Cleaning water jackets 225

Before going any further:

If the '69 never showed overheating.... did you take the temp sensor out of the '69 and put it in the '65 ? If not, then the '65 has a different temp sensor and THAT needs to be checked out first thing. There are a LOT of temp sensors that look the same but have totally different sensor resistances. There were some tech bulletins on the very early /6 temp sensors and gauges because they tended to show high temps.

Same thing about the thermostat.... I doubt the '69's was pulled out and put in the '65. If the '65 has a bad one or it is installed backwards, then it will overheat.

So there are 2 items that need to be checked before you go any further.

Also, either borrow an infrared heat gun to check the top of the radiator to see the real coolant temp or use a candy thermometer in the top of the radiator inserted in the rad cap's hole.

As Bill sez, I doubt the engine would not have been hot tanked. But as an FYI hot tanking does not get all the rusty crap out of the water jacket. An acid tank dip is needed for that.

If the t-stat or temp sensor don't fix this, then ask the builder if the head and block were checked for cracks. And ask what head gasket was used and if it was installed unmodified (as it should be).