Looking to buy a new Harley gas cap (or two) for your motorcycle? Want to know what to look for when digging through gas caps at a swap meet?
The use and differences between vented and non-vented Harley gas caps is straight-forward, once you know. If you don't already know, you are about to find out!
WATCH VIDEO: HARLEY GAS CAPS 101 VENTED VS NON-VENTED
In this video Tim shows you the major difference between three styles of gas caps and how you can tell if they are vented or non-vented.
Each style of gas cap (screw-in, bayonet, and weld-in custom gas caps) have their own characteristics that help identify if they are vented or not.
Also, what as Tim shows what happens when you try to put a vented cap on a non-vented Shovelhead gas tank, and vice versa.
SCREW-IN STYLE HARLEY GAS CAPS
A very common style Harley gas can is the screw-in style that fit most 1982 to current Harley-Davidsons. Harley actually changed the thread pitch, with earlier 1982-1995 bikes using a different thread cap than 1996 to current.
All aftermarket gas tanks use the 1996 & later fit gas cap. However, if you are looking for a replacement gas cap for a stock 1982-1995 Harley, be sure that the gas cap you purchase is meant for that year range.
LEFT-HAND VS RIGHT-HAND THREADED HARLEY GAS CAPS
Screw-in style gas caps Right and left hand thread.
While they look the same at first glance, a closer look reveals that the threads are at different angles. The cap on the left is a right-hand thread, in that the cap will tighten and pull down into the gas tank when turned right, clockwise.
The cap on the right is a left-hand thread, in that it tightens when turned to the left, or counter-clockwise. Left-hand threaded gas caps are always unvented, and are located on the left side gas tank on Harley-Davidsons with 'split tanks'. That is, a large gas tank that is comprised of two halves, a left and a right, typically with a metal 'dash' in the center.
VENTED VS NON-VENTED HARLEY GAS CAPS
If your gas tank has only one filler neck, It will require a vented style gas cap.
If you are running split tanks or your gas tank has two filler necks, the right side filler neck will use a vented style gas cap and the left side filler neck will use a non-vented gas cap.
Most of our screw-in style gas caps we sell you will find an "R" for right hand thread or "L" for left hand thread. Right hand thread is always vented Left hand thread is always non vented.
The main reason Harley uses a non-vented gas cap on the left side is so gas will not spill out of the filler neck when the bike is parked and leaning over on its kickstand, with a full tank of gas.
The indicator is lookin in on the inside of the gas cap threads and you can plainly see vented and not vented.
Another way to check if a gas cap is vented or not is just by looking on the bottom of it in the center of the threads. If there's a tube coming out the middle most likely its vented. If there is nothing there at all it is most likely non-vented. Easy peasy.
BAYONET GAS CAPS AKA CAM STYLE
Bayonet style, or what some refer to as "cam style," gas caps fit 1936–1982 Harley-Davidsons and are available in two different diameters: 2" for 1936–1972 models (Harley OEM 61103-36) and a larger 2.5" for 1973–1982 models (Harley OEM 61102-73a).
2" diameter gas cap on the left is a replacement for Harley OEM 61103-36 and fits 1936-1972 Harley models. The 2.5" diameter gas cap on the right fits 1973-1982 Harley models, and replaces Harley OEM 61102-73a.
Both the 2" and 2.5" diameter gas cap fit the same style gas filler bung; only the diameter of the outer cap is larger.
However, beyond appearance, there are functional considerations that must be taken into account when making your selection.
One noticeable difference between vented and non-vented Bayonet style gas caps is the center profile on the underside of the cap.
When looking at a Bayonet style gas cap, flip it over to look at the center where the two tabs lock the cap into the tank. If the tabs are about flush with the edge of the gas cap, when viewed from the side, it most likely is a vented cap.
If the center profile is raised and the tabs are higher than the outside edge of the gas cap, when viewed from the side, then it is most likely a non-vented gas cap.
Here are some more determining factors to look for when identifying a vented or non-vented Bayonet style gas cap.
A couple more way to spot the differences between a vented and non-vented Bayonet style gas cap is look for a hole on the center profile. If it has a hole it most likely vented where one with out a hole is not vented.
Some gas caps like the ones we sell will also have an "R" which stands for, right side tank and these will be vented. We also have gas caps labeled with an "L" which stands for left side tank and will these will non-vented.
Both the screw-in and bayonet style gas caps are not interchangeable on a split style, 2-piece gas tank or a gas tank with 2 filler bungs. Non-vented caps are for the left side, and vented caps are for the right side.
CUSTOM GAS CAPS
Here is just of the many styles of custom gas caps we offer the Aluminum Gas Filler Cap with weld-in steel bung vented and non-vented.
With making a custom gas tank or modifying an existing gas tank and installing your own custom gas cap and filler neck, you need at least one vented gas cap, unless the gas tank has a separate vent (such as a small 1/8" NPT bung with an NPT hose barb and hose, for instance).
This can also create issues if you have more than one cap. For example, we offer a small custom gas cap with a filler neck bung in both vented and non-vented versions (both are available with an aluminum or brass gas cap).
Flipping over our custom gas caps you can tell the difference immediately by looking at the bottom. The mesh filter is the vented gas cap and the no filter is the non-vented.
To tell the difference between vented and non-vented siply flip the caps over and check for the mesh vent filter on the underside. The cap with the mesh filter should always be installed on the vented right-side filler neck. It just requires a little extra attention when reinstalling the caps.
Small, custom vented caps will typically have a small hole visible either on the top of the gas cap, or sometimes as a small hole on the edge of the gas cap, which connects at a 90 degree angle internally to a hole drilled from the underside.
Related Products
