Is there a reason the MK3 primary steam engine has different specs than the MK3 secondary steam engine? The other tiers, MK1 and MK2, have the same specs for primary and secondary so I didn't know if this was intentional or not.

Not intentional...

Fixed a typo:

primary_load_steam_steam_engine_mk3.fluid_usage_per_tick = 0.8
primary_load_steam_steam_engine_mk3.maximum_temperature = 265
primary_load_steam_steam_engine_mk3.maximum_temperature = 600

Should have read:

primary_load_steam_steam_engine_mk3.fluid_usage_per_tick = 0.8
primary_load_steam_steam_engine_mk3.maximum_temperature = 265
primary_load_steam_steam_engine_mk3.max_heath = 600

Fixed, updated version submitted.