Does this help ya?
From AHRMA site and for AHRMA races
http://dev.ahrma.org/?page_id=45
VINTAGE CLASSES:
Note: All classes are further divided into Novice, Intermediate and Expert rider ability levels.
Premier Lightweight: Certain pre-1965-era machines up to 250cc.
Premier 500: Pre-1965-era machines up to 500cc
Premier Open Twins: Pre-1965 twin-cylinder machines 600cc and larger.
100cc Motocross: 88-100cc two-stroke and 88-120cc four-stroke production motorcycles manufactured as up to 1974 model year machines and like-design.
Classic 125: 125cc two-strokes and four-strokes up to 150cc, manufactured through December 1971 and any like machines.
Classic 250: Specific 250cc and smaller two-stroke machines introduced just after the Premier Lightweight era (i.e., four-speed Spanish motorcycles, twin-pipe CZ), as well as the heavy 350cc non-unit motorcycles, such as Matchless and BSA.
Classic 500: Specific motorcycles from the late 1960s and early ’70s.
Sportsman: 125, 250 and 500 classes for machines up to model year 1974.
Sportsman Open Twins: Twin-cylinder machines up to model year 1974.
Open Age: Any AHRMA-eligible rider on an AHRMA-eligible vintage machine.
40+: Riders 40 years of age and older on any AHRMA-eligible vintage machine.
50+: Riders 50 years of age and older on any AHRMA-eligible vintage machine.
60+: Riders 60 years of age and older on any AHRMA-eligible vintage machine.
70+: Riders 70 years of age and older on any AHRMA-eligible vintage machine.
Women: Riding any AHRMA-eligible vintage machine.
POST VINTAGE CLASS'S
CLASSES
Note: All classes are further divided into Novice, Intermediate and Expert rider ability levels.
Historic 125: Certain 125cc and smaller motorcycles built up to and including the 1977 model year as the first-generation of long-travel machines.
Historic 250: Certain 126-250cc motorcycles built up to and including the 1977 model year as the first-generation of long-travel machines.
Historic 500: Certain 325-460cc two-strokes and four-strokes up to 636cc built up to and including the 1977 model year as the first-generation of long-travel machines.
Historic Four-Stroke: Certain four-stroke machines built up to and including the 1978 model year with up to 636cc. Overhead cam machines are limited to 500cc.
Gran Prix 125: Certain post-1977 motorcycles up to 125cc.
Gran Prix 250: Certain post-1977 motorcycles 126-250cc.
Gran Prix 500: Certain post-1977 motorcycles 325-500cc two-stroke and up to 580cc four-stroke.
Gran Prix Four-Stroke: Certain 185-580cc four-stroke motorcycles that came just after the Historic Four-Stroke period.
Ultima 125: Certain motorcycles up to 125cc that do not have disc brakes or power-valve type mechanism built directly into the engine.
Ultima 250: Certain 126-250cc motorcycles that do not have disc brakes or power-valve type mechanism built directly into the engine.
Ultima 500: Certain 325-580cc motorcycles that do not have disc brakes or power-valve type mechanism built directly into the engine.
Ultima Four-Stroke: Certain 200-580cc four-strokes that were produced either with a single-shock rear suspension or a newer engine technology.
Open Age: Any AHRMA-eligible rider on any eligible machine.
40+: Riders age 40 and older on any eligible machine.
50+: Riders age 50 and older on any eligible machine.
60+: Riders age 60 and older on any eligible machine.
Women: Female riders on any eligible machine.
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And for AMA races, view following link:
http://www.americanmotorcyclist.com...ments/AMA_Vintage_Motocross_Classes.sflb.ashx