fuel tank sag

[rydfly]

In the midst of my annual right now, one squawk we're addressing is a condition where the left-side fuel drain can just barely rub the top of the door sometimes as it swings past. After spending nearly all day removing the fuel tank cover (screws painted in place for the past 24 years), we were finally able to lay eyes on the tank itself this afternoon. It looks like there's some slack on the rear hold-down strap, while the front is fairly tight. There appear to be wood blocks with a cork layer that attempt to support the rear of the tank. I assume these aren't original as I can't find anything like them in the IPC. I've attached some pics of the wood blocks and the slack in the rear support strap.

So my questions to you knowledgeable folks... what are the likely culprits that would allow the tank to sag about 1/4" or so at the rear more than it should? Are the wood/cork blocks supposed to be there? How tight should the straps be and is there a common fix to this problem so we don't waste time reinventing the wheel on this?

Thanks,

-Kennet

[jlwild]

All the items are in parts catalogue. Just keep looking mine is at hangar or I Would
give you Figure number. For example, blocks of wood are to keep tank from moving fore and aft. But your tank hold down straps are very loose; plus there should be rubber strap under steel band to keep from chafing tank. See parts catalogue.

On your drain issue, it's not tank IMHO. check to make sure you have correct drain in tank. It takes a special drain for 170's. Correct drain is a SAF-AIR product CAV-170, 7/16-20 thread for Cessna 140A, 150, 170,170A, 170B, 172, & 175.

[rydfly]

I have the same drains on each side, but only one door makes contact. Plenty of clearance on the other side (and yes, door hinges and such all check ok). As it sits now, you can lift the rear of the tank about 1/4" before it gets tight on the rear strap. If it can be shimmed or otherwise adjusted by that amount, I should have enough door clearance again.

I'll try and post some pics of the drains I'm using specifically. I don't remember the part #, but they were both replaced 2 years ago, I think.

[jlwild]

For wing figure, see page 17, Fig. 8, items 8, 9, 13, 14, 15, 16 etc. If your fuel drain is correct part number, than another area to check is the neoprene pad, Fig. 8-15 & 16 that is located under tank. If it is missing or worn out, the tank will sit lower. Send me a PM with your email address and I will send you pictures of mine when I removed tanks.

[ghostflyer]

The clearance between the fuel tank drain valve isn't a set distance. this clearance can change when you change the wings angle of incidence with the concentric bush. I pulled out my tanks and found a heap of corrosion where the wood blocks had been glued on to the spar. there is now closed cell foam instead. Due to my head hitting the drain valves when accessing the cabin ,I changed to the flush fitting drain valves. The lid of the fuel tanks had some old rubber glued to it also. this was removed and some plasticine was placed on the tank and the ribs of the tank cover fitted to gauge the clearance. Closed cell foam rubber was glued to the ribs of the tank cover to gently hold the tank in position.

[rydfly]

For wing figure, see page 17, Fig. 8, items 8, 9, 13, 14, 15, 16 etc. If your fuel drain is correct part number, than another area to check is the neoprene pad, Fig. 8-15 & 16 that is located under tank. If it is missing or worn out, the tank will sit lower. Send me a PM with your email address and I will send you pictures of mine when I removed tanks.

PM sent. Thanks!

[rydfly]

SAF-AIR CAV-170 is what I currently have installed. Are those the correct drains?

[Bruce Fenstermacher]

SAF-AIR CAV-170 is what I currently have installed. Are those the correct drains?

Yes they are correct. As you suspect the tank has saged in my opinion. That and maybe that wing eccentric bushing is adjusted so the wing is lower than the other side.

I would use appropriate material as mentioned by ghostflyer to shim the tank for valve clearance.

BTW this is not an uncommon problem. Many 170 doors, including mine, have a slot filed in the door edge to clear the valve. Shimming the tank properly is a nicer fix.

[daedaluscan]

You certainly see more of the tank boss visible in that photo than I do on my tanks.
When I bought my plane it had (reportedly new) flush drains. When i took them apart I saw that a) they would not come apart and b) they had a steel seat which of course rusted. They constantly dripped and I changed to the CAV 170 (which is brass). The pilot side sometimes caught the door, and soon that drain valve was dripping also. I put in a new one and and I relieved a bit of the door with a file - it already had a small relief here - I just deepened it. I hesitated to do this, but nothing is that straight around my pilots side door.

[rydfly]

So what's the best option to replace the neoprene pads with? (Genuine Cessna part no. - if available still? some bulk neoprene stock from Spruce or some other supplier? a different material completely?)