Double Aluminum Rollers 15398: What 13–12 mm Means
The ‘13–12 mm’ in Tamiya item 15398 describes two integrated contact sections in each aluminum roller, not a choice between two separate packs. Tamiya says the 13 mm section contacts the course fence on straights while the 12 mm section comes into play in corners. The package includes 520 ball bearings and attachment hardware. Read the geometry before deciding that a stepped roller belongs on your current stay.
Inspect the mounting space
Check the selected stay's roller holes, the available screw length and the height of the wall contact point. A double roller occupies more vertical room than a simple flat roller. Dry-fit with the car's body and tires installed, then spin the roller by hand; neither section should rub a screw head, shell or another roller. Use the supplied attachment parts in the order shown by Tamiya.
Measure the finished setup
Tamiya's broad chassis list includes AR, MA, VZ and older families, but the assembled width depends on your chosen stay and hole position. Measure across the outer roller edges, then check height and all other event dimensions against current rules. A roller can fit a chassis and still breach a class or venue requirement when paired with a wide plate.
Test the contact rather than the label
Start with a clean, freely spinning bearing and record the old roller layout. Change one position, make several runs on the same course and note where the car contacts the wall or derails. The official description explains the intended two-diameter action; only your measured runs tell you whether the added assembly helps your build.
Check current availability and the exact variant on the product listing before ordering.
Related reading
Frequently asked questions
Are there two diameters in one roller?
Yes. Each aluminum component integrates 13 mm and 12 mm sections.
Are bearings included?
Tamiya says 520 ball bearings and mounting parts are included.
Sources checked 26 September 2026: https://www.tamiya.com/english/products/15398/index.html, https://www.tamiya.com/english/mini4wd/regulation.htm. Recheck product availability before ordering.