Mini 4WD Stock Class Setup
Tamiya's official Stock Class rules do not do what most builders assume. They barely touch motors — Dash-series motors are explicitly permitted — and instead lock down where extra parts can come from, what the chassis may become, and how tyres, rollers and mass dampers may be prepared. Stock Class reads closer to a "no aftermarket parts, no cutting the chassis" rule than a "slow motor only" rule, and that distinction is the difference between a car that clears technical inspection and one pulled at the table.
What Stock Class actually restricts
Per Tamiya's official Stock Class regulations, chassis cutting, shaving and homemade chassis are all prohibited outright, and you cannot reverse a chassis' front-to-rear orientation. You also cannot drill new screw holes anywhere on the chassis — but you may countersink an existing hole and drill an existing hole out to 2mm, and you're free to combine any chassis with any A-parts set even if Tamiya never sold that pairing as a kit. Every additional part fitted to the car must come from the Tamiya Mini 4WD, R/C Mini 4WD or Dangun Racer ranges — no generic third-party hop-ups — and modifying any part other than the body is prohibited. The body itself gets more latitude: it can be shaved down and cut for clearance, it does not need a driver figure or rear wing attached, you are not disqualified if body pieces or a driver figure fall off mid-race, and clear/polycarbonate bodies are allowed as long as they're cut per the kit manual rather than split into multiple pieces.
Motors, tyres, rollers and dampers under Stock Class
Dash-series motors are legal in Stock Class, so the class is not the place to look for a motor cap — see the motor guide for what's actually banned tournament-wide (Ultra-Dash and Plasma-Dash, neither of which appears on the permitted-motor list). What Stock Class does restrict is what you do to a motor: taking one apart and swapping internals is not allowed. Tyres cannot be cut or shaved, though mixing tyre and wheel sizes within a set is fine. Ball bearings are permitted, but you cannot replace the bearing inside a bearing-type roller with a different one, and a roller worn down to a "markedly small" diameter fails inspection at the staff's discretion — the same discretionary standard applies to mass-damper holes that have visibly widened through use, while deliberately widening a damper hole yourself is banned outright. FRP and carbon stays may be cemented together or reinforced with cement, but may not be mounted loosely or have their edges tapered down.
Stock Class quick-reference table
| Item | Allowed | Not allowed |
|---|---|---|
| Chassis | Countersink/drill-out an existing hole to 2mm; mix any chassis with any A-parts set; race without the battery holder fitted | Cutting, shaving or building a homemade chassis; new screw holes; reversing chassis direction |
| Motor | Any current Dash-series or Tuned motor | Using parts from a disassembled motor |
| Body | Shaving/cutting for clearance; clear or polycarbonate bodies; racing without a driver figure or rear wing | Splitting the body into separate pieces; using leftover sprue/offcuts as parts |
| Tyres/wheels | Mixing tyre and wheel sizes | Cutting or shaving tyres |
| Rollers | Ball-bearing rollers | Swapping the bearing inside a bearing roller; markedly worn-down rollers |
| Stays/plates | FRP or carbon, cemented or reinforced with cement | Loosely mounted plates; tapered plate edges |
| Mass dampers | Standard fitment | Deliberately widened damper holes |
The baseline every class, including Stock, still has to clear
Stock Class rules sit on top of Tamiya's general race regulations, not instead of them. Those set a minimum car weight of 90g including the battery and motor, an overall size cap of under 165mm long, under 105mm wide and under 70mm tall, and a minimum ground clearance of 1mm. Tyres must fall within 22–35mm diameter and 8–26mm width, and only Tamiya-branded AA batteries are permitted — no third-party cells. The permitted machine types span Racing, Super, Fully-Cowled, Aero, Laser, Mini 4WD REV, Mini 4WD PRO, Mighty, R/C and Truckin' Mini 4WD, so a Stock Class build still has to start from one of those families before the class-specific restrictions above apply on top.
The May 2026 forbidden-zone addition
Tamiya's Stock Class page carries a change dated 11 May 2026 that adds a "forbidden zone": the area between the front and rear shafts and bearings, plus the rear half of the front wheels and the front half of the rear wheels. Only the body and driver figure are permitted inside that zone — nothing else may be mounted or positioned there. In practice this closes off tucking extra plates, weights or brackets underneath the chassis between the axles, a spot builders had used to add ballast or reinforcement outside the usual roller-and-stay area. Rules and their effective dates change, so check the current wording on Tamiya's own Stock Class page before you build around it, rather than this article.
Where the real gains are with parts fixed
Once the parts list is capped, the field compresses onto build quality rather than shopping. Deburring gear teeth and cleaning moulding flash off the gearbox — covered in the gear guide — reduces drivetrain drag for free. True axles, evenly seated bearings and consistent roller height do more for lap-to-lap consistency than any legal motor swap, since Dash motors are already on the table for everyone. Tyre trueness matters more here than usual too, precisely because you can't cut or shave a tyre into round — you have to select and mount carefully instead. None of this shows up on a spec sheet, which is exactly why it's where Stock Class races are actually decided.
Frequently asked questions
Does Stock Class ban Dash motors?
No. Tamiya's official Stock Class Q&A confirms Dash motors are permitted; only using parts salvaged from a disassembled motor is banned. The tournament-wide motor ban (Ultra-Dash, Plasma-Dash) applies regardless of class.
Can I drill a new screw hole to fit a lower roller stay?
No. New screw holes are prohibited in Stock Class. You may countersink an existing hole or drill an existing hole out to 2mm, but you cannot add a hole that wasn't already there.
Is there a Stock Class-specific weight or size limit?
Tamiya's Stock Class page does not publish its own weight or dimension figures; the general race regulations' 90g minimum weight and length/width/height caps apply. Bring a car that clears those and complies with the Stock Class part-sourcing and chassis rules above.
Organiser rules and on-day technical inspection always take precedence over this summary — check the current official race regulations and Stock Class regulations before an event, and see the technical inspection checklist for how that check plays out at the table.