This might help a bit.

the Gen numbering system is something i introduced as a way to identify the different changes made to the engines. As i was asked to help develop the range it was a way for me to help categorise the evolution of the engine.

The first pic is of an early Gen 3 , the way to tell is the top of the engine has a solid finish. It had B21 on one side and the original Go logo on the other. No serial numbers yet. C 2005 to Dec 2006.

Pic number 2 is of a Gen 4 Case , change made was milled top , carb neck dimension change. exhaust port expanded. introduced SKF front bearing. New Carb , B21 and Original Go Logo still no Cerial number , head design change. C 2007 to may 2008.

Pic number 3 is a gen 5 case , Slightly larger exhaust porting side flutes. Sealed carb restrictor bolt, new case material introduced, cerial numbers introduced on right of engine then later on the left side, New Go Logo and R.21 on the other side , carb mould changed, New rod , introduced tini coating on pin and carb for some models, head design change. C May 2008 to June 2009.

Pic 4 is of Gen 5.5 case & MG case Change seals,, Carb neck changed, Coating of case, back plate coating , New carb material , seals , manufacture process , New lightened rod , new rod and crank dimensions , tini coating on pin , carb slide and crank , tolerance fit PS set from 6.5 to 6 mm , new head design from 10 to 11 fin except for MG model still 10 fin. June 2009 to present.


Some of the improvements have over lap to the dates supplied as far as internals are concerned. generally as an improvement was made and tested it was implimented.

Hoep this helps !

Cheers MM