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Another stipulation of the rules is flat-top pistons. Combined with the limited number of legal cylinder heads, compression ratio of the class's engines typically falls in the 10:1 to 12:1 range. Thomson makes the most of the rules, however, by using lightweight Diamond aluminum pistons that have a friction-reducing Teflon coating on the sides. Combined with lightweight GRP rods Thomson keeps the weight of the reciprocating assembly as low as possible, which allows the engine to rev higher and faster.