Need help identifying what gas cap will work

[Seattle]

I just got this 78 C20 and there is no gas cap- can anyone identify the cap that would fit based on the numbers in the pic?  I've called around to parts stores and online suppliers and they all say they have no way to reference that number.

I've bought multiple 78 gas caps, an 81 gas cap, and a 74 gas cap- all the gas caps the local stores had in stock, and non of them fit.  Unfortunately, the truck has a bunch of other issues so isn't drivable...  And there aren't any wrecking yards near by...

Here's another view of the set up:


For some reason the image links aren't working...

It has a dump bed on it- so it was set up that way (custom) to make it easier to fill-up.

Any suggestions would be greatly appreciated!

[VileZambonie]

Why don't they fit? Vented is 6800 non vented is 10800

[Rapid Roy]

If the tank is from an original fleetside  bed, then this is the one I use. If from a step side, It would be a different cap.

https://www.ebay.com/itm/MotoRad-MGC800-Fuel-Tank-Cap-for-10800-11800-12F39-31743-40800-6800-703-1835-il-/295143304797?_trksid=p2349526.m4383.l4275.c1#vi-desc-maincntr

[Seattle]

Why don't they fit? Vented is 6800 non vented is 10800

Thank you!  I'm sure I tried both vented and non vented..

The size of the locking tabs on the fuel cap is different, so I can't tighten/lock the cap on. Or, for other caps, the depth of the cap is wrong- so if it were a shallower cap, if feels like it would lock...   

[Seattle]

If the tank is from an original fleetside  bed, then this is the one I use. If from a step side, It would be a different cap.

https://www.ebay.com/itm/MotoRad-MGC800-Fuel-Tank-Cap-for-10800-11800-12F39-31743-40800-6800-703-1835-il-/295143304797?_trksid=p2349526.m4383.l4275.c1#vi-desc-maincntr

Thank you!

Since it was a random Chevy truck fuel tank neck sitting on a spare tank, I don't know what year or model Checy truck it came off- I'm hoping there is some way to reference those number on the filler part of the neck???

[VileZambonie]

I would guess it had a locking gas cap and someone pried it off and caused some damage. The cap #'s listed will work, so look again more carefully.

[JohnnyPopper]

Can you post a pic without the gagging rag?