forward Fuse Skin Thickness?

[jon s blocker]

George,
Do you know if these reinforcements are still available? If so I would like to get some and put them on my cowl. I tried without success to run the part numbers, but didn't get any favorable responses. I would be interested to see if any others on the site have them on their plane, and if they have helped. Thank you, Jon

[GAHorn]

I'm not aware of any source for them, but just looking they should be easy to make out of 2024 aluminum.

[c170b53]

Just pumping some life back into this thread, it might be helpful to look at your lower cowl install and verify whether there’s a nut strip installed on the fuselage skin. The nut strip is depicted by item 21. In many cases the nutstrip has been discarded and individual nutplates have been attached directly to the skin. I strongly recommend that a doubler strip be fabricated with indivdual nutplates attached to it much like as Del has described to strengthen the attach points and to prevent cracks forming in the fuselage skin.

[DaveF]

Thanks for bringing this back, Jim. There's some good info in here!

[c170b53]

Any time Dave, I think the collective group (as you have thks) over time have added to old posts for the benefit of all. Its no surprise that an old problem resurfaces for owners or new owners might benefit from a rehash of intel and an older guy is finding old info suddenly new until their memory is stirred (this guy).
I’ll just add I’ve had this problem myself and ended up changing the skin (I posted that as well I think). I have also had the other side crack, which I stop drilled with good success. I did recommended not to stop drill but I’ll correct myself here in that every crack has to be assessed for what it is and where it is. I replaced the skin because there were two cracks that merged. I might add cracks develop at the bottom section of the skin often just above the lower engine mount. Often a crack will likely start at a 3/32 rivet retaining the individual nut plate or the screw hole cracks to that rivet then from the rivet hole, the crack propagates aft into the fuselage skin.
Solution is to reinforce the skin with a doubler, with the individual nut plates on it.

[IA DPE]

I saw someone post in another recent thread to replace the nutstrip with anchor nuts and machine screws. Thoughts?

[c170b53]

See the 3Rd post in this thread. In my opinion Del was spot on. I used .040 material to make my strip. Consider the strip sacrificial, its easily replaced as are the individual TINNERMAN anchor nut plates (NAS444) when they break (which is often). If the plates are directly attached to the skin , after several replacements the holes are likely to get sloppy, edge distances are encroached and bad things start to happen.

[n2582d]

Jim,
What are your thoughts of using AN525-832 screws and floating 8-32 nutplates on an .040" doubler in place of the tinnerman NAS444 plates? Sheet metal screws have always struck me as being a bit rinky-dink for cowling screws. Another alternative I've considered is to use Skybolt's C172C800P conversion kit of 1/4 turn fasteners attached to doublers. This kit for the early C-172 does not appear to be STC'ed (although a rep at Skybolt said it was). Their STC SA3285SO is for the 1967 and later C-172s. It doesn't include the earlier C-172s which have a cowl attach style very similar to the C-170. At over $440 this conversion kit is probably overkill.

[c170b53]

Gary I think you’re right on the money to keep it simple and cost effective. For myself I do the simple, simple nut plates and “B” sheet metal screws. There’s nothing wrong with machine screws and floating nut plates (possibly lighter.) Only thing is as they are on the vertical, when they wear, the nut tends to sink and you have to hunt for it. But then you just replace the strip again, all done in under an hour (if its any norm hangar with the usual visitors disrupting work after coffee breaks).

[GAHorn]

Although not in this particular location, I’ve found these Monadnock Clip-nuts to be more durable than tinnermans:
https://www.aircraftspruce.com/catalog/ ... -00153.php

[DaveF]

My cowl is attached with AN526-1032 screws and floating nutplates attached directly to the fuselage skin, without a sacrificial strip. A former owner did the change. I use nylon washers on the screws.

Until I read this thread, I never worried about stressing and cracking the fuselage skin. Thanks a lot!


Edit: Nutplates are attached to the fuselage skin, not the cowl.

[c170b53]

Monadnock Clip-nuts

Well, these have been abandoned in commercial, even after a nylon insert was added to prevent the steel clip from fretting on aluminium surfaces. If you can make them work, great, they are very easy to use and install well on flat surfaces (a bit hard to find on a Cessna). They also require a hole that would agree with the clips throat dimension .
I would not use them on floor access holes to provide cover retainment, as they won’t sit flat and will eventually tear the skin’s flange around the hole

[c170b53]

Dave, I don’t like the wrinkles that stress and age has given me either.

[GAHorn]

Monadnock Clip-nuts

Well, these have been abandoned in commercial, even after a nylon insert was added to prevent the steel clip from fretting on aluminium surfaces. If you can make them work, great, they are very easy to use and install well on flat surfaces (a bit hard to find on a Cessna). They also require a hole that would agree with the clips throat dimension .
I would not use them on floor access holes to provide cover retainment, as they won’t sit flat and will eventually tear the skin’s flange around the hole

They are what are called for on my 172C and used on the lower cowl (on a doubler) . The Cessna PN is S4213-18 (but less expensive via the Spruce link above.)

[c170b53]

like i say if the hole is designed for them (and you have the right part), they maybe ok but I’m not a fan. I dislike hunting for the nut and sooner or later the screw will cross thread.