Older EMD diesel engines ("B" and older) had a tendency to leak coolant into the crankcase when cycled between operating and storage temperatures and common anti freezees would damage the engine bearings if the coolant came in contact with them.

The "C" liners and heads overcame the problem by modifying the coolant passages and a modification was available to convert older engine blocks to accept the "C" power assemblies resulting in an "AC" or "BC" configuration many of which are still serving on shortline and industrial applications.

Sammie