The radiator from my parts truck is a Nissin in excellent shape. Everything seems good so far, but I want to switch to a 180F thermostat. I have two of them but whenever I install either of them the temps start to go crazy like they're not opening. I need to check them in hot water.

 

When I drain and refill the coolant, observing carefully the bleeding instructions in the RAVE, the temp spikes to 220 or so for under a minute on the test drive, then comes down to where it should be. A minute at 220 or so won't cook the headgaskets. I don't know how long you let it run with it "overheating", but you might want to let it run a little longer to see if the temps come down as they did on my truck. And drive it after start up; don't just leave it to idle. Just sayin' ...

 

This was while driving, about 10 minutes in. Went up to nearly 150. Not fun.
Has been stable since putting the 195 thermostat back in.

 

The temp overshoot on initial warm up, then settling back down to normal ... is a classic symptom of thermostat opening too late ... or a thermostat that's not working properly... Did you replace the t-stat?

 

As stated, I tried two different 180F thermostats. One that I had used last year, and one new out of the box. Same behaviour with both. I'm going to test them.

 

An update on temps. Driving around Midwest temps in the 90s AC on I haven't gone over 186. Flowkooler with new fan clutch and fan definitely worth it. Fan is much louder though.

 

Thanks for updating us.