The Zenoah 34mm bore, with the stock stroke crank.
Championship Porting available, HR (New porting for 2018).
This cylinder is about maxed out, but still structurally strong at the exhaust port. Comes with the High Rev Piston and engraved water jacket. This kit has tight squish, so you will want to check it on your engine before you start it.
Note, some racing classes allow up to 27cc displacement which this kit will also work well with, using a 1mm stroked crank and a .020" added cylinder base gasket, making the engine a 26.33cc engine.