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My wife and I just returned from the 10th Annual Woodies in the Valley show in Visalia CA yesterday but weren't sure we were going to make it home without a tow truck I've been driving the '40 Plymouth now for a couple of years and I've never had an overheating problem, including driving around in the summer at temperatures over 100°. I got the motor from an old Mopar racer who had rebuilt it, new distribution tube, etc. and it always ran like a top. It had 160° thermostat, which after reading various posts here, seemed like I was asking for sludge trouble in the future (apparently the engine doesn't get hot enough to burn off the moisture which creates problems), so I did what you're not supposed to do right before a trip and changed the thermostat to a 180° unit from Napa. Immediately the gauge registered 20° hotter which made sense. We left Saturday morning, outside temperature in the 50s, and drove 60 miles on the freeway with no issues. We had a lot of fun at the show, caught up with old friends, tc., and then caravanned through the countryside, which included driving through some small local downtown areas very slowly. Outside temps were in the low 70s. Immediately it pegged the temperature gage – I couldn't even see the needle it was so far off the scale. We pulled over and clouds of steam were coming out of the bottom area of the radiator. After cooling down we added water only, and filled it to overflowing. As I was looking among the group for some anti-freeze, a couple of the old-timers told me that unpressurized flatheads do better in hot weather with pure water rather than an anti-freeze mix. Now with more water than coolant,in the stock, honeycomb radiator, the gauge went back down to about 170° and stayed there the whole rest of the trip. I had never heard this before- I would think that adding anything to water would change its boiling point and be better. Thoughts?