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The reason for the overbore was to allow for taking a production and remachining o your own racing accuracy. The maximum over bore for homologation was 0.040". So it's for Cooper S 970 + 0.040" = 999. 1071+ 0.040" =1098, 1275+.020"=1293. For Ford twin cam it's 1558+0.040"= 1598. the Ford sizing is why there were Elan 1500s. The other Ford tweak was to take the max tolerance sizes on a 1600 BDA and getting 1601 cc, which allowed the engine to be bored out to around 1800cc as the class entered was then 1601 to 2000cc
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