Thunder Stock Rules

All drivers are required to have their own Raceivers for one-way communication with race director official.

Top 5 feature finishers will be required to go to tech area after the race.

  1. Inspections may occur anytime at the discretion of a race official.
  2. All cars must be strictly stock for make and model of car from front bumper to rear bumper. Minimum wheel base of 107″. 
  1. 3200 lbs. with driver

    2. Ballast allowed, but must be attached with (2) two steel ½ bolts. Ballast must be painted white with car number and must meet safely rules contained herein.

  1. Complete bumper-to-bumper OES stock frame for make and model of car. 2″x 3″ framing may be used from stock rear shock mount (rear kick up) to rear bumper. “2 x 3″ framing MUST appear in stock location. Cars with actual stock rear subs, 2″x 3” bar bracing may be used but still must retain stock appearance.
  2. Unibody cars. Unibody cars that have been welded to create full frame car permitted.
  3. 107″ minimum wheel base.
  4. Rust spots may be repaired.
  5. Any repairs to front stub must retain exact OEM measurements for make and model of car. These repairs are subject to track official approval.
  6. No switching or shifting of any body part or frame.
  7. Roll cage must be composed of a minimum 1½” OD .090 thickness round steel tubing.
  8. A four-post roll cage must be installed with minimum 3 door bars on driver’s door. Driver’s door must have 1/8″ steel plate welded.
  9. Minimum 2 bars for passenger side door.
  10. Bars may be added from firewall forward to protect frame and radiator.
  11. Main roll cage can be braced rearward to the kick up of the rear frame.
  12. Drive shaft hoop is mandatory and must be installed 6-8″ behind transmission tail shaft. No chains allowed.


  1. Stock appearing OEM steel body for make and model of car. Sheet metal skins allowed for fenders, doors, hood, and trunk ONLY and MUST have body lines. Sheet metal skins MUST be same thickness or as close thickness as stock OEM steel bodies. No aluminum body pieces allowed.
  2. Complete firewall must seal trunk from driver’s compartment. 16 gauge steel sheet metal minimum.
  3. No Mirrors
  4. A fuel cell is mandatory. Fuel cell must be mounted in trunk area between frame rails. Fuel cell must be above center line of rear end differential housing or 12″ above ground, whichever is highest. For more specific detail, see Fuel Cell Installation section.
  5. Trunk floor may be cut to accommodate fuel cell. Minimum of two, 12 gauge 1½” safety straps.
  6. 4″ clear spoiler optional. 3 braces mandatory.
  7. Roofs must look oem & steel
  8. No flat roofs
  9. back tail section must be closed, no open tail sections
  1. All glass, headlights, tail lights must be removed prior to arrival at the track.
  2. No broken glass shall be in the car. Car must be swept clean.
  3. 4 bars minimum in front of driver in window. Bars must be 3/8″ minimum with wire mesh fastened.


  1. OEM stock suspension only for make and model of car.
  2. Lower A-arms must be OEM stock for make and model. No aftermarket ball joints. Front upper A-arms may be non-adjustable tubular steel in stock location. No welding or altering of upper A-arms allowed.
  3. Rear suspension arms must be OEM stock for make and model. No aftermarket rear suspension arms permitted.
  4. Stock steering box. Stock pitman arm. Stock center link. Stock tie rod ends. Heim joint style tie rod ends/steering linkage permitted.
  5. No coil over, overload, cargo ratchet or air assisted shocks permitted.
  6. Racing shocks permitted. MSRP $200 Max. per shock, No bulb shocks, Non-adjustable shocks. No Schrader valves. Steel shocks only. Shocks must maintain stock OEM for front.
  7. No screw jacks or adjustable cups in front.
  8. Racing springs allowed.
  9. Metric cars may reinforce OEM upper A-frames only.
  10. No lift bars or recovery shocks or spring recoils.
  11. Steering quickener permitted.
  1. Steel brake calipers only.
  2. Cars must have functioning brake on each wheel.
  3. Rear disc brakes allowed. Brake assemblies must be in stock location on all wheels.
  4. No brake bias or proportional valve permitted.
  5. After market pedals allowed.
  6. Steel hubs only.
  1. Fuel cell mandatory. OEM gas tank must be removed.
  2. Bracing in trunk permitted
  3. Trunk floor plan may be cut out to accommodate fuel cell.
  4. Fuel cell must be mounted in trunk area between frame rails with at least 1½” clearance on all four sides. Minimum of two 12 gauge 1½” wide safety straps mandatory.
  5. No electrical fuel pumps.
  6. Fuel line from trunk to engine compartment must be steel or steel braided. No copper or rubber tubing allowed. Any fuel line that runs through driver’s compartment must be enclosed in rigid tubing from inside trunk area to inside engine compartment and must be sealed on both ends. Fuel line system must be approved by race official.
  7. Fuel cell will be minimum of 12″ above the ground or above center line of rear differential housing, whichever is highest.


  1. Engine block will be stock OEM small block passenger car for make and model of car.
  2. No 400 blocks.
  3. Chevy/GM 360.4 c.i.d max displacement, Ford 362 c.i.d max displacement, and Chrysler 367 c.i.d max displacement, 360ci max. No aftermarket blocks.
  4. Blocks numbers must be legible upon inspection.
  5. Engine must remain stock. (see below specifics)
  6. All engines must be stock stroke. 5.7in rods mandatory on Chevy engines.
  7. Engine studs allowed.
  8. Solid or hydraulic cams, no roller cams allowed. Roller Rockers permitted. No stud griddles
  9. Engine will be mounted in stock OEM position. No alterations of front sub or cross member.
  10. Solid steel nonadjustable engine mounts may be used.
  11. No engine swapping between make and model of car. (no Chevy engines in Ford cars etc.)
  12. Dome pistons not allowed. Flat top pistons allowed.

Cylinder Heads

  1. Steel stock heads only. Camel hump heads and vortec cast iron heads legal.
  2. No aftermarket or bowtie heads allowed.
  3. No angle plug, or any type of factory performance heads allowed.
  4. No Ford SVO heads.
  5. No aluminum heads allowed.
  6. No port work allowed.
  7. Aluminum valve covers with raised crank case breathers/filters permitted

Intake Manifold and Carburetor

  1. One Holley 2 bbl 4412 500cfm with stock 1.378 max venturi bore ONLY.
  2. Manifold must be cast iron or Dual Plane Aluminum and unaltered.
  3. Intake Manifold must be strictly stock for make and model of car. No alterations, porting, or polishing.
  4. No high-rise truck or marine type intake manifolds .
  5. Carburetor may be pulled for intake inspection per race official’s discretion.
  6. No fuel injection allowed.
  7. Aftermarket air filter housing and filter allowed.
  8. No carb spacers unless cleared by tech.
  1. Cast iron exhaust manifolds or headers allowed. No cross over or 180 headers
  2. No center dump exhaust systems.

Fuel System

  1. No electric fuel pumps allowed.
  2. Stock manual fuel pump mounted in stock location on engine.
  3. Fuel shut-off valve highly recommended.
  4. E85 fuel allowed. Race Gas is permitted, No additives of any kind allowed. No pure alcohol

                                             Electrical / Battery

  1. No dual point distributors.
  2. Stock coil in stock location. Stock HEI distributor only.
  3. Battery must be securely mounted inside plastic battery safety box.
  4. Battery box will be securely mounted to frame or roll cage bars behind driver’s seat.
  5. Plastic safety box will be securely mounted to car.
  6. Ground wire will be directly attached to frame or roll bar.
  7. A battery disconnect switch/kill switch with clearly marked on/off label is required within reach for safety officials.

Cooling System

  1. Stock radiator or Aluminum Racing Radiator single and dual pass  in stock location permitted
  2. Aluminum water pump allowed.
  1. Stock OEM manual or automatic allowed.
  2. OEM stock turbo 350 or 400 automatic with 3 forward gears and 1 reverse gear allowed. Stock torque converter only.
  3. Stock OEM manual transmissions allowed with stock single disc clutch inside an explosion-proof steel bellhousing. No aluminum transmissions. No Double or Triple Disc Clutches.
  4. Aftermarket clutch pedal assembly permitted no dual or triple disc clutch assemblies allowed.
  5. No aftermarket made-for-racing transmissions allowed. (i.e. Bert, Brinn, Falcon)
  6. Powerglides ok.
  7. Steel flywheels only
  8. Transmission must function at engine idle.
  9. No direct drives
  1. Steel Ford 9″ floater rear end allowed.
  2. Stock steel mounts must be in stock location for year and make of car.
  3. Must remain stock.
  4. Posi-traction limited slip, locked or welded, spider gears allowed.
  5. Full spool or mini spool only. 5″ Trailing Arm Brackets MAX.
  6. No locker rear ends.
  1. Reinforced racing type steel wheels mandatory.
  2. Bead locks to be used only for right side.
  3. 1″ lug nuts mandatory on all wheels.
  4. Maximum wheel width is 8″.
  1. Tires must be 60 or 70 Series DOT approved radial tires.
  2. 15″ DOT street tire allowed.
  3. Grooving, siping, and/or soaking is not permitted.
  4. Absolutely no bias-ply or racing tires disguised as DOT street tires.


See “General Safety Rules” page for track specific safety rules.


Plymouth Speedway reserves the right to change, delete or add to rules throughout the racing season. The management reserves the right of interpretation of all rules and the right to inspect and reject any car and/or driver, crew member, etc. during the racing program.


For any questions on the Plymouth Speedway rules, please call Tech Director Steven Woods 574-406-5615