In a Japanese wheel listing, a size such as 7J or 7JJ is often read as a width with a unit attached. It is not. The number is the rim width in inches, and the letter or letters after it name the shape of the rim flange, the lip that meets the tyre. This guide covers what the letters mean, what the two main shapes (J and JJ) measure on the sources we could find, and where we could not find an answer, which includes the 7JK in this guide’s title.
The Short Version
- The letter is not a unit. Japanese sources describe J as a type of rim-flange cross-section, not a measure of width. The width is the number in front of it.
- J and JJ differ by a small amount. Two Japanese motoring sites give the flange height as 17.5 mm for J and 18.0 mm for JJ. A third source gives 17.5 mm for J and 14 mm for the older B shape.
- J or JJ is not a fitment test. Toyota’s Gazoo site says a different flange shape does not by itself prevent a wheel from fitting the car or being paired with a tyre.
- We found no definition of JK. One English-language page names JK, K, P and D as profiles without giving any detail. We could not read the standard that would define them, so we do not define JK here.
How to read the size
Take a listing such as 17×7.5JJ. The 17 is the rim diameter in inches, the 7.5 is the rim width in inches, and JJ is the flange shape. One Japanese guide describes J as “a kind of rim-flange cross-section” and stresses that it is not a unit symbol. Another says the flange shape is set by the wheel maker’s design, so a buyer does not choose it. The same two-part reading appears in the listings we have cited in our own guides: the Pulsar GTI-R guide quotes a registry entry of 14×6JJ at +40, and the Aristo guide mentions a database entry of 8JJ×17.
What the letters measure
Flange shape is defined by height, width and thickness. The sources we found give a height for three shapes, and a thickness for those same three from one source only.
| Shape | Flange height, as reported | Flange thickness, as reported | What the sources say |
|---|---|---|---|
| B | 14 mm | 10 mm | Older shape. One source says it is found mainly on older cars with 12 and 13-inch wheels and rim widths up to 5.0 inches. A Japanese guide says only a small number of wheels still use it. |
| J | 17.5 mm | 13 mm | Named as the most common passenger-car shape by one source. Two Japanese sites say most current wheels use J or JJ. |
| JJ | 18.0 mm | 13 mm | Slightly taller and larger than J. One English source says it is common on 4×4 and SUV wheels. |
| JK | Not found | Not found | Named without detail in one source. Not covered in any Japanese page we read. |
The J and JJ heights agree across two Japanese motoring sites, and the J height and the B height also appear on a third, English-language site. The thickness figures are from one Japanese site only. One of the Japanese sites says a JJ flange is thicker and larger, so on a tight fit it may protrude. The other says a J flange makes a wheel look larger and a JJ flange makes the tyre look slightly thicker. Neither claim comes with a measurement, so treat both as descriptions of appearance.
The standard behind it
Japan’s standard for this is JIS D 4218, titled “Automobile parts – Wheels – Contours of rims”. The current edition is dated 21 September 2021 and replaces the 1999 edition. Its public preview says it is based on ISO 4000-2:2013 and ISO 4209-2:2020, with changes for Japanese conditions, and that it covers rims for passenger cars, small trucks, trucks and buses, including a section on flange shape for 5-degree deep-well rims. The preview shows only headings, not the designations or any dimension table. The related JIS D 4102:2007 covers the vocabulary, designation and marking of wheels and rims and is based on ISO 3911:2004. We have not read the full text of either, so the figures in this guide come from the secondary sources above and not from the standard.
What This Means When Shopping
- Compare the number, not the letter, when you compare width. A 7J and a 7JJ have the same 7-inch rim width. The letter is the flange shape.
- Read the letter as part of the maker’s design. The sources we read say the buyer does not choose the flange shape. If a listing gives J on one wheel and JJ on another, the difference is something the maker or the listing has stated, so ask which is marked on the wheel.
- Do not use the letter to judge fit. Gazoo’s explanation is that the flange shape alone does not prevent fitting. Offset, bolt pattern and hub bore are what decide whether a wheel goes on. See our hub bore reference and bolt pattern changeovers.
- If a listing says JK, ask what it means. We cannot tell you what it is. Ask the seller for the marking as stamped on the wheel and treat the answer as the seller’s statement.
- Do not copy the letter between listings. Databases and registries sometimes differ on the letter for the same car, as we noted in the Pulsar GTI-R and Aristo guides, so confirm the size marked on the wheel in your hands.
What We Could Not Verify
What JK means, and any dimension for it: we found it named once, without a definition, and in no Japanese page. The flange height and thickness figures from the standard itself, since the public previews of JIS D 4218 and JIS D 4102 do not show the tables and we did not buy the full text. The thickness figures, which come from one site. Where on the flange the 17.5 mm and 14 mm heights are measured. Whether JJ is chiefly an SUV and 4×4 shape, which one English source says and the Japanese sources do not. Whether the flange letter is stamped on a given wheel, and where. Which makers use JJ on which wheels. Whether a J flange and a JJ flange change how a balance weight sits, which none of the sources address. What the letters stand for. The ISO equivalents of the Japanese letters. All figures here are as reported on the pages we cite. Confirm anything that matters to a purchase against the wheel itself.
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