Here is what I have done to date on my 31:
- rebuilt water pump
- new fan belt
- new radiator core (3 rows)
- timing is 18 degrees
- new thermostat 160 degrees
- temperature gauge was rebuilt
- plate behind water pump is intact and good condition
- radiator has a 50% mixture of antifreeze and it covers the tubes.
Here is what happens. I drive along temperature operates at normal (just a little above dead center on the gauge). I slow down and go around a corner and the temperature spikes to the max on the guage. Drive along and temp drops back to normal. If I rev the motor that helps to bring the temp down. Why am I getting this spikes? I think I have pretty much checked everything that I can think of.

is the engine freshly rebuilt? if so, it'll take a thousand miles or so to loosen up and hence heat up less. i wouldn't trust the rebuilt temp gauge. TIF Instruments, Inc sells a digital thermometer that you can install and then run the car while observing the temp fluctuations at various road conditions. the sensor is inserted at the radiator hose outlet on the head, its got a cord that connects to the digital gauge that is inside the car. you can also use the tool to measure tire temperatures(if you are into autocrossing/road racing/drag racing) as well as measuring the temps of exhaust gases at tailpipe. believe the tool is a couple hundred bucks new, but they show up on ebay for less that 100 bucks. mike

running a 2 or 4 blade fan?

I run a 4 blade and no t'stat and rarely does the needle go past straight up

First the temperature gauge bulb in in the back corner of the head which is the hottest place on the engine. It is also the furthest from the water pump and so gets less circulation that other places. So it is normal for the temperature gauge to rise when the engine is slowed. What is not normal is the rise coupled with a loss of coolant or temperature rising to the maximum on the gauge.

The problem can have one or more of several causes. You have already addressed the radiator and water pump which are critical to the major heat transfer to the air. Coolant circulation within the block and head are also critical. Any restrictions to flow in the block or head will cause heating particularly in the parts of the engine furthest from the water pump or in the immediate vicinity of the crud. Internal engine deposits from hard water can coat the metal surfaces reducing heat transfer to the coolant. The more heat in the area the more likely the deposits forming.

Another consideration is air or exhaust gas in the coolant. Obviously exhaust gas, which is HOT, is the worst. It adds heat and also displaces coolant, like a double whammy. Air typically drawn in past the water pump shaft (but also past hose connections, yes even if no water or coolant leaks out.) It displaces the coolant and tends to accumulate in the highest, hottest, lowest flow areas. That is exactly where the temperature probe is located. Both of these result in coolant loss. Small losses may not be obvious.

The head was recently rebuilt and there is no air in the fluid and I am running a 4 blade fan.

I have two rebuilt heads on running engines that were magnifluxed prior to rebuilding. Both are now suspected to have cracks most likely in the exhaust passages. They don't leak water into the combustion chambers (yet) but one looses water and overheats especially when going down a hill. Yes going down a hill. The other is on a test engine that so far has not seen the road. I have helped them some but never fully cured the problem. Hopefully your head does not suffer the same problem. Will let all know when I take them off and pressure test them if they are indeed cracked.

Against popular belief there is little benefit to a 4 blade fan. It only differs in cooling at idle or slow speed operation when compared to a 2 blade fan. If you do many parades they are good otherwise they use horsepower with little to no actual benefit.

when you stated it indicated hot when you slowed then cooled when the engine was reved I thought it might have sometihing to do with air flow thru the radiator. but it shouldn't fluctuate that much it that period of time.

confirming the gauge's functioning would be the first step. an infrared thermometer works great for this. it can pinpoint temps at different areas of the engine. I also have a 100-700 degree thermometer with a magnetic base that works also. I've seen a lot of hard work and grief expended over nothing more than a faulty gauge. eliminate the easy stuff before attacking the engine.

was it doing this before the head work?

Ok, I borrowed an infrared thermometer gun and the temperature at the top of the radiator is 160 degrees and 110 at the bottom of the radiator. The block measures around 170 to 190 at various points. During this process the temperature gauge is just a little past dead center. This is idling in the garage. Temperature outside is only about 60 degrees. Took it for a ride and no indication of overheating or getting hot. Cool day. Will have to test on a hot day.