Corrosion of the Rear Spar Carry Through

[blueldr]

I made a comment to one of the locals the other day that the rear spar carry through in my C-170B had rarther early in its life been replaced for "corrosion".
When I was asked how the corrosion was detected, I had to admit that I had no idea since the repair was done long before I had acquired the airplane. That part is an upside down hat section and any moisture that gets in there is going to be difficult to get dried out.
Has anyone else ever had this problem?
I do not remember ever really inspecting in there other than when I had the wings off for a wing change. I would think the same might be true for the Front Spar carry through also.

[GAHorn]

The "hat section" is sealed by the upper cabin skin and the only way to look in there is to remove the wings and look through or drill off the upper skin.
Mine was cleaned out by using a bristle brush and scotch-brite when it was restored as the upper skin was completely replaced.
If you remove the wings, pull out the blocks (two more long AN3 bolts for/aft thru the blocks) you can scrub it out using a long bristle brush or 27mm cannon cleaning kit.
Get a toilet "loop" brush and straighten it out and you can do pretty well with it, and can even get more aggressive if you use abrasive cloth wrapped around the brush. You can treat it by tying a rag dipped in chromate or primer and run thru it and let it dry....then put it all back together.

[bagarre]

Last year when I pulled the headliner, I got a long straw and emptied a can of ACF50 into the hat section thru every hole I could find. (seat belt mounts, visor rivnuts, corners where I could barely fit the little straw...

That will at least stop any further corrosion. Did the same with the attach blocks.
It's not the perfect solution and wont cover everything but, it's better than nothing.

[DaveF]

Good question, BL. A while back there was a thread here with a story about a guy who bought a 170 that needed a new forward spar. I'm sure we all said, "I'm glad I'm not THAT guy!", but I wondered, how do I know I'm not that guy ... ?

[blueldr]

I saw pistures of a badly corroded part removed from a C-172 where someone had installed shoulder harness mounts and had cut what looked like half inch round installation holes in the top skin for access. To close up the holes, they had used "Push Plugs" that leaked water into the hat section. It didn't say how the corrosion was detected.
It did show pictures of the push plugs and they were apparently steel and were rusty.

[correamoon]

I was searching for this topic as my mechanic was installing a set of BAS inertial harness, on the passenger side the nut on the spar failed, when he checked with a boroscope through the hole he found corrosion, looks like I am lucky and is not deep and we are going to fill it with ACF-50 my airplane has been always in California and in hangars so I was a little bit surprised about this, so if possible guys remove the headliner and try to take a look there....

[melcessna]

I am in my second restoration. I believe there is really one solution. I have taken the top skin off the cabin and then got at the corrosion in both spars. I then epoxy primed and coated the primer with Dinol in both spars. This is a lot of work, but the model is old and if you are worried, I recommend this approach.

[bagarre]

How much work is it to pull off and replace the top skin of the roof?
Any tricks to bucking any of the rivets?

[c170b53]

It's probably the easiest skin to fabricate / replace on the plane (wings removed). You'll need to remove the front spar cap to get at the spar channel. Likely you will make a new spar cap, again pretty easy but you need to find the material.
It's a classic repair in that if you're removing the spar cap, the top front window retainer will be removed as well......then heck, might as well change the front windscreen and then under the panel......

[bagarre]

It's probably the easiest skin to fabricate / replace on the plane (wings removed). You'll need to remove the front spar cap to get at the spar channel. Likely you will make a new spar cap, again pretty easy but you need to find the material.
It's a classic repair in that if you're removing the spar cap, the top front window retainer will be removed as well......then heck, might as well change the front windscreen and then under the panel......

In my case, the wings and windscreen are already off and the roof has some hail damage. So if it's no worse than a belly skin (which I'm already in for) I might as well add it to the list along with a baggage door.

It's gonna be hard to keep this project under three years

[melcessna]

I am restoring a 48, so the oldest and subject to more corrosion, and did not have to fabricate/order any new parts. Drilling just the top off the rivet and then removing the remainder of the rivet with good metalworking practices will save all your skins. You should be able to remove any corrosion on the top skin and cap by using a scotch brite wheel. The rear spar you'll have to do by hand and the front spar you'll be able to wheel. Be careful to not comprise existing rivets inside the spar through too much abrasion. The only row of rivets that will be problematic will be the front row above the tack strip for headliner against the windshield. I took the windshield out. I suppose you could place several layers of tape against the windshield and also on the bucking bar in case you slip off the rivet and maybe that will protect marring the windscreen.

So, you'll need to remove the wings, preferably the windscreen, the top skin, the front spar cap, and the blocks that anchor the wing attach points in the spars. The stiffners for the headliner attachment also need to be removed to access any corrosion on the top skin and also to remove the top skin. You can't remove the top skin with these stiffners in place. The fuel vent should also be removed and blasted at low pressure so as not to bend the steel. It is held on by only six rivets and you'll have rust and corrosion under it from years of fuel overflow and humidity. Prime the skin and re-rivet and seal the base of the vent. As to the bucking bar, the front row will require a narrow bar (1/4" face) to fit. You should be able to purchase easily. This project is probably 200 rivets to drill out and replace, but worth it in my opinion based on the issue. Hope you find this helpful. Take care.

[bagarre]

Finally getting back into this and almost ready to pull the roof.

Is the roof skin square and flat on all sides or should the left and right sides have cutouts?
On mine (photos) the left and right sides have cut outs in the skin but I can't find another 170 roof skin that looks like this.
The cut outs are not perfectly smooth which leads me to believe it's not original. No log book entries to say either way.

Not sure how to proceed. Straight sides or cut outs?

IMG_3351.jpg

IMG_3352.jpg

IMG_3353.jpg

[melcessna]

I have never seen a 170 cabin top like you have pictured. I have done two, and both had right angles on all four corners. Not sure why yours looks the way it does. If you look in your parts manual for the year of your model you'll have a picture which will verify what your's should like. I am not comfortable telling you until you verify it in the parts manual. If you don't have a parts manual, I am sure there is probably a PDF copy on the internet. Both times I took the cabin top off I was happy with the access to the carry through spars. I was able to clean, epoxy prime, and seal with Dinol. Hope this helps. Take care and Merry Christmas. Mike

[bagarre]

I have a copy of the parts catalog but I don't 100% trust the illustrations in it as flaws have been found over time.

The parts manual shows all 4 sides square and flat, as you describe and as I've seen all other roof skins.

I'll do more digging for sanity's sake but I suspect mine is a deviation.

[c170b53]

Sorry David what year? I replaced my roof and it looked just like yours, mines a 53. On another post I found my belly skin aft of the gearbox was made in two pieces. Found out; some planes have two piece belly skins some one piece. So looking at just one or two planes may be misleading and the IPC sometimes tells the truth sometimes also misleads.
It's an interesting question, the shape of the roof, as I think there's two rivet lines to consider. The spar rivet lines extend past the rib rivet lines. If it's a square panel then there's a large overhang along the ribs from the fore and aft rivets attaching the ribs.