[lust4speed]

I checked and the only heater nipple on my shelf is the new style. So while I can supply photos of it, I can't run the test on the old style spring to see how it reacts to heat.

Steve, if you get a chance drop the old one you have in a pan of boiling water and lets see if the spring changes shape. I'm still thinking that the spring is only a buffering mechanism to smooth out any pressure pulses generated.

I think that almost all heater cores came with a crimped in area on the copper inlet tube that should pretty much insure that more coolant can flow out than in, but seems like the engineers wanted a little more insurance to protect the heater core.