Contents: 01. Quarter-turn 27mm thin wing
Contents: 02. Quarter-turn 30mm
Contents: 03. Quarter-turn 30mm thin wing
Contents: 04. Quarter-turn 40mm
Contents: 05. Quarter-turn 40mm thin wing
Contents: 06. Quarter-turn 40mm wide wing
Contents: 07. Quarter-turn 40mm external wing
Contents: 08. Push-in xxxx 28mm
Contents: 09. Push-in xxxx 30mm
Contents: 10. Push-in xxxx 32mm
Contents: 11. Push-in xxxx 36mm
Contents: 12. Push-in xxxx 40mm
Contents: 13. Screw-on M30-1.75 external thread
Contents: 14. Screw-on M40-1.75, 2.0 external
Contents: 15. Screw-on M42-1.50 internal thread
Contents: 16. Screw-on M50-1.50, 2.0, 2.5 internal
Contents: 17. Screw-on “bottle-cap” thread 45mm
Contents: 18. Pop-up (moped)
Contents: 19. Information and Explanation
1. Quarter-turn 27mm
This is a special size gas cap, only found only on some Trac models:
1983-90 Clipper, 1982-86 Eagle, 1984-88 Escot, 1984-89 Liberty, 1986-89 Olympic, and maybe 1988-90 Blitz.
It is part number 21204 or 17620-M56-8900.
A 30mm quarter turn cap is too big and cannot be made to fit by simple grinding.
2. Quarter-turn 30mm
For quarter-turn gas caps, the named size is the inner diameter of the filler neck it fits into. The actual size of the gas cap is a little less than the named size. That’s why a 28mm gas cap on the internet is called 30mm here, or a 38 is called 40 here.
3. Quarter-turn 30mm thin wing
Some of these have thinner wings, and some have slightly different wings than “regular”.
4. Quarter-turn 40mm
For quarter-turn gas caps, the named size is the inner diameter of the filler neck it fits into. The actual size of the gas cap is a little less than the named size. That’s why these are called 40mm here, but they are called 38mm elsewhere.
5. Quarter turn 40mm thin wing
6. Quarter-turn 40mm wide wing
7. Quarter turn 40mm external wing
8. Push-in 28mm
The early Vespa Ciao, 1967-68 up to frame #154099, had a light gray plastic 28mm push-in gas cap. Myrons does not have any, not even a photo.
After that it was a 32mm push-in.
9. Push-in 30mm
The named size of push-in gas caps is the inner diameter of the filler neck, 30mm. The actual size is smaller, 28 or 29 so it can slide in. But the seal lip or rubber ring is bigger 31mm, so it can get squeezed as it pushes in.
10. Push-in 32mm
The push-in gas caps 10a, 10c, 10d, 10f have a thick rubber ring that is made too large, 34-35mm. Right out of the package it is too big to enter a 32mm gas tank filler neck, even with grease and care. The rubber ring must be ground down to reduce the outer diameter to 33mm. This makes the price a few dollars more.
It’s amazing how much that Derbi gas cap shrank, from 32 to 30mm. And how the one from the newer model shrank, but the older model one did not. So a badly shrunken one could fit nicely on a Peugeot! Other soft plastic push-in gas caps with lips, not rubber rings, also shrink, like Peugeot. Only those 35 year old NOS ones that have been kept in a sealed plastic bag are not shrunk.
11. Push-in 36mm
12. Push-in 40mm
13. Screw-on 30mm
14. Screw-on 40mm
15. Screw-on 42mm
These gas lids are not shrunk because they were kept in a plastic bag for 40 years. Others that are shrunk are offered at half price. Those were exposed to the air for 40 years.
Almost all moped gas caps are vented. Some have a vent switch. When you transport the bike laying on it’s side, gasoline drips out of the air vent hole in the gas cap. With a vent switch you can close the vent hole during transport or storage, and open it during regular use.
16. Screw-on 50mm
This cap 16b has the 50mm thread tops on the tank, like 16a and 16c, but the pitch is in between those two.
This cap 16c looks similar to 16a, but threads are coarser. The ridges on top are evenly spaced, but on 16a they are in pairs.
17. Screw-on “bottle-cap” thread 45mm
This tank has bottle-cap type threads, not machine-type. The inside of the gas tank neck is 34.4. The outside diameter is 44.6. Each thread is 4.0 mm wide.
The Allstate Compact scooter also has this gas cap, as do most 1950’s-60’s Puch mopeds and their clones.
18. Pop-up gas caps
19. Information and Explanation
More gas caps, not for sale, for your information:
Gas caps come in 4 types. 1. Quarter turn, 2. Push in, 3. Screw on, 4. Pop up. Each type has different sizes.
Quarter turn has two “wings” on opposite sides that enter two slots in the gas tank lip. Then when the gas cap is rotated 90 degrees to the right, which is one fourth of a turn, it locks on and seals the outer edge by the spring tension in the wings. The spring has a certain amount of “travel” that allows the lid to work (seal) on a range of gas tank lip thicknesses, or as the rubber or cork gasket gets thin. But sometimes a thicker rubber washer is needed to get the wings to push harder and seal better. In the 30mm size an extra 2mm thick rubber is sold here, but not for 40mm, at this time.
Push in lid gas tanks have a smooth round wall, about as deep as it is wide. The lids are usually black plastic with thin flexible circular ridges that press against the wall. The entrance has a taper for easy operation. In the 1977 USA moped standards, push in gas lids were supposedly required to be quarter turn. Puch, Cimatti, Garelli, and Sachs switched from push-in to quarter-turn but not everyone else did. Again in 2010, quarter turn gas caps are mandated in the USA, this time not for safety reasons, but for environmental ones.
Screw on lid gas tanks have threads. The lid must be screwed on one or more full turns clockwise. The threads on the lid can be either female (internal) or male (external). Most modern screw on lids are more like bottle cap threads, more coarse than machine threads. Also the tops and bottoms of the threads have wide plateaus. Some are at right, not listed.
Pop up gas lids are generally on top tank mopeds and modern motorcycles. Turn the key or push the button and the lid pops open.
Vent Hole: All moped gas caps are “vented”. That means they have a tiny pinhole or channel that lets air get in or out. If the pinhole is blocked, after awhile a suction will develop, the gasoline supply will stop, especially with a full tank. They all have a vent design that prevents gasoline from shooting out, like a wall or a maze, but lets air get through. Some gas caps have a vent switch, “on” or open for riding, “off” or closed for transport and storage. All vented gas lids will leak gas if the tank is too full or leaned over too far.
Shrinkage: Unless they have been kept stored in an air tight bag, many of the vintage moped plastic gas caps shrink after 20 or 40 years. The Peugeot and Colombia black plastic push-in 30mm lids become 29 or 28mm. The 32mm late Derbi black plastic push-in lid becomes 30mm. The Garelli 42mm screw-on black plastic lid becomes 41 or 40. When a push in lid shrinks, it gets loose and falls off. One remedy for that is a “leash” made out of a twist tie. Another remedy is a thin rubber o-ring, but it’s hard to find the correct size. With push-in gas caps, the actual diameter will be slightly more, about 0.5 millimeter, than the tank hole inner diameter, to make a light press fit. So a 30mm lid should be 30.5mm wide, a 40 should be 40.5. With screw on male lids there can be shrinkage and the lid will still function, because it does not seal on the loose threads. But when a female (internal thread) screw-on lid gets smaller, it becomes tight and difficult or impossible to screw on.