When the cylinder is bored it is "hopefully" perfectly round and parallel. However, when the head is bolted down there is distortion introduced if there are bolts that run through the barrel from the crankcase to the head. The Rotax max has base bolts and is not as prone to this distortion. There are other things that also influence the distortion but basically what you want is a bore that is perfect when everything is bolted down. If you can bolt the cylinder down and then bore it while it is under "tension ", you have a chance of having a round cylinder to give a gas tight seal on the ring when everything is bolted up. Bearing in mind that apparently everything that is written on here is 80% wrong, may I direct you to this link which will explain the further niceties of bananas and other assorted fruit cakes. http://www.realclassic.co.uk/techfiles/tech05031600.html
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