hidden corrosion on the spar

[ghostflyer]

Have removed a fuel tank due to old age,cracking near the rear edge welds and internal corrosion. well I am up for a new tank but this is NOT the big problem. I found 2 little wooden blocks screwed to the main spar and to the rear spar in the fuel tank cavity .They centralize the tank in its position. My little wooden blocks have been painted silver and look like a couple of aluminium blocks. They have been screwed to the spars with 2 self tapering mild steel screws on each block. I removed each block and found deep corrosion where the blocks were attached[contacted] to the spars. It looks like they have been there since new [1949] A repair will have to be done to the rear spar as too much metal[corrosion] had to be removed. The wood was absorbing the moisture and the mild steel screw and a corrosion cell was formed. So when you do a detailed inspection with your tanks out look at these wooden blocks. We have just looked at a Cessna 172 A and they have similar tanks and they also have these wooden blocks. It seems its a Cessna part. In the Cessna 170B IPC page 16 fig 8 index no.36 p/n 0523524 this is the wooden spacer.

[bagarre]

Wow, I'm surprised this hasn't come up before. Being a block of wood I would think this would be a common problem.

Is it possible to inspect this area with a bore scope or other means (other than removing the tank)?

[ghostflyer]

it can't be inspected without removing the tank,the block has to be removed also to see what's under it. I have examined the wings in detailed on ouryearly inspections and never thought of removing this block.everything looked great until the block was removed. there is nothing written from Cessna about problems. the 172 thatis in for a check has some corrosion under this block.