The Cessna 120-140 Association Forum

I just got plane back from annual. Had the gasket changed for oil seeps that seemed to show up on the side of reservoir.
However now there is an obvious leak at the rear of the tank from the gasket, middle of flange right above the alternator.
This leak was not there before. I have looked for the proper torque for the 6 ( I think) bolts , I can’t find those on the usual searches. Any suggestions on a securing technique to get this leaking gasket to stop with proper torque values maybe?

This is pretty common. It's exacerbated by folks cranking down on the nuts in an attempt to stop the leak, which leads to a warped flange, and more leaking. To make things worse, the tanks are prone to cracking at the weld to the flange and it's hard to tell exactly where the oil is coming from with all the air swirling around under the cowl.

They are 1/4-28 nuts so the prescribed torque is 75-85 in-lbs (C-series overhaul manual, page 119), but that's assuming a fiber gasket. Those silicon-rubber gaskets are pretty compressible, so you're sort of on your own.

Johns correct and I'll ad that if the tank is removed and set on a flat surface you'll probably notice air or daylight where its been over torqued. This is between the stud holes.

In your experience is the fiber gasket a better solution. Part 3577. ? Since you have a known torquing value. If the flange is warped is it repairable? The big ol hammer method.?
Thanks.

redbud40 wrote: ↑Thu Sep 18, 2025 7:03 pm In your experience is the fiber gasket a better solution. Part 3577. ?

If you use the rubber gasket be sure to torque evenly and not so much as to distort the gasket.

If the flange is warped is it repairable? The big ol hammer method.?

Belt sander. If you use the hammer, watch for cracking!

I replaced the tank recently and used the silicone gasket. Basically I used the little warped locknut because the gasket will not tolerate anything approaching normal torque values. I want to say that the recommended value was insanely small, like 15 in-lb. So you need a nut that will grab the stud rather than any sort of lockwasher. And mark them all with orange goo so you can see if they back off. I basically tightened them all until the gasket was pooching out slightly all the way around from being compressed. It seems insanely loose in terms of nut torque, but it doesnt leak at all so far. .