BUILD IT RIGHT.
BRING IT RACING.

The current website labels Junior as in development. Its sheet also says live-axle/centrifugal-clutch gearing is to be determined. Confirm eligibility and gearing with the club.

01Type

Full-size lawn tractor, front engine. Mid-engine and rear-engine chassis are not allowed.

02Engine

Governed engine: 3,650 maximum RPM, no load. Single-cylinder overhead-valve engine must be stock with a stock carburetor, not exceeding a factory rating of 33 cubic inches.

03Exhaust

Design is open, but the exhaust must terminate away from the driver and competitors in a rearward and downward direction, presenting no safety hazards. A muffler is recommended but not required.

04Driveline

Transmission or transaxle must be shiftable and of lawnmower origin. Gear ratio in the highest gear must be no less than 8:1. Maximum motor pulley: 6″, no exceptions. Live axle and centrifugal clutch allowed; gearing to be determined.

05Chassis

Stock stamped-steel frame rail or 1″ × 2″ steel, 16 gauge minimum, allowed. Reinforcing the frame is allowed and recommended. Minimum ground clearance: 4″ from ground to frame.

06Body

Must be stock and securely fastened to the chassis. Rear fenders may be trimmed to clear tires or widened to cover tread, but may not be wider than the tires.

07Deck

SoCal exception — all classes: A mower deck or simulated deck is not mandatory. Proper running boards with safety nerf bars are required. Running boards without the deck appearance are permitted. We encourage every member to make their racing mower look as much like a riding lawnmower as possible. Cutting blades must be completely removed.

08Front axle

Must remain in the stock location. Stock front axle must be bolted or welded solid; no articulation allowed.

09Rear axle

Transaxle or live axle allowed. Live axle must be steel; no aluminum axle allowed.

10Brakes

Axle-mounted rear brake.

11Wheelbase

39″ minimum.

12Track width

40″ maximum, sidewall to sidewall.

13Wheel size

6″ tall front wheel; 8″ tall rear wheel.

14Tires

Lawnmower tires only. Rear tire: 16″ maximum tire size as written on the sidewall.

15Kill switch

Mower must be equipped with a kill switch that turns off the ignition if the driver leaves the mower. An ATV or jet-ski wrist-lanyard kill switch is recommended. A stock seat-pressure switch is not allowed.

Updated September 29, 2026. Based on LSMRA GPS/GPT governed rules, with the SoCal centrifugal-clutch and chain-drive exception. Single- and twin-cylinder mowers share the local Stock class. The shared requirements below and referenced rulebook also apply.

01Type

Full-size lawn tractor, front engine. Mid-engine and rear-engine chassis are not allowed.

02Engine

Stock single- or twin-cylinder four-stroke lawnmower engine, governed at 3,650 RPM maximum with no load (clutch engaged, transmission in neutral where fitted). Twin engines may not exceed the factory rating of 810 cc / 49 cubic inches; no big blocks. Original stock flywheel and stock air-filter assembly required. Direct stock-replacement consumable filters and spark plugs are allowed. No internal engine modifications, overhead-cam engines or liquid cooling.

03Exhaust

Design is open, but the exhaust must terminate away from the driver and competitors in a rearward and downward direction, presenting no safety hazards. A muffler is recommended but not required.

04Driveline

A centrifugal clutch and jackshaft arrangement may replace the shiftable lawnmower transmission or transaxle. Chain drive is permitted using #35 chain from the engine to the jackshaft and #35 or #40 chain from the jackshaft to the drive axle. An axle-mounted rear brake and guards over exposed chains and sprockets are required. Torque converters are prohibited.

05Chassis

Original factory stamped-steel, front-engine lawnmower frame. Discrete reinforcement is allowed; a fabricated replacement frame is not allowed. Minimum frame clearance: 4″.

06Body

Must be stock and securely fastened to the chassis. Rear fenders may be trimmed to clear tires or widened to cover tread, but may not be wider than the tires.

07Deck

SoCal exception — all classes: A mower deck or simulated deck is not mandatory. Proper running boards with safety nerf bars are required. Running boards without the deck appearance are permitted. We encourage every member to make their racing mower look as much like a riding lawnmower as possible. Cutting blades must be completely removed.

08Front axle

Must remain in the stock location. Stock front axle must be bolted or welded solid; no articulation allowed.

09Rear axle

Transaxle or live axle allowed. Live axle must be steel; no aluminum axle allowed.

10Brakes

Axle-mounted rear brake.

11Wheelbase

42″ minimum, unaltered.

12Track width

40″ maximum, sidewall to sidewall.

13Tires

Tires must comply with the LSMRA Prepared-class tire provisions and match in size, brand and tread design per axle. No kart tires, slicks, Snow Hog, tiller, ATV, commercial-mower or performance-compound tires. Metal wheels must match in size per axle; no dual or tandem wheels.

14Kill switch

Commercial racing tether kill switch mechanically attached to the driver and mower required. It must stop the ignition when the driver leaves the mower. No Velcro, pressure switches, homemade or magnetic switches, or bypasses. Engine must remain off until the driver is mounted.

15Seat

The bottom of the seat must be at the top of the rubber of the rear tires, or at least 13″ above the ground.

Updated September 29, 2026. One combined local Modified class for single- and twin-cylinder engines. LSMRA Prepared-class engine-modification requirements apply, with the SoCal drivetrain, seat, deck and width exceptions below. This local class is not a separate national CP or BP classification. Shared Prepared-class requirements below also apply.

01Type

Full-size lawn tractor, front engine. Mid-engine and rear-engine chassis are not allowed.

02Engine

Single- and twin-cylinder, four-stroke lawnmower engines compete together in one Modified class. Aluminum billet flywheel required. Engine must be originally manufactured for lawn-mowing equipment. Internal modifications are permitted. Engine must remain stock in appearance except for the air filter, air cleaner or velocity stack. Crankshaft must remain in its original orientation and frame clearance hole. Original cylinder block, cylinder heads, sump and side covers must be retained. Billet side covers, billet sumps and aftermarket cylinder heads are prohibited. Billet or aftermarket valve covers are permitted for roller-rocker clearance. External ignition systems are permitted. No overhead-cam or liquid-cooled engines.As the Modified class grows, and enough single-cylinder mowers participate to form their own class, single-cylinder and V-twin mowers will be separated into their own classes.

03Exhaust

Design is open, but the exhaust must terminate away from the driver and competitors in a rearward and downward direction, presenting no safety hazards. A muffler is recommended but not required.

04Driveline

A centrifugal clutch and jackshaft arrangement may replace the shiftable lawnmower transmission or transaxle. Chain drive is permitted using #35 or #40 chain from the engine to the jackshaft and #35 or #40 chain from the jackshaft to the drive axle. An axle-mounted rear brake and guards over exposed chains and sprockets are required. Torque converters are prohibited.

05Chassis

Full-size, front-engine lawnmower with its original factory stamped-steel frame. Discrete reinforcement is allowed. Minimum frame-to-ground clearance: 4″.

06Seat

The bottom of the seat must be at the top of the rubber of the rear tires, or at least 13″ above the ground.

07Body

Must be stock and securely fastened to the chassis. Rear fenders may be trimmed to clear tires or widened to cover tread, but may not be wider than the tires.

08Deck

SoCal exception — all classes: A mower deck or simulated deck is not mandatory. Proper running boards with safety nerf bars are required. Running boards without the deck appearance are permitted. We encourage every member to make their racing mower look as much like a riding lawnmower as possible. Cutting blades must be completely removed.

09Front axle

Must remain in the stock location. Stock front axle must be bolted or welded solid; no articulation allowed.

10Rear axle

Transaxle or live axle allowed. Live axle must be steel; no aluminum axle allowed.

11Brakes

Axle-mounted rear brake.

12Wheelbase

42″ minimum, unaltered.

13Track width

SoCal maximum: 42″, measured sidewall to sidewall.National USLMRA Prepared-class rules specify a 40″ maximum. The LSMRA reference includes a 42″ exception specifically for CP; it is not a universal allowance. A mower wider than 40″ may be ineligible to race outside Southern California. Check the host organization’s class rules before entering.

14Tires

Lawnmower tires; no kart tires or slicks. Tires must match in size, brand and tread design on each axle. Minimum rim diameter: 6″ front and 8″ rear.

15Kill switch

A commercially available racing tether kill switch must shut off the ignition when the driver leaves the mower. Tether must be mechanically attached to driver and mower. No Velcro attachments, pressure switches, magnetic or homemade switches, or bypass switches. Engine must remain off until the driver is mounted.

Updated September 29, 2026. LSMRA Factory Experimental (FXS/FXT) requirements apply within the combined SoCal Super Modified class. SoCal specifies a 42″ minimum / 52″ maximum wheelbase for every mower and allows 2″ minimum frame ground clearance. Engine-specific requirements remain applicable. See Section 18 for SoCal driver approval and out-of-area eligibility.

01Type

Full-size lawn tractor, front engine. Mid-engine and rear-engine chassis are not allowed.

02Engine

Lawnmower engines or direct replacements permitted under LSMRA FX rules. FXS configurations include single-cylinder OHV or flathead engines and opposed-twin flatheads up to 46 cubic inches. FXT V-twins must be small-block engines, maximum 810 cc / 49 cubic inches; no big blocks. Internal and external modifications are permitted within the FX restrictions. Retain the original engine block assemblies; billet side covers and sumps are allowed, but aftermarket cylinder heads are prohibited. Aluminum billet flywheel mandatory. Cover rotating assemblies. No electric fuel pumps, overhead-cam engines or liquid cooling.SoCal Outlaw engine class: The SoCal Mower Racing Club also offers an Outlaw engine class. You must start with and retain a small-block lawnmower engine of 49 cubic inches or less. No big blocks. Beyond those requirements, engine modifications are unrestricted. This engine exception applies to the Outlaw class; chassis and safety requirements still apply.

03Exhaust

Design is open, but the exhaust must terminate away from the driver and competitors in a rearward and downward direction, presenting no safety hazards. A muffler is recommended but not required.

04Driveline

Live axle, belt or chain drive, custom gearboxes or transaxles, 90-degree gearboxes and jackshaft arrangements are permitted under LSMRA FX rules. Centrifugal and traditional belt clutches are permitted. Chain guards must be at least 1/8″ steel or aluminum. No ATV drivetrain, suspension or all-wheel drive. Driveshafts must be covered, connect to a gearbox rather than directly to the rear axle, and have a clutch/disconnect system.

05Chassis

OEM-based or fabricated tube chassis permitted under LSMRA FX provisions. Rails between front and rear axles must be continuous, matching-size tubing and parallel, no more than 15″ apart under the source wording. Weld the front axle to the full tube dimension; no pinching or swaging the frame at the axle. A third right-side bearing support is recommended. SoCal minimum frame-to-ground clearance: 2″. Confirm the rail measurement method with club technical inspection before fabrication.

06Body

Must have a stock-style hood, securely fastened to the chassis. Rear fenders should resemble lawnmower-style fenders to retain lawnmower appearance.

07Steering

Direct steering mandatory. Push-pull steering is permitted only when the steering wheel turns in the same direction as the front wheels. Lock nuts required on connections; ball-type or spherical threaded rod ends required. Plastic steering wheels must be steel reinforced.

08Rear axle

Rear wheels must be secured with lock collars. No axle or wheel-mounting projections beyond the tire sidewall.

09Brakes

Hydraulic brakes required, acting on the rear axle. Front brakes are not mandatory, but are allowed if the driver so desires. A clutch lockout is required if there is no neutral gear so the mower can be pushed by hand. With a centrifugal clutch, start only with the driver seated or rear wheels securely off the ground.

10Wheelbase

42″ minimum and 52″ maximum for all SoCal Super Modified mowers. LSMRA’s unaltered-wheelbase wording otherwise remains applicable.

11Track width

42″ maximum, sidewall to sidewall. Maximum total axle offset: 4″. The Lone Star rulebook illustrates this with measurements of 23″ from frame center to the outside of the right tire and 19″ to the outside of the left tire. Refer to page 23 of the Lone Star reference rulebook linked above for this measurement example.

12Tires

Lawnmower, dirt-kart and mini-sprint tires authorized under LSMRA FX rules; no slicks. Match tire sizes per axle. Minimum rim diameters: 5″ front and 6″ rear. Custom steel or aluminum rims permitted; stock lawnmower wheels prohibited.

13Kill switch

Commercial racing tether kill switch mechanically attached to driver and mower required. It must stop ignition when the driver leaves the mower. No Velcro, pressure switches, homemade or magnetic switches, or bypasses.

14Minimum weight

Including driver: 450 lb for FXS engine configurations and 500 lb for FXT engine configurations. Added ballast or parts installed for weight must be painted white.

15Bumpers and side bars

Side/crash bars and a closed-loop rear bumper are mandatory. Follow the LSMRA FX construction and clearance requirements. No sharp edges. Front bumpers may protect the hood only, not the front tires. Confirm conflicting bumper-clearance wording in the source with technical inspection.

16Front axle

One-piece beam axle; no aluminum beam, suspension or shock absorbers. Minimum spindle diameter: 5/8″. Spindles must be drilled and cotter- or safety-pinned. Eliminate stock pivoting. Custom axle must be near the original position, not on top of the chassis.

17Seat

Seat must be centered side-to-side on the frame. Go-kart seats are permitted.

18Driver eligibility

For events outside Southern California, follow the host organization’s driver eligibility, experience and licensing requirements. The LSMRA FX rules generally specify age 18 or older with more than one full year in Prepared racing, with applicable younger-driver exceptions and approvals.SoCal exception: The SoCal Mower Racing Club does not have an official licensing system. All Super Modified drivers will be authorized or denied participation by club management on a case-by-case basis. Local approval does not establish eligibility for events run by other organizations.

19Running boards / deck

SoCal exception — all classes: A mower deck or simulated deck is not mandatory. Proper running boards with safety nerf bars are required. Running boards without the deck appearance are permitted. We encourage every member to make their racing mower look as much like a riding lawnmower as possible. Cutting blades must be completely removed.

SHARED REQUIREMENTS

SoCal exception — all classes: A mower deck or simulated deck is not mandatory. Proper running boards with safety nerf bars are required. Running boards without the deck appearance are permitted. We encourage every member to make their racing mower look as much like a riding lawnmower as possible. Cutting blades must be completely removed.

For Stock, Modified and Super Modified, the applicable general mower, safety, inspection and class provisions in the linked 2026 LSMRA reference apply unless expressly replaced by a SoCal exception above. Class-specific exceptions take priority over general requirements.

• Two throttle-return springs with separate attachment points; secure throttle cables.
• Cover exposed chains, sprockets and flywheels. Secure batteries, insulate positive terminals and clamp fuel lines.
• Remove cutting blades. Secure all body panels and eliminate sharp edges and dangerous projections.
• Use compliant tether kill switches, steering fasteners, wheel retention and axle-mounted rear brakes.
• Fuel must comply with the source’s pump-gasoline provisions. No additives, prohibited induction/injection systems or electric fuel pumps.
• Stock and Modified: no offset or stagger, no front bumpers, actual mower seats, and the Prepared-class wheel-retention and body requirements apply.
• Full-face DOT- or Snell-approved helmet, goggles or face shield, approved racing neck support, long pants, long sleeves, full-finger gloves and leather over-ankle footwear required. Bring a full 10BC fire extinguisher.
• Technical inspection is required before practice or racing. Driver eligibility, visible race numbers and applicable licensing requirements remain in force.

Where the reference has inconsistent deck, tire, bumper or licensing wording, obtain technical clarification before building.