Mini 4WD Stock Class Rules: What the May 2026 Amendment Added

Tamiya's Stock Class only became its own published ruleset in December 2025, and it was amended on 11 May 2026. That amendment is not cosmetic: it added two clauses that between them decide where parts may sit on the car and how firmly they must be attached. If your Stock car was built before May 2026 and has not been re-read against the current text, it is worth ten minutes. This guide covers what the ruleset says now, what specifically changed, and what both mean for how you build.

Stock Class is newer than it looks

The main Mini 4WD race regulations record their most recent structural change as December 2025, when the Stock Regulations were added as a separate document. Before that, Stock existed at events without a dedicated published Tamiya text to point at. That matters for a practical reason: a lot of the received wisdom about what is and is not allowed in Stock predates there being an official answer, and some of it is now simply out of date.

The Stock document does not replace the main regulations. It says explicitly that Stock follows the Official Regulations with alterations to section 6, Modifications, and that everything else is governed by the main text. So it is a diff, not a standalone ruleset — you still need the general rules for scrutineering, battery and motor eligibility and race conduct, which the rules and tech inspection guide walks through.

What the 11 May 2026 amendment actually added

Tamiya marks amendments by colouring the new text red, and the page header says so: sections in red were added on 2026/5/11. There are exactly two red passages on the page. One is the header note itself. The other is clauses 12 and 13 in their entirety — so those two clauses, and nothing else in section 6, are what May 2026 introduced.

Clause 12 defines a "forbidden zone." It covers the area between the front and rear shafts and their bearings or eyelets, plus the rear halves of the front wheels and the front halves of the rear wheels across the full width of the tyre. Inside that zone, only the body and a driver figure may be attached. No other part may be mounted there — and the clause closes the obvious loophole by adding that this applies whether a part sits entirely in the zone or only partly in it.

Clause 13 requires wall-contacting parts to be rigidly mounted. Anything that touches, or could conceivably touch, a course wall must be fixed to the chassis, or to a plate that is itself fixed to the chassis. The exceptions are only for parts that need to move: wheels and tyres, rollers, and the moving components inside mass dampers and sliding dampers.

Read together, these two clauses tighten Stock considerably. Clause 12 rules out anything slung under the middle of the car — the region where a lot of mass damper and stabiliser mounting naturally wants to go. Clause 13 rules out anything that reaches a wall while floating on a pivot or hanging off a body rather than being anchored to structure. Neither is a ban on dampers as such; both are constraints on where and how.

What did not change

It is worth being clear about this, because the no-cutting rule is the one people most often assume was the recent change, and it was not. Clause 1 — no cutting or shaving the chassis, no homemade chassis — is not marked as new. Nor are the other long-standing restrictions in section 6:

Additional parts are limited to Tamiya Mini 4WD, R/C Mini 4WD and Dangun Racer parts. Non-body modifications are prohibited, with countersinking existing holes in FRP/carbon plates and the chassis, and pushing shafts through wheel holes, as the named exceptions. Body work is limited to shaving and cutting away obstructions, and body catch parts must be used. Roller combinations and drive gear combinations must follow the regulations. HG aluminium setting boards, setting gauges and pinion pullers may not be attached to the car — they are bench tools, not parts. Battery terminals must be kit-standard or grade-up parts installed as their manual instructs, with soldering and double terminals prohibited. Modifications must not risk hand injury or track damage, and grease must not be allowed to dirty the course. Batteries go in the right way round, and plates must be securely fitted without wobble.

That last one is worth pairing with clause 13 in your head: "securely fitted, must not wobble" and "must be fixed to the chassis or an affixed plate" are pointing at the same failure mode from two directions.

What this means for how you build

The practical consequence of clause 12 is that mounting real estate has moved outward. Anything you want to hang on the car now needs to live ahead of the front shaft or behind the rear one, on stays and plates anchored at the ends. That pushes Stock builds toward front and rear stay work and away from centre-slung solutions.

Clause 13 then constrains how those stays behave. A brake stay, a wide front plate or a rear plate that carries rollers has to be genuinely anchored — screwed to the chassis or to a plate that is screwed to the chassis — not resting, clipped or tied. Slide dampers remain legal because the clause names their moving components as an exception, but the damper assembly itself still has to be mounted to structure.

There is a chassis consequence too. Because clause 1 forbids cutting the chassis, a bumper-less build has to be reached without a saw. That is not new — Tamiya's own VZ copy notes that the front bumper separates as well as the rear roller stay — but the ME chassis, introduced in January 2026, is built around it: Tamiya describes its front and rear bumpers as independent, separately moulded parts, “making them easier to switch out or add plates to,” and shows the car without them. Japanese racers have been noting since early 2026 that this is a large part of why the ME suits Stock and B-MAX, along with it being the least stiff of the dual-shaft chassis. That is community assessment rather than a Tamiya claim, and it comes with a community caveat worth repeating: the roller-stay screws strip easily if overtightened, and the usual field fix is a longer screw with a nut behind it. The ME chassis overview covers the platform itself.

Finally, the obvious but necessary caveat: local organisers set their own rules, and many Malaysian and regional events run house variations or B-MAX rather than Tamiya Stock verbatim. Read the event's own regulations before you build to this one. The B-MAX rules guide covers the most common alternative.

Frequently asked questions

What exactly changed in the Stock Class rules in May 2026?

Two clauses were added, 12 and 13. Clause 12 defines a "forbidden zone" between the axles and across the inner halves of the tyres where only the body and driver figure may be attached. Clause 13 requires anything that could touch a course wall to be fixed to the chassis or to an affixed plate, with exceptions only for wheels, rollers and damper moving parts.

Does the amendment ban mass dampers in Stock Class?

No, but it restricts them. Nothing other than the body and driver figure may be mounted in the zone between the axles, so a centre-slung damper is out; damper moving components are explicitly exempted from the fixed-mounting rule in clause 13, so dampers themselves remain legal when mounted outside the zone on anchored stays.

Is cutting the chassis a new restriction?

No. Clause 1 prohibits cutting or shaving the chassis and homemade chassis, and it is not among the passages marked as added in May 2026. It predates the amendment.

Can I run a bumper-less car in Stock Class?

Only if you get there without cutting, since clause 1 forbids that. The ME chassis suits it, because Tamiya moulds its front and rear bumpers as independent parts meant to be swapped or removed; the VZ separates its front bumper and rear roller stay too. Confirm with your event organiser first — local rules vary, and the organiser has final authority.