The International Cessna^® 170 Association

Makes sense to reason why the cracking took place. In my case the individual nut plates were attached to the skin whereas I believe there should be a nut plate strip at the forward skin lip on both sides below the top mount down to the lower end of the skin. If anyone with a B wants to confirm that or dispute it please feel free. My cracks eminated from an individual nut plate hole.
If any loads other than skin loads are imposed on the skin likely it will crack. So if your cowls fit poorly like Richard suggests then it's likely you'll get cracks somewhere at the attach points. Fatigue in the skin is also a possibility if your vibration devices have deteriorated.
As for skin replacement, depends on which skin and which model in terms of difficulty.

Gary
The rivets are a NAS1097AD. Aircraft Fasteners have some. You can go on their website and get tech info and dimensions. Used for countersinking in thinner material. I think that skin is .032 thickness. Check the rivet head height in a -4 diameter to make sure it is less. Talk it over with your AP/IA
Good luck
Mike Roe

Decided to replace the skin. Had fun and learned a lot.

That had to be good feeling and experience

What if anything was done to keep the problem from re-occurring? What about the motor mount to firewall bushings? Was gang channel used in place of single nutplates? That sounds like a good idea to help spread the apparent load on that bottom screw. My A Model has tinnerman nuts with PK screws instead of nutplates and machine screws.

hilltop170 wrote:What if anything was done to keep the problem from re-occurring? What about the motor mount to firewall bushings? Was gang channel used in place of single nutplates? That sounds like a good idea to help spread the apparent load on that bottom screw. My A Model has tinnerman nuts with PK screws instead of nutplates and machine screws.

It would help Richard. Especially if the screw holes in the skin(s) were enlarged a bit to allow the cowling to sort of "self adjust". The tinnermans would "self align" (slide) with the screws/screw holes in the cowling, which should remain original diameter. I have seen both individual nutplates, tinnermans, and gang channels at these locations. I think gang channels would cause more problems than individual tinnermans.
I like the "bumper" you have installed. Does it cause any noticeable vibration issues?
Another tip for cowling installation (and I'll bet most already know this), is to install all fasteners without tightening any of them. Once all are installed begin tightening the bottom cowl from top edge to bottom, top cowl from top center to bottom aft edge, and then the remainder from aft to front. An occasional "jiggle" of the cowl assy during the process helps. And nylon washers under the screw heads are also desirable

Anyone have a picture of these cracks or their location?

A couple of pictures of a doubler installation would also be appreciated. Nice job on that skin replacement!

What was the skin thickness did you use ? Reason why I am asking ,we have one in the shop at the moment that the lower skin has to be replaced due to cracking and some one drilling oversize holes for a cowl attachment. The cowl attachment is totally wrong . The cowl attachment is totally illegal. They have used tinnerman washers and self tappers screws for cowl attachment. YIKES!!! Its[ the skin] is only 25 thou thick with paint. I am thinking 32 or 40 thou and 2024 T3. This one is one real roughie. Cessna isn't that helpful on this one.

I think the cowl attachment described is quite common and may or may not cause a problem but could just as well cause cracking if not installed as Del mentioned. Having said that I think there's supposed to be a channel with nut plates for the lower cowls sides resting inside the skins replaced in this post.
I just ordered both sides for my plane (as I have been putting this off for awhile) due my plane has a doubler on one side and a stop drill on the other.

My original intent was to patch the area but my friend helping me did not agree. Skin thickness was .32 The original cracking seemed to start on the lip because there was nothing else but the skin. After installing the skin we flush riveted a piece of thin steel to the lip where the cowling attaches. I reinstalled the cowl with tinnermans as previously installed.

I pulled the nuts off the rear side of the engine mount bolts to inspect the rubber pieces. All of the rubber was in acceptable condition.

I asked my buddy who does a lot of work like this (also is an FAA DAR) what would stop it from cracking again. He beleives that the lip needed a doubler where the cowling attached and the fact that original skin was 60 years old before it cracked the first time. We plan on changing the other side when the weather warms.

Brad

Spot on stuff Brad.
In my case it cracked out a chunk, the crack originated from a nut-plate hole went into the fuselage about two inches, stopped and reversed course at a 45 Deg. angle to another nut plate hole. I cleaned the damage, put in a filler, placed a doubler on the outside to pick up the holes at the edge and put a doubler on the inside to support the filler.
Yes it looks ugly especially with my paint job which was a bit thin and runny but has done the job for the last two conventions.