For a more detailed description of the classes you can check out our 2012 USLMRA Official Rule Book
KS (Kids Stock): 6 1/2 horse power, rotary engine, 15:1 gear ratio, 5 mph max speed. For kids 6-10 years old.
Stock: Just that, remove the blades, drive it out of the shed and bring it to the track. 3650 rpm max. (Low or No Budget)
JP Class: Class for kids ages 10-15. IMOW rules apply.
IMOW (International Mowers Of Weeds): Same rpm, limited chassis mods, 8:1 overall gearing. A faster “Stock” class. Engines up to 15 hp. (Low Budget) Front engine; highly regulated.
Prepared Class: Full-bodied mowers with allowed modifications and engine size rules. Engines can be modified.
AP: 8 1/2 hp limit, usually older mowers that come with engines under 8 1/2 hp.
SP: 12 hp flat head engine limit. More common in late model lawn mowers.
CP: Over 12 hp OHV single cylinder.
BP: Up to 20 hp block v-twin cylinder engines. This class sounds like a pack of Harley’s. The v-twins can be built to over 40 hp with aftermarket engine parts. They have lots of power but are big, heavy and a handful to drive. Fast and loud.
FX (Factory Experimental): Engines limited to a single cylinder up to 465cc (12-14 hp). Must be a mower engine. Lots of modifications allowed. Tran-drive train open. This class, while restricted power-wise, is usually light and fast. It can have a kart clutch or torque converter drive systems. Major modifications allowed.