Gas cap does not fit on CJ7 1979 304 QT

Hi and good evening,
I am now back with my first question here, I have to replace my gas cap,
but unfortunately none of the new ones fit, that I have bought.
The old one is #1 in the pic
#2 NOS AMC #5350828
#3 Stant #10583
#4 no name non-lockable aftermarket
#5 FRAM RG-792



#1 has an inner diameter of about 38 mm and a diameter at the noses about 43 mm
#2 - #5 have 34/40mm, so they do not sit firmly
I am a bit confused, because I did not find one with that diameter, seemingly the filler neck is different here.
Maybe somebody knows the part # of the correct one.
Or should I exchange the filler neck?
Thanks in advance for the help.

 

What is the reason for replacing the original cap? Seems by the picture that the seal or gasket is dry rotted!! Is that the only thing wrong?

 

Exactly, the gasket is rotten.
Can this gaskets be bought somewhere? I never have seen such an opportunity, respectively never thought of this...

 

What I additionally found out in the mean time is that seemingly I have an 1 1/2" filler neck,
it should be 1 3/8". Can this be for export models?

 

I would suggest trying to use a gasket from one of the other caps that you had purchased to see if something works for you!! I can't answer the question of the filler neck size difference. I'm sure someone will chime in that knows.

 

Do you have the filler neck where the smaller filler vent tube (makes the tank fill easily - no glug-glug effect) connects directly to it? The older type has a separate bulkhead fitting in the filler hose.

From '78 on, Jeep used the filler neck where the filler vent tube connects to a nipple on the filler neck. For that year range, there is only one filler neck for domestic production. There is, however, a filler neck for export, and a filler neck for Canada. The only cap that is different is the '78-79 Export, not ECE (emission control exempt), 5356793 vs. the standard 5358207.

If you have an oddball filler neck, the easiest route might be to replace it with an aftermarket or junk yard part from a CJ. You have plenty of gas caps to test the replacement filler neck with .

 

Thanks for this valuable advise, I think, I found everything in the parts book, I needed the Explanation to understand everything.
Now it seems to be clear:
80/90 1978 Canada, 1979 Export 304 ECE and 1980 Canada share the same different filler neck (5357921).
I have an 1979 304 Export...
So I have to look for a 5357894 filler neck, which is noted for 80/90 78/, where my NOS cap should fit...
Sorry for the complications, I am trying to keep everything correct for the year...

 

I slightly moved ahead, bought some different filler necks and more caps,
I know it's silly, but somehow I am to much a perfectionist ;-)
I found three different inner diameters of filler necks and compared the fitting
of various gas caps.

Does anyone know which part number belongs to the respective inner diameter
of the filler neck?

Any advise is appreciated...

 

Another option might be that the difference between 38 and 40 mm is just the tolerance
in the manufacturing process?