[img]/phpBB/images/smiles/icon_cool.gif[/img]one reason is to meet class rules, another is to gain a better rod/stroke ratio, with attendant advantages and disadvantages. a third reason is to prevent and engine that is built right at class displacement allowances from growing to break the displacement rules. most engines will grow an inch to an inch and a half in displacement when they get hot, so you reduce the stroke a bit, usually a couple of tenths, so that when the engine grows, it meets class requirements.
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