Mini 4WD Stock Class vs B-MAX
"Stock Class" and "BMAX" get used almost interchangeably at some tracks, and that's the problem. Stock Class is a Tamiya-published rulebook with a dated revision history. BMAX is not a Tamiya class at all — it's a name organisers and hobby-shop circuits apply to their own, locally-defined ruleset, and no two BMAX nights are guaranteed to run identical rules. This post explains what each actually is, on paper. For which parts and setup choices are legal once you know your class, see Stock Class vs BMAX setup and part choices.
Stock Class: a Tamiya-published rulebook, last revised 11 May 2026
Stock Class is one of Tamiya's own official race formats, on a regulations page separate from the general race rules, currently dated 11 May 2026. The rule that defines it: cutting and shaving down the chassis is not allowed, and homemade chassis are prohibited outright. Existing screw holes may be countersunk or drilled out to a maximum of 2mm, but no new holes may be created, and reversing the chassis is forbidden — the car stays recognisably the kit it came from.
Body work is narrower still: modifications are limited to shaving the body down or cutting away sections that obstruct fitment. The body cannot be split into multiple pieces, though it may be cut and repositioned for chassis clearance; driver figures and rear wings stay optional. A defined "forbidden zone" between the front and rear shafts/bearings and specific wheel halves may hold only the body and driver figure — nothing else, even partially.
Parts sourcing is restricted to Tamiya Mini 4WD, R/C Mini 4WD, and Dangun Racer parts only. Ball bearings and cemented carbon plates are permitted, but modifying any part other than the body is otherwise prohibited: no altered gears, no roller combinations outside the regulation list, no motor disassembly, no tyre cutting. Battery terminal parts must be kit-standard, or a Grade-Up part fitted exactly per its manual, and HG setting boards, gauges and pinion pullers can't be carried on the vehicle. A closing safety clause bans anything risking hand injury, track damage, or grease on the course.
The general Official Race Regulations Stock Class sits inside
Stock Class is a stricter subset of Tamiya's wider Official Mini 4WD Race Regulations, covering Laser Mini 4WD, Mini 4WD REV, Mini 4WD PRO, Racing Mini 4WD, Super Mini 4WD, Fully-Cowled Mini 4WD, Aero Mini 4WD, Mighty Mini 4WD, R/C Mini 4WD and Truckin' Mini 4WD machines. These baseline limits are what most organisers reference regardless of class:
| Spec | Official limit |
|---|---|
| Width | Under 105mm |
| Height | Under 70mm |
| Length | Under 165mm |
| Ground clearance | At least 1mm |
| Minimum weight (with motor and batteries) | At least 90g |
| Tyre diameter | 22–35mm |
| Tyre width | 8–26mm (tyres must be fitted) |
| Batteries | Tamiya-branded AA (R6/UM3) only; alkaline only for R/C and TR-1 chassis cars |
Outside Stock Class, the general regulations permit more: modification is described as "cutting away plastic material or cutting holes out of the stock chassis" using only Tamiya parts, provided "the original part shape must be clearly identifiable" — chassis alteration Stock Class bans is, within limits, part of the open format. Permitted motors include the kit motor plus Rev-Tuned, Torque-Tuned, Atomic-Tuned and the Dash variants; bodies must carry stickers or paint and be securely attached, and homemade bodies are banned either way. See the motors guide for which motors are excluded from official events entirely.
What "BMAX" actually is — and isn't
BMAX does not appear on Tamiya's own regulation pages at all. It's a name independent circuits and hobby-shop organisers — common across Malaysia and the region — apply to their own house ruleset, run under their own name rather than as "Official Stock Class" or "Official Open Class." A BMAX night's rules are whatever that organiser has written down for that event, and they're free to be looser than Stock Class, stricter than the general Open regulations, or a custom blend — permitting setups Stock Class forbids while still banning things the general regulations allow. There is no single "BMAX rulebook"; there's only the organiser's rules for the meeting you're entering. Assuming BMAX inherits Stock Class restrictions, or Open Class allowances, is a common and avoidable mistake.
Stock Class vs BMAX at a glance
| Tamiya Stock Class | BMAX (organiser class) | |
|---|---|---|
| Published by | Tamiya (current: 11 May 2026) | The event organiser, not Tamiya |
| Chassis cutting/shaving | Not allowed | Depends on that event's rules |
| Parts sourcing | Tamiya Mini 4WD / R/C Mini 4WD / Dangun Racer only | Whatever the organiser permits |
| Consistency across events | Same rules wherever "Official Stock Class" is stated | Can differ meeting to meeting |
| Where to verify | Tamiya's Stock Class regulations page | Organiser's rule sheet + on-day inspection |
How to find out which class you're actually racing
- Ask the organiser directly whether the event runs Tamiya's Official Stock Class, the general Open regulations, or a house BMAX-style ruleset — never assume from the name on the flyer.
- If it's Stock Class, treat the 11 May 2026 revision on Tamiya's page as your baseline and re-check it before the event in case it's updated again.
- If it's BMAX or any organiser-named class, ask for the written rule sheet, not a verbal summary — specifically on chassis modification, permitted parts brands, and motor limits, since that's where BMAX diverges most from Stock Class.
- Bring your machine to technical inspection before racing regardless of class; see the rules and technical inspection checklist for what officials typically check.
- Photograph your build before any irreversible modification, as a record if a borderline part gets questioned.
Frequently asked questions
Can I run the same car in both Stock Class and BMAX?
Sometimes, but don't assume it. A car built strictly to Stock Class rules — no chassis cutting, Tamiya-only parts, unmodified gears and rollers — is a safe baseline that clears most BMAX rule sheets too, since BMAX is rarely stricter than Stock Class on chassis integrity. But a car built around BMAX's looser allowances will very likely fail Stock Class inspection. Check both rule sheets before entering both classes with one machine.
Does Tamiya define BMAX anywhere?
No. BMAX doesn't appear on Tamiya's official regulation pages. It's organiser terminology, and its rules exist only where that organiser has published them — there's no central Tamiya reference to fall back on.
What chassis platforms can I use in these classes?
Any chassis on Tamiya's current compatible-parts catalogue — MA, MS, ME, VZ, AR, FM-A and others — is eligible under the general machine-type list; Stock Class doesn't exclude chassis by name, only certain modifications to whichever one you choose. See the chassis compatibility list before picking a base kit.
Rules and revision dates above reflect Tamiya's Official Mini 4WD Race Regulations and Stock Class Regulations as reviewed for this article; BMAX rules are set entirely by individual organisers. Regulations change, and officials retain final authority to disqualify at their discretion — confirm the current official pages and that event's own rule sheet before building or entering, and treat on-day technical inspection as the final word.