I just rebuilt the top half of my 502, rebuilt the heads due to compression loss. I removed the old rusty looking T-stat and installed a new Siera 163' T-stat. Temp is fine on hose and at idle. Under load anywhere above 1500 rpm, temp climbs up and will go to 240'! As soon as I return to idle it drops back to 160' I removed and checked the belt driven water pump ( I have a cross over, so its a belt driven pump that runs off the same pump as the fuel pump) and it looked fine. I didnt pull the lower impellor apart, I was at the lake so that was out, I went to the T-stat next. I removed the T-stat, the overhitting problem is cured, but now it never goes above 100', way too low. Was that T-stat bad, wrong, or what? Should i look into a restricter plate instead? Any help would be great, thanks, Derrick

Did you match up the old head gaskets to the new?

yes, all gaskets are exact same part numbers with macthing holes

IS T-STAT IN UPSIDEDOWN,HOW MUCH WATER PREASURE DO YOU HAVE AND IS IT AT THE BLOCK?

no, T-stat was installed correctly, ides fine so it is opening, just maybe not enough, I dont know, this is a weird situation. Cools down fast when i idle engine, i was thinking maybe one of the pumps is going out, but after removeing ( I should say gutting and leaving T-stat without middle pieces as a restrictor) the temp stayed at or below 100' so i ruled out a bad pump or gaskets or head, but Im not sure.

CHECK FOR RESTRICTION IN COOLER,STRAINER ECT.

Im not sure what my water pressure is and Im not sur what you mean by "is it at the block"? I doubt there is a restriction in the coolers, It ran cooled fine a few weeks ago, other than the head rebuild i changed nothing but the t-stat, if is wasnt the t-stat then why does it cool so well with it removed? Im not sating it isnt anything else, im just curious what else would only be efected with the t-stat in or out.

I just had the same trouble, it was the sea strainer.

ok, this is going to be a stupid question, but whats the sea strainer?

WITH NO T-STAT THEIR IS NO RESTRICTION TO THE BLOCK & HEADS.WATER CAN FLOW STRAIGHT THRU AND YOU WILL NOT GET ANY WATER TEMP.THAT'S WHY I SAY THEIR IS TO MUCH RESTRICTION FROM SOMETHING.YOU JUST HAVE TO FIND IT.