E-9900A This drawing roughly illustrates the oldest series of FDL engine as used in the U25. 
E-13461 Change to steel cylinder heads, and single pipe exhaust manifold.  Applicable to U28 and U30.
E-16188 Change to use of cylinder heads held in by the replaceable liners instead of being directly bolted.  Applicable after introduction of U36 but before steel capped pistons introduced. 
GE FDL Diesel Engine diagrams