Part 5 - Component by Component

9. Spark plugs

Must be standard and unmodified from the following list only, no other can be used. The standard Honda resistor spark plug cap - as supplied with the engine - must be used where a non-resistor spark plug is used, otherwise plug cap is free.

Permitted spark plugs


Nippondenso W20EP-U W20EPR-U W16EP-U W16EPR-U

10. Bodywork / ducting

All of the engine bodywork and ducting must be standard and unmodified except for the drilling of a small hole to accept one end of a throttle return spring or security fixing. The pull-cord mechanism must be standard and unmodified, although the pull-cord starter may be rotated on its standard mounting holes. The on/off switch must be fitted and be capable of stopping the engine. A shroud may be attached to the casing to protect the switch, if required.  All or any of the bodywork / ducting can be painted or chromed.

11. Rocker cover

Rocker cover must be standard and unmodified, although it may be painted or chromed. Its valve must be present and in working order. The breather pipe must be in position and intact, of suitable length that it is securely fixed in both the rocker cover and the outlet of the airbox, and have no perforations or leakage points. Cover interchangeable between all engine types.

12. Valve Gear

The valve rocker studs must be standard and unmodified. On the T1 engine only, the inlet valve collet (PN 14771-ZE1-000) may be replaced with an exhaust valve collet (PN 14773-ZE1-000) and an exhaust valve rotator (PN 14781-ZE1-000). If this modification is performed to the inlet valve, it is permissible to fit one 8mm washer between the cylinder head and the base of the valve rocker post to raise the post and ensure that the adjuster locknut sits on a full thread. The valve spring used must offer no mechanical advantage over a standard GX140 valve spring (PN14751- ZE1-000) i.e. a force of 5 kg will compress the spring to less than 25mm overall length, or a spring which offers no mechanical advantage over a standard QHG4 spring (PN 14751-ZH8-9400) i.e. a force of 8 kg will compress the spring to less than 18.5mm overall length. Valve rockers, cam followers and pushrods must be standard and unmodified.

13. Valves

Valves will be standard and unmodified. Valve-seat grinding and cutting is allowed, to the single standard profile only (45°) and 30° cut to restore seat width, as specified in the latest Honda manual for the engine type. Valves of T1 and T2 engine are not interchangeable. 

T1   T2
25mm Inlet valve* 26mm
24mm exhaust valve* 23mm
62.2mm exhaust valve length* 63.7mm