Although the most expensive route, for reference, LN Engineering manufactures underbores of all mahle cylinders in billet nikasil, including those that were originally cast iron or biral. Makes it really easy to dial in an extact displacment for classes such as 2.5L, without sacrificing cooling or reliability.
Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance