Locking gas cap under OEM flip down.

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4mm

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1971 Mustang 351C 4V Fastback
Is it possible to fit a slim/small locking gas cap under the flip down cap? Have any members succeeded in doing this and what brand did you buy?

 
Hello, just wondering if you ever got any replies on your question, I'm looking for the same item.

 
Twist on cap will not fit under original Mach 1 cap. The pop open cap had a vent that actually protruded into the opening of the filler pipe when closed. The Feds had come down on Ford over some issues with the pop open cap. In some roll over and rear end collisions the Mach1 cap was "Popping" open and spilling fuel. That's why you see the standard Mustang twist on cap production installed on all 72-73 Mustangs including Mach 1's. There is a new setup on the market that satisfies people like me that like the look of the Mach 1 cap but has the safety of a regular twist on cap. I have this setup on my car and will be on my other Mach1 when I put it back on the road. I would check with Don (OMS) as he seems to have the best price. I've seen some on other sites for $130.00 https://www.ohiomustang.com/store/order_page.asp?itemid=3293

 
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Thanks for the info. Im aware of what you posted and I am wondering if a small "locking"' twist on gas cap would fit under the aftermarket flip gas caps. I'm sorry I didn't make myself clear enough. :)

 
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Looks like you can use the one offered by Don @ OMS that secluff posted then use a locking cap instead of the twist on cap. Might have to check the outside diameter of the locking cap to make sure it will fit.

I'd buy the one from Don then take it with me to NAPA store and just try some until I got a locking one that would work.

Ray

 
I've been wondering the same thing - I'll see if I can remember to take my OMS-purchased repop Mach 1 cap with me to the parts store to see if they have anything that locks.

The lid itself is fairly low profile - there's not a lot of room in there at all. The cap is also very minimal, but has a small, flat round section in the middle that I suppose could be big enough for a locking mechanism (as in, maybe the folks that stamped the caps have more than one customer who might put locks in their caps. They also have a knurled edge to help grip for twisting on/off.

I'm pretty sure those standard caps with the 'fins' [that help in twisting the cap on/off] will have issues fitting in there because they might stick out a little too far for the lid to fully close.

 
Remember 'our' pop-down caps are VERY popular and used on many other modded Mustangs etc.

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http://forums.vintage-mustang.com/vintage-mustang-forum/589239-locking-gas-cap.html
Well that looks like a locking gas cap right there. :whistling:

 
A trip to your local parts store or even a message to Don (OMS) would probably work. He's probably been asked about a locking cap before. Since the kit he offers has the inside cap with ears which takes up a little more room, and with no vent mechanism under the Mach1 cap there should be room for a small locking cap of some sort. He might even have a cap similar to the one in the picture Boss1Ray posted. I don't have one on mine so the info here hopefully will motivate me to get off my tail and get one!

 
I tried stant cap #10559 which looks like it might have worked, but no luck. In case anyone was wondering.

 
I was considering that exact one. Was it too wide or thick?

 
I thought there were pop-open repros with locking caps already?

If you look at the original, can the seal assembly be removed to give additional clearance? I have an original Autolite with the seal and the 72 Motorcraft which does not, but the pop-open cap looks identical, as if they just did not assemble everything on to it. If you try to disassemble an original, you might break the screws.

Or, maybe stack a couple gaskets between the body and the pop-open if there is enough clearance between the locking cap and the pop-open base.

 
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