small gas cap for side aux. fuel tank - help needed

[dsy1]

I need to find a small gas cap for my side aux. fuel tank. The neck inside diameter is 1.25 inches but I can't find anything that small. This is a cam-style cap (two tabs fit into slots on the top of the neck and it twists 90 degrees to lock on). The neck that comes up from the tank has a rolled top with the two notches for the tabs on the cap, with 1.25" inside diameter and about 1.75" outside diameter.

So far I have been using a push-in rubber cap meant for an oil cap on a valve cover.

I have tried a twist-on 1.25" oil filler cap with tabs but the tabs extend in too deep so it just spins and won't engage to lock. I could maybe add a thick rubber washer to make it tight enough, but I would like to find something correct instead. Maybe a cap for a motorcycle or something small? Any ideas?

[Steeveedee]

I have saddle tanks on my truck. Post a pic of your cap (both sides) with a steel scale next to it, and try to find a part number on the cap. I'll eventually have to replace my caps, so it will be interesting information for me as well as others. What year is your truck, and does it have Federal or California emission controls? We can steer you through the various emissions requirements, don't worry about that!

[dsy1]

I have no original cap for that tank, the truck didn't have one when I bought it. It seems to be an aftermarket saddle tank, 18 gallons on the drivers side, the neck comes up to a lockable door in the bedside about a foot behind and below the normal tank location. The neck is smaller in diameter than the one on the behind-the-seat tank. This is on a '71 C20.

I bought what is in the link below to use while I search for something correct (just a push-in rubber valve cover cap, but it doesn't seal as well as I'd like):

https://www.amazon.com/dp/B00062ZCSW...v_ov_lig_dp_it

The smallest cam-style twist-on gas cap with the proper style lugs that I have seen is 1-3/8, I need 1-1/4 diameter. It needs to look like this one, but smaller:

https://content.speedwaymotors.com/P...64b7982d81.jpg

[Steeveedee]

OK, can you take pix of the filler neck, with some size reference? I'll help you all I can, because I'm sure that at some point I'll need (probably) the same thing!

[dsy1]

Here are some pics...This morning I used a valve cover cap and cut some donut gaskets from an old inner tube to make a trial cap, the extra rubber thickness helps it fit tighter. It sort of works but a real cap would be better if anyone knows of where to look.

http://home.sandiego.edu/~yandell/truck/

[Steeveedee]

OK, you have the same caps I have. I just went and measured one. Looks like I need new ones, too. I'll check into it, hopefully tomorrow.