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Junior BamBam

Junior Pro 4.2

Junior Gearbox 125cc/140cc

Senior Stock Pitbike 140cc

Vets Stock Pitbike 140cc

Senior Open

Senior MiniGP/Moto 4

Stock Classification - Additional Information


These must be as supplied by the engine manufacturer and can only be replaced with a like-for-like product. You CAN lap the valves in using a grinding paste but you CANNOT cut the valves in or re-seat them other than lapping in.


Must be standard. High compression pistons are not allowed.


Must be as supplied as standard for your engine type and can only be replaced with a like-for-like component.


Can be replaced with a lightened inner or outer rotor kit along with a lightened oil spinner if required depending on your class regulations. The oil spinner cannot be removed.


The crank must remain STANDARD and must NOT be altered in anyway.


The timing sprocket must be non variable but a variable stator plate may be fitted if required, unless stated otherwise in your individual class regulations.


Only either the standard cam supplied with the original engine, a like-for-like replacement or a Z40 Cam. No other profile cam will be allowed.


The compression level must be below the level set for the class and will be taken post race.


Can only be replaced with a like-for-like product as supplied with your engine type; i.e. a Zongshen Z140 gasket cannot be fitted on a YX140 engine or vice versa.


All Machines must have adequately packed and baffled exhausts to obtain an exhaust noise level of below 98db when tested on a track side drive-by test and below 105db when tested in a static text.


The engine must remain as supplied by the manufacturer and components from one engine cannot be used with another. For instance using a Z140 Barrel & Head on a YX140 bottom end is
NOT permissible. You can only install a replacement part that is like-for-like; i.e. you cannot install a TB head on a Stock 140 or 160 engine.


YX140 & Z140 = 143.26cc, YX160 = 160.4cc

Z155 = 155.88cc, YX150 = 150.15cc, YX88 = 88.90cc

YX110 & Z110 = 107.0cc.

These figures allow a 0.025% tolerance over the actual capacity, this is to allow for the build quality of the Chinese engines used in the classes.


All engine serial numbers and markings must be present and cannot be removed or defaced.

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