Kel,
The FS-21139 is for the 69 to mid 72 engines with 10 bolt heads & 3 bolt thermostat mounts to the head.
FS-21182 is for the late 72 - 75 engines with 12 bolt heads & 2 bolts to mount the thermo housing to the head.
Tom C
The FS-21139 is for the 69 to mid 72 engines with 10 bolt heads & 3 bolt thermostat mounts to the head.
FS-21182 is for the late 72 - 75 engines with 12 bolt heads & 2 bolts to mount the thermo housing to the head.
Tom C