GAS TANK SURPRISE

[k0al]

During a just completed annual, I addressed a maintenance issue that had bugged me and I found a surprise that I would like to share with others.

My 170A is fully polished and it irritated me to no end that a small amount of fuel seemed to seep out around the filler necks at each refueling and spoiled some of my polishing efforts. I searched various postings in the archives on this forum and decided to tackle the task of replacing the gaskets. I ordered the required gaskets (3 per side), replacement steel screws, and one of the oft mentioned Brown Tool Co. "knockers".

I would NEVER attempt to do this WITHOUT one of the Brown "knockers".

The SURPRISE was, once the wing panels covering the tanks were removed, I discovered that one of the retaining straps on EACH side of the aircraft had failed. This necessitated complete removal of the tanks (I should note that the fuel gauges gave to be removed to get enough clearance for removal) to allow re riveting of the 'owner produced' replacement straps. I should note that it was a different mode of failure on each side. While there were "shiny" spots where the loose straps had vibrated against the tanks and where the tanks had moved ever so slightly against the fore and aft locating blocks, thankfully, there was no appreciable amount of metal removed which would have required remedial repair of the tanks.

To be sure, it is a two person job to get the tanks back in place and even more so to reinstall the wing panel as there is very little tolerance when aligning the panel to the filler neck and the panel with respect to the 50 or so screws around the perimeter of the panel. I found that you can NOT just tighten the straps down and trust that the panel will align. It was a back and forth process of nudging the tank in one direction or another, tightening the straps some, rechecking and repeating until it was a "go".

I would also recommend running an 8-32 tap through all the nut plates BEFORE starting to reinstall the wing panel. I found that a few tapered wooden shims (lumber yard items) were crucial in getting the wing panels started back in place under the adjacent overlapping panels.

[wingnut]

The nutplates are designed to be self locking. It's okay to partially tap the nutplates to offer a good start for the screws, to avoid cross threading. If you run the tap all the way thru, you'll eliminate the manufactured crimp that serves to provide the locking feature of the nut plate. The screws attaching the tank top panels are structural and therefore require proper torque and the locking means to retain that torque since no lock washers are employed in this installation.
For an owner who may only occasionally remove these panels, your method of on/off a couple times to properly locate the tank for alignment is the best way. When you do it often, as in our shop, we have made a template from a good tank top which has sections cut away to allow access to the tank retaining straps so they can be secured while the tank is positioned. Then the locating templates are removed and tank tops installed. Make sure your tank retaining straps still have the rubber anti-chaff strips glued to them, and that the hat channels in the bottom of the tank bay, that the tank sits on, are still glued in place. When these rubber strips get some age (60 years) they tend to dry, get compressed and brittle and therefore strap tension is reduced, allowing the tank to shift somewhat. Since the tank is still fastened securely to the tank top at the filler neck, any shifting could cause some pressure/flexing to the neck with possible cracking after a few years (6 decades).

[Robert Eilers]

Uhmmm, 6 dicades. I have a 1952 C170B - now let's see 2008 minus 1952 equals........

[wingnut]

How does 5.6 dicades sound for yours Robert?