ok, here is my problem-I just put in a new 16 gallon gas tank in my 1965 coupe, along with a new sending unit/o-ring and all new hardware also, but it is leaking out thru the sending unit. I looked it over, the o-ring is ok, even had a second new o-ring tried it also still leaks - the sending unit seem loose, will wriggle when installed. I was wondering if any one has any ideas, I thought maybe the locking ring might be wrong size or some thing, but I could use any ideas or helpful hints. By the way I bought all the stuff from mustang corral so it should good, never had any problems with them yet.

 

Yea if the sending unit is loose then the o-ring isnt tight. Might be able to bend the ring up on the 3 spots that it is held in front of the tabs on the tank. This might bring you in tighter to seal.

Another thing would be to look at the o-ring you got. If it was cut too short and sits below flush in the groove then it will never tighten up and seal.