be either one. So, looking at the table, did Cessna use all steel nuts or nuts with fiber inserts on the C-170 and C-17OA? Looking at the numerical index in the IPC's of all three 170 models there is no "A" on any AN365 nut listed. My guess (without any proof) is that there must have been a different standard when Cessna listed these nuts. It seems to me that Cessna equates AN365-XXX nuts without any suffix to be the same as AN365-XXXA.

In their Q&A section of this nut it seems to me that Aircraft Spruce has got its facts wrong as they write

Q: I would like to know the difference between: AN365C-1032 and AN365-1032C
These are both the same as the "C" represents that they are stainless steel.

and again

Q: What is the difference between a AN365-428C and an AN365-428A?
The C designates stainless steel. The A will be cad plated steel.

[sfarringer]

"A" suffix is not a steel nut with non-metallic insert, it is a copper-base alloy with non-metallic insert.

[n2582d]

No, in that example the "B" (for brass) in place of the dash designates it’s a “copper-base-alloy” nut, the “a” at the end shows it has a nonmetallic insert . I've edited my entry to add the source document for that screenshot.

[sfarringer]

Sorry for my brief confusion!

[n2582d]

No apology necessary. In studying this, I've almost concluded that the "C" stands for confusing or maybe crazy, thus the thread title. In actuality, when the "C" indicates the nut metal, I'm guessing it stands for "CRES" which is an acronym for Corrosion REsistant Steel, another word for stainless steel.

[cessnut]

Yes, this is confusing. However, an AN365-720C is and was equivalent to an MS20365-720C which is a cad plated all metal locknut. When the "C" is at the end it apparently has a different meaning than when it is in the middle(AN4C-4 for example). Several prominent companies have this confused, including Textron like I mentioned. For example, the Caravan parts manual used to call for an AN365-720C on the engine mount. Now all new EX 208s come with nylon locknuts on the engine mount ring, which goes against everything we were taughts as AP/IAs.

[GAHorn]

…. For example, the Caravan parts manual used to call for an AN365-720C on the engine mount. Now all new EX 208s come with nylon locknuts on the engine mount ring, which goes against everything we were taughts as AP/IAs.

Slightly-off-topic…but just another example of incorrect hardware… when my B-model was restored…. it was found that the engine mounts were attached to the firewall using BRASS self-clenching nuts.

(unknown if it left El Salvador that way…or if it was done for the recertification/Airworthiness-inspection which FAA approved.)