New Member
Hi, I read from a thread back in 2009 someone was building a H100 racer with a 125cc head, does anyone know where I can get this? mine is a 1992 s2 model, also I've already messaged him but haven't gained a response, lastly how else could I increase bhp on this little smoker to gain maximum top speed, UK laws only permit for a 125cc until you are 19 and I had a H100 S2 around so that's why I'm using this project as a base

The Limey

Evil English Villain
I had an H100 as a sprog. They're funny bikes - a decent 11BHPwhen new, though you can bet 3 or 4 of those have leaked away over the years. They're very low geared, which means they're great for the urban cut and thrust but top out in then low 50s on the open road, so upping the gearing helps a lot.

A lot of olde air-cooled CR parts are a direct swap, so it may have been a CR head that the chap was referencing.

Transfer/inlet ports can be raised about 2mm, exhaust as much as 5.

Bending the reed valve stops to 10-12mm helps, though requires some dremmel work to the barrel casting to make room for this. No one makes carbon fibre reeds for these, but there is another reed for a totally different bike (Garelli perhaps?) that is the same, and are also avalalable in CF.

No one makes spannies for them, though a few were made for the MB80 which fit, requiring only a bracket to adapt the rear mount. You'd be lucky to find one now though.


With the CB500 you have to be careful as the top crankcase is designed to fit the sleeve size of the 500. That is why you can't simply put the barrel from a CB550 onto a CB500, the sleeves are too big. There were aftermarket kits made for the CB500 that took them to 572cc, 591cc and 600cc but these ALL came with new sleeves that were smaller at the bottom to fit into the crankcase of the CB500.

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