Well i have had a chance to try the cap mods with full tanks over a 350 mile fast run last weekend and a big thank you to Mr Ed Smith it worked a treat NO spillage !!!

Awesome Carl, I need to inspect your handiwork now and take lots of photos.  Thanks!

Finally got around to working on the Mini today, which required full throttle accelerations and I'm happy to report that even with a full tank of gas only a touch of fuel leaked out of the cap. Not enough to even reach bumper level which in my eyes is quite acceptable.

The "Smith Seal" is here to stay!

We have tackled this one differently,but the effect is the same. (sorry no pics) We took some plastic round stock and lathed a Top hat shaped plug. 1.75 in long -the top section is 1.75in wide and 3/8in thick ,The bottom part of the plug is 1.5in and has a 1.5in O ring butted up against the top section. We bored a 1in center hole almost all the way through but left some meat at the bottom. Then we clocked the part ,we drilled a small hole in the bottom corner. At the top and opposite to the hole in the bottom we drilled a hole and inserted a small dowel. I have Aston alloy flip lids on my tanks. So the fuel smashes up the tank filler tube ,hits the almost on size plug and calms down any fuel that gets around the plug now hits the O ring. The plug its self is held under tension by the alloy cap and its spring loaded gasket. When a small amount of fuel shoots up the drilled hole it will drain back to the tank and will allow the vent to work.  The small bit of dowel is a finger hold to help remove the plug for fill ups. Also for clocking the drain hole on the bottom. I sanded the top of the plug to get the right feel when closing the flip lid. It really works well!!
A picture is worth a thousand words!! Cheers

WOW Brian!

Image Gallery

Or you could do this.

Smith Seal take two.

Removed the ultra thin bicycle inner tube that started rose pedaling for 1/16" thick rubber cut to the same diameter as the gas cap to make a nice uniform tight seal in the neck of the tank.

Did it work? Well you'll just have to wait until I drive it to find out

All my cars have the locking gas caps. I have two SPI minis and one carb Elf. What NON LOCKING caps would I need for each car to do this modification to prevent the leaking caps?

Which cars take the vented caps and which ones take the non vented ones?

Mine are standard non locking vented gas caps. I don't see how you could attach the screw with the locking mehanism.

I noticed that the caps used with this seal are all the standard non-locking cap. I don't imagine this method would work on a locking cap because of the locking mechanism???

Sorry about that, I continue to drive my car with no leaks on either tank. My gas caps have new gaskets which leaked before I installed the seal . The rubber was from a  truck tube. I hope there is feed back from others so we can see how this works on a wide range of cars. 

Filled the tank for the first time in as long as I can remember and well I still had fuel spillage from the gas cap.

I'm thinking the inner tube I used was too thin because it looked more like a rose pedal shape when I took the cap back off.

I'll look for a thicker rubber and give it another try.

Well here's my shot at the "Smith Seal".

I found that I needed a locking washer in front of the nut on the gas cap side to keep the machine screw/nut from spinning but other than that it went together just as esmith diagramed it. And along with firm tightening I applied a good amount of Locktite to be on the safe side.

Tomorrow I'll try it out with a full tank of gas!

I think the older mini's like mk1's, dont have a cap with a detent, they just have two locking tabs that cause the ruber seal to press against the top of the filler neck to make what amounts to poor seal.

I have found on my '79 that if you tighten the gas cap all the way to the second detent it leaks.  But if you tighten it to the first detent it does not leak.  I think tightening to the second detent deforms the gasket (which is brand new) & allows it to leak.

Those are great pictures. Thanks for posting the info.

And of course if all else fails, remember the "BIB" attachment that you can buy to help save the paint in a perpetual leaking situation-

BIG AL

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You should have the pictures.

I will go to the shop tomorrow after church and see what I can do.