I got this rebuilt 440 hp .030 engine ---
Fires up runs great. BUT under normal
driving conditions she overheats - temp goes thru the roof.

Tried changing the water pump and the thermostat had no effect.

The radiator is hot on the bottom and cold at the top.

TOOK the thermostat completely out and temp stays at 170 degrees -
but I know that is not the correct solution.

What is going on?

sounds like no circulation thru the radiator,, did you have it cleaned or recore, did you put a air conditioner water pump on the car, weather you have ac or not, also I put a 165 thermostat in mine. the way to check to see if the radiator is stopped up is leave the radiator cap off rev up the engine and if coolant come out the top, it's stopped up..

Have to look at each piece of the puzzle separately. Fresh engine was it hot tanked and cooling passages cleaned and then recleaned and visually inspected. The Water pump and housing are they mated and just what water pump are you using. The rad, is it new has it been recored, what rad is it and how many cores. Thermostate I run 160 but other temps are fine when your system is working perfectly. Is this motor completely stock with no adjustable rockers and everything like cam timing and distributor timing set correctly. All thing I have mentioned can affect how hot the motor runs. There are a lot more things that affect motor heat. Look at one part of the cooling system at a time. Start with the RAD since you say it is cool on the top and hot on the bottom.

I know this sounds crazy, but it may be the wrong block for the heads.Clean the rad first. Tell me the numbers on the top pad and the casting date. I have a block that was a motorhome block with the extra cooling holes and it would overheat with 906 heads. A buddy had the exact same problem. they have to be matched up with the correct heads and head gaskets.

With no stat & back out in the same driving conditions the overheating problem dissappeared? That'd almost sound like the stat was upside down or something in the stat housing was keeping the stat from completely opening when it was in there or just a bad stat & possibly the second stat could be bad also. Assuming the timing is in the ballpark & I'm thinking that it is I would have a rad shop pump it up to 15 lbs when hot & see if it holds pressure (head gasket/cracked block or head(s) & if good there then as said the radiator. I'm assuming it runs hot around town and out on the hiway?. Holler how it goes. EDIT FastMark may be on to it (but I would say head gasket cooling holes) especially if it did not overheat before it was rebuilt & now after the rebuild it does overheat & the same rad was used. PS how hot is it getting?

The key is it ran fine without the thermostat. So it's not the head gasket or block or heads or radiator, it's the thermostat. If you must, leave it out until you can find one that works or how to install it.

R.

make sure youre not installing your thermostat upside down; seen that before

Check out this thread and it'll probably cover what you need.

Hi John - Yes -- what is the difference between AC water pump and non ac water pump? I put an A/C water pump in - I thought it would not make any difference. thanks
The pump on the left is NON-AC and the pump on the right was in the engine when overheated. The fins are in a different direction.

Looks like you found your issue. Don't take this wrong but I'm glad I'm not the only one this stuff happens to.

Heck, just took apart a "fresh" stroker shortblock to "make sure" it was good to go & found that the oil ring expanders were way to big & scratched a couple of bores on install. Must have been a mfg screwup but machine shop missed it. Fun hobby....

I don't think the impeller is critical which way it's made? It's going to do the same thing either way it's turned.
I like your original pump better than the one with a cheaper stamped impeller.

I bet some of your heat is coming from being lean in the cruise or power mode. Today's gas needs to be jetted much richer than straight gas and that surely will add heat to your new motor being lean. Also you sure you have enough timing, that will cause it to run hot also.

Good luck.

Those are both non AC 8 blade pumps (& I prefer an 8 blader) & the one on the right appears to be more rugged with slightly more vane area to capture water. Unless the impeller is slipping on the shaft (not likely) then the problem is elsewhere

The temperature pegs - not just running hot
I think there may be something to this A/C water pump vs non AC water pump, (I hope) Going to install 160 deg t-stat and this non-ac pump tomorrow-see if that does it. hate to think head gasket issue.
thanks for your insight and comments.

The direction of the impeller matters not?
What is the diff between ac/non ac?
Thanks

Sorry but that won't fix it, either pump can keep it cool. What carb?

One has 6 and the other has 8 paddles on the impeller.

Holley 770 street avenger

Jet up, it's cheap and easy to do. Easier than changing a water pump and see if it helps.

Fuel and timing makes them run hot if everything else is in order. I would run a 180 or even 195 stat.

Stats do seem to be hit or miss as I installed a bad one in a big truck recently . Had to do the job twice.

big block pumps draw coolant in at the center open side & sling it outward & with the clockwise rotation in mind it would appear that the pump on the right in addition to having larger vanes (at least in the pic appearance) & that the horizontle back support part of the vanes is in front rather than trailing like in the left pic which would lead me to believe that it would capture/sling more water. But the problem IS elsewhere. To answer your Q an AC pump has 6 vanes

Yes - the 26in radiator is brand new from year one.
You mentioned AC vs NON ac water pump - will that make a diff?