1966 427 radiator cap overflow

[Joe T.]

I have a 1966 427/425 HP. New Harrison radiator and new GM restoration cap on the radiator. After running several miles, the Corvette will be between 180 and 200 degrees with motor off and parked. The overflow will spit approximately 1/2 pint. I have checked the temperature with the dash gauge and a calibrated temperature gauge on the radiator. I have not pressure checked the cap.

Any recommendations and/or advice?

Thank you,
Joe T.













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[Mark M.]

Re: 1966 427 radiator cap overflow

Joe I had the same issue with a 67 427 car. The radiator is the same as your 66. These copper Harrison radiators have a fill line about 3-4 inches down from the cap embossed on the tank. The system needs the space for expansion or the cap lets coolant out usually soon after turning the engine off. It could purge enough out on it's own or you could lower the level to the line. I noticed if on a hot day parking with the front right side of car lower, it would spit some coolant out the overflow. These cars also run hotter on the original ported vacuum advance but that's another subject.

[Joe T.]

Re: 1966 427 radiator cap overflow

Thanks Mark.. Your reasoning sounds correct to me. My radiator was probably fuller than it should have been. Seems to be at the correct level now. I will be running it this week. On that different subject, I have gone through the timing, vacuum, distributor spring and so on learning experience for running hot on my 427. Runs great now. 180 on a 80 degree day.

[Patrick H.]

Re: 1966 427 radiator cap overflow

In addition, the restoration caps are not known to work all that well.
If you have the $ I'd recommend having Joe Perri sell or restore a real one for you.
To drive, buy a new RC-26 which will function perfectly well and not puke coolant.

[Mark L.]

Re: 1966 427 radiator cap overflow

I'll second Pat's comments. I almost failed a PV due to using a restoration cap. Fortunately got the spill cleaned up before the team leader came back to see the fix on the one failure.

[Daniel S.]

Re: 1966 427 radiator cap overflow

Joe, install a Milodon 180 degree thermostat. These are modern editions of the origin Stant units. Second, install a real/OEM RC15 from Joe P. He is the best in the business.

https://www.summitracing.com/parts/mil-16406?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-milodon&gclid=CjwKCAjw27jnBRBuEiwAdjQXDPtMjzFbiNI4UQoL7BCr6-u5ae8FZDOIhRxwYRlrAi5mzIfbEX0VuBoCpTwQAvD_BwE

Joe - corvetteillinois@gmail.com

Good luck,
Dan

[Joe T.]

Re: 1966 427 radiator cap overflow

Thanks for all of your advice and recommendations. Greatly appreciated..

Joe