Fully assembled S&S Cycle heads for stock and 3-5/8" bore Evolution engines were originally designed for engines using stock crankcases 3-1/2" bore heads can be used with stock cylinders Although these heads are machined with the stock Evolution cylinder pattern and may be used on stock crankcases, special pistons are required with a "pop up" dome shape that matches the contour of the S&S Cycle combustion chamber for maximum performance Natural finish