Magnetic tank bag: why it won’t stay on your motorcycle and how to fix it

You buy a magnetic tank bag, place it on the motorcycle, and nothing happens. It stays there, moves when you push it with a finger, and by the first corner it is already askew.

It is not a defect in the bag. It is that your motorcycle's tank is most likely not made of iron.

Why magnets no longer have anything to grab

Magnetic tank bags originated in the days when tanks were made of sheet metal. On a motorcycle from that era they worked: four magnets, downward pressure, and the bag stayed where it was.

On most recent adventure and touring motorcycles, that assumption no longer applies. The tanks are plastic, or they are metal but covered by side panels and fairings that keep the bag away from the magnetic surface. The result is the same, and no bag brand can fix it.

This is where the two wrong solutions seen everywhere come from. The first is to run straps under the seat: they hold, but after a few thousand kilometers they mark the side panels, and at every stop at the gas station you have to fumble with four buckles. The second is to give up the tank bag altogether, losing the only place where documents, your phone, and gloves are truly within easy reach.

The solution is a ring, not a better bag

The system that has become standard on modern motorcycles moves the attachment point: instead of attaching to the tank, the bag attaches to a mounting ring bolted around the fuel cap.

The ring stays on the motorcycle and replaces the original fuel-cap trim ring. The bag rests on it, the magnets guide it into position at four points, and four mechanical clips lock it in place. To remove it, lift it off, and that's it.

The point that changes everything: the clips, not the magnets, provide the secure hold. The magnets only align the bag as you place it. That is why the tank material becomes irrelevant — metal or plastic, the ring is mechanically secured and does not care what is underneath.

The standard ring measures 122 × 104 mm and weighs 85 g, made of aluminum alloy and nylon. The version for the Harley-Davidson Pan America is larger, 147 × 122 mm, made entirely of aluminum, and weighs 130 g.

The ring is not universal: the complete table

It is the most common mistake, and the one that leads to a return: you order the bag and assume the mounting ring is included or will fit anyway. The ring is purchased separately and is specific to the motorcycle family, because the flange design follows the number and position of the fuel-cap bolts.

On the ring page, you will find a make → model selector that returns the correct code, along with the price and a button to add it to your order.

The Honda fuel cap trap

It deserves a section of its own because it is the only case where knowing the motorcycle model is not enough.

Some Honda models—CB400X, NX400, NX500, and CB500X—come with two fuel cap versions, with five or seven holes, and the two rings are not interchangeable. Same bike, same year, different fitment depending on the trim.

The check takes ten seconds: go to the garage and count the screws around the cap. Five holes means BTBZJ291; seven holes means BTBZJ281.

The two measurements to check before ordering

Besides the motorcycle model, there are two physical checks that apply regardless of the bag brand.

The space between the handlebar and the cap. Larger bags require at least 70 mm between the handlebar mount and the tank cap. Below that measurement, the bag either will not fit or will touch. You can measure it in one minute with a tailor's tape measure.

Full steering lock. This is the maneuver where the bag gets in the way, not when riding in a straight line. Bags with a sliding base solve the problem: the plate moves back and forth over 11 cm by lifting a buckle, without tools. If the handlebar touches at full lock, move the base back instead of giving up on the bag.

Installation, step by step

1. Unscrew the original screws from the fuel-filler cap ring.
2. Position the ring flange so that it aligns with the holes.
3. Screw it back on with the supplied hardware, tightening in a crisscross pattern and observing the torque specifications provided by the motorcycle manufacturer.
4. Check that the cap opens and closes freely and that the key inserts without forcing it.
5. Set the bag in place, adjust the sliding base, and check that the handlebar does not touch when the steering is fully turned.
6. Recheck the fasteners after the first 100-200 km.

No drilling, fully reversible, just a few minutes.

Which bag to choose

All the tank bags in the range use the same mount: none attaches with straps. Install the ring once, then switch bags depending on the trip.

The fabric is 2520D double-yarn ballistic nylon, the weave used where abrasion resistance matters: on a bag that rubs against your jacket and knees throughout the trip, it is the factor that determines how long it lasts. The BTB12 also has an EVA upper shell that helps it retain its shape even when half empty, plus SBS zippers with pulls sized to be gripped while wearing winter gloves.

A clarification about rain

This is the point on which there is the most misinformation. The fabric is water-repellent and easily withstands rain while riding, but it is not waterproof when submerged, and the manufacturer does not declare any IP rating for these bags. In a downpour, protection comes from the one-piece rain cover included in the package, which should be fitted before the rain starts coming down heavily, not afterward.

How much does it cost

The ring costs 26 € for most models and 31 € for the four larger versions (the two Honda versions with 5 and 7 holes, CFMOTO 450MT, and Pan America). This is the expense that makes a bag costing over one hundred euros usable: adding them to the cart together is the only way to avoid being stuck on the day of delivery.

Find the ring for your motorcycle →

Frequently asked questions

Does the magnetic tank bag work on a plastic tank?

No, not by itself. The magnets need a ferrous surface. With the mounting ring screwed onto the cap, this is not an issue, because the bag attaches to the ring rather than the bodywork.

Is the mounting ring included with the bag?

No. It is purchased separately and is specific to the motorcycle family. The bag cannot be mounted without the ring.

Do I need to drill the tank to install the ring?

No. The ring replaces the original cap ring and uses the screws already in place. Installation is completely reversible.

Can I use the same ring with bags of different volumes?

Yes. All tank bags in the range share the four-point attachment, so once a ring is installed, you can switch from the small bag to the 10-liter one without changing anything on the motorcycle.

How can I tell whether my Honda has a five- or seven-hole cap?

Count the screws around the fuel cap. Five holes correspond to code BTBZJ291, while seven holes correspond to BTBZJ281. The two rings are not interchangeable.

How much space is needed between the handlebar and the cap?

At least 70 mm for the larger-volume bags. Below that measurement, the bag may touch the handlebar when the steering is fully turned.

Is the tank bag waterproof?

The fabric is water-repellent and withstands rain while riding, but it is not waterproof when submerged and has no declared IP rating. For prolonged bad weather, use the rain cover included in the package.

Can the ring be removed if I sell the motorcycle?

Yes. The original ring is unscrewed and refitted: no holes or permanent modifications remain.

If your model does not appear or you are unsure about the model year, send us the make, model, and year: we will check the manufacturer's documentation before you order.