I know it sounds paranoid, but before I put a thermostat in anything I always dunk it in boiling water. I'd rather take the 5 minutes to test it than have to drain & refill the cooling system again after having to change a defective thermostat!
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1969 Chevelle
Old setup: Procharged/intercooled/EFI 353 SBC, TKO, ATS/SPC/Global West suspension, C6 brakes & hydroboost.
In progress: LS2, 3.0 Whipple, T56 Magnum, torque arm & watts link, Wilwood Aero6/4 brakes, Mk60 ABS, Vaporworx, floater 9" rear, etc.
I'm gonna get another thermostat (not a Mr Gasket) soon since I dont want it running that cool, but it will work for now....... I think I'm gonna pre-test this one first