Those regs are pretty good Firko. My additions/alterations in blue, and deleted stuff in green - most alterations are simply to cull unnessacary words and thereby simplify the rules:
THE 100cc FLY CLASS .
The class is open to all pre-1975 trail or enduro style machines that were originally road registerable. No machines originally designated for motocross competition are eligible. All street equipment such as lighting, side and centre stands, switchgear and superflous brackets and gizmos should be removed. Owners are encouraged to present the bikes in neat and well maintained, race-ready condition. in an endevour to instill some pride of ownership and to present a professional attitude to outsiders who are quick to throw the “shitbox” tag at our beloved vintage rides at the best of times. They’d have a field day with these low budget bikes if they weren’t presented in the best light. We don’t envision spending shitloads on anodizing and chrome bling, that’s not what the dungers are about. We know from experience that you can achieve miracles with a couple of $2.99 spray paint cans, a tube of metal polish and a bit of tasteful lateral thinking.
*The MA Manual of Motorsport rules for pre 1975 motocross and dirt track should be the overriding regulatory document if a particular point isn't mentioned in the following regs.
CAPACITY: 100 cc two stroke
125 cc four stroke.
Overbores to factory maximum only. No stroker cranks.
ENGINE:
* Aftermarket heads not allowed, cylinders from designated motocross models are not allowed. although GYT and Suzuki Race Kit cylinders and heads designated for the modification of trail type bikes are allowed.
*Ignition can be changed but it must be from the same parent manufacturer and be manufactured prior to 1975. For example, Yamaha points ignition can be replaced by MX/YZ CDI ignition. No post 1975 ignitions are allowed unless they are the same as the pre 1975 version. No PVL, Krober, Interspan or other aftermarket ignition allowed.
*Exhaust can be changed but replacement 2 stroke pipes must be of period design. No tapered header pipes or “fatty” style centre sections or pipes from later era bikes allowed. Four stroke pipes must be made from steel. All bikes must meet the MA 96db sound limit.
*Maximum carburetor size is 28mm. All carbs must be of round slide, period design. No power jet or later style carbs allowed.
*No race fuels, methanol or nitromethane. (don’t laugh. Nitro used to be a very popular fuel with some SL125 dirt track racers in the early vintage days!) Unleaded pump fuel only.
CHASSIS:
*Suspension travel is limited to 7” front and 4” rear. Maximum fork diameter is 32mm. No “Gold Valve” style cartridge emulators are allowed. Forks can be swapped from one make to another but forks must fit the pre 1975 cutoff criteria.
* Modern style gas shocks with external dampening adjustment (Ohlins, YSS, Falcon, Reiger etc)are not allowed in the interest of keeping costs down. All shocks must be of period appearance and design. Works Performance must use early finned alloy body. Early Marzocchi style piggyback shocks allowed.
*Chassis modifications are allowed but must be sympathetic with engineering parameters of the time. Swingarms can be lengthened a maximum of 2”(50mm). Engine cradles can be lowered a maximum of 1” (25mm). Swingarms must be steel, and of period design no aftermarket alloy swingarms. Steering head modifications are not allowed in the interest of safety. Steering geometry can be changed by the use of different offset triple clamps. Triple clamps must be manufactured prior to 1975.
*Engine swaps are permitted but all components must fit the Dunger class and Pre 1975 criteria. Engine swaps are only permitted where the replacement engine is a direct bolt-in fit to an unmodified frame. Both the frame and the engine must be from Fly-class eligible bikes.
* In order to make tyre choice easier, front wheel size can be changed to 21”. Back wheel is limited to 18” unless a different size came standard with the bike. Rims can be changed to alloy. Single leading shoe brake only unless a twin leader fitted as standard at the factory.
MISCELLANEOUS:
Period aftermarket plastic, fibreglass or alloy fuel tank and period fenders such as Preston Petty, Pacifico, etc are allowed provided the period look of the bike is maintained. Aftermarket mods that change the period integrity of the bike are not permitted.
No titanium components are allowed. Rubber footpegs are encouraged to be changed to serrated style in the interest of safety.
The most important thing is to have fun and respect the spirit of the era. If you feel the need to cheat and get caught out, cop it sweet and fix whatever is wrong with the bike. This is a fun class that we hope can exist without bad attitude.
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Stuff like the 21/18" wheel thng is redundant and simply add words to the regs. Ditto specifying that bikes must meet the noise limit.
My understanding is that engine swaps were very uncommon in these sorts of bikes back in the day, so I don't think that they should be allowed now. However, given that many of these bikes shared their frames/running gear with other bikes (eg: DT100/125/175), then there's no reason to prohibit the use of a 175 or 125 frame if a 100 frame cannot be found. My re-wording attempts to reflect this, but currently fails.
Apart from anything else, my gut feeling is that a DT100 motor in a TS100 frame would be the hot ticket - but allowing such swaps takes two bikes out of the pool.
I'm dubious of the shock thing as currently written, but greater minds are already on the task, so I'll keep my mouth shut on that topic.
The 'original ignition thing' is just a nightmare waiting to happen, IMHO. For example, if you're going to build a Yamaha, the only available electronic ignition would be the YZ125A bit - which are hardly thick on the ground.
