so your car is overheating? before we start, out of all the nailheads, the 425 doesnt cool the best. in fact it can be very hard to cool in custom applications such as fenderless hot rods, where you are really limited to fan size and radiator size, and really narrow engine compartment early 50s and earlier narrow engine compartment vehicles. the same goes for bagged cars which have ride height 3 inches off the ground. the lower the car, and the narrower the engine compartment, the harder it is for air to leave the engine compartment. air cant efficiently enter the engine compartment through the radiator, if it can't leave it at the same or similiar rate. . 264s, 322, and 364s can typically cool no problem in a model A with a 4 bladed mechanical fan and no shroud. those engines can actually run too cool if you run too cold of a thermostat... and the reality, is in a car like that, often the earlier engines are more period approriate for the theme of the build, and the difference in power would hardly be noticable. 364s make pretty much the same HP as a 425, built the same way. guys also think they need to run the biggest nailhead and then undercarborate it with an offy tripower that has HALF the CFM the engine needs to make power, so a well carborated 264 could probably smoke it.
first off, you MUST run a thermostat. theres a lot of mystery on internet forums surrounding how a thermostat works. if you're curious, just throw one in a pot of water and put it on your stove with a thermometer and watch what it does. a thermostat does 2 things. it creates restriction with it's small hole to slow the speed of traveling water. it takes X amount of time for the heat to dissapate through the radiator, and X amount of time for the water to absorb heat from the block. if you leave the thermostat out, the engine will run too cool, until it "catches up" and then temp will continue to clime and overheat. the second, is it insures the coolant temp doesnt drop below the specified temp once it reaches it. these engines were designed to run 180 degrees, when all metals have expanded to proper clearances. an engine that never reaches operating temp will recieve a lot of premature wear and tear. the reality is anywhere between 180 and 190 is ideal operating temp. you arnt going to hurt your engine if it gets 210 or even 220 in a blue moon. as long as it doesnt boil over, you won't hurt it. the factory idiot lights didnt even read hot until 235. not a good idea to put your engine through that abuse day after day. the cooler the temp thermostat, the sooner the cooling system starts cycling, and the cooler the engine is likely to run. a thermostat doesnt garuntee that it runs the specified temp, ONLY garuntees it wont drop below it. for instance on a perticular application, a car may not run under 200 with a 180 thermostat, and a 160 is all it needs to run 185, for instance. i always drill a tiny hole in the thermostat during instal so it doesnt create a mess if you ever have to drain coolant and remove the upper hose, and it helps bleed air pockets in the system during coolant install.
the next thing is plugged radiator and or plugged block via rust or corrossion. if your engine only runs hot on the highway, the majority of your issue is going to be that the radiator is either too small, or the radiator/block is plugged full of rust. on fresh engines, i always run panty hose as a filter in the upper radiator hose to catch any broken loose debri during cam break in. you can't reach in between the cylinders and lifter valley with the bead blaster or tank to really clean and remove 100 percent of the debri. if you have a LOT of rust that never stops coming out, then you need a clear servicable filter that you can continuously monitor and clean. panty hose is NOT for long term filtering. it will eventually melt and fail and you wont know if its plugged solid. if you're building an engine that has an extreme amount of rust, ill put freeze plugs in, stand it on end on a stand and soak the jackets with evaporust.
so your engine only over heats sitting in traffic, and the temp drops back down once you get moving? the majority of your issue is going to be an air flow issue. stock buicks with a fan clutch MUST run a shroud, but the reality is a shroud is better for every application. a shroud turns a fan into an air pump by creating a housing for it to pump air, rather than just create turbulance. how the fan is fitted to the shroud is equally important. fan should be as big as possible. ideally no more than an inch of distance in diameter in difference between fan size and shroud opening. fan tips should be halfway in the shroud, and halfway out, or slightly further out. a fan that is too far inside the shroud will create turbulance instead up "pull." ive had the best luck with 6 or 7 bladed riged fans, as big as you can fit. those 7 bladed huge flex fans for custom applications where you are dealing with very tight clearances ive had huge success with. if you are not running a shroud, the mechanical fan needs to be as close to the radiator as possible.
electric fans? i'm not a fan of them (pun intended). they do have benefits, however. they spin fast when the engine isnt, and they dont rob HP from the engine. other than that, theres no benifit. you can't pull half the air, as you cant pull with a mechanical fan and shroud. you cant run as steep of a pitched blade. if you run one as a pusher, you lose 30-40 percent of the fans efficiency via turbulance. my experience over putting electric fan as "helper" pusher on the front of radiators is they do litterally nothing except look ugly. theres a reason why large cubic inch rear wheel drive trucks to this day, still run mechanical shrouded fan clutch set ups despite electric fan technology. they do have a time and a place on custom applications, where you are limited on fan placement. but otherwise, they can be the one thing that ruins the entire look of a period look or classic car. your stock buick ran cool from the factory without an electric fan. if you think you need one to resolve a cooling issue, then you are misdiagnosing the problem.
pulley sizes? theres a rediculous amount of different pulleys for these engines. theres pros and cons of running a larger water pump pulley, vs a smaller one. on certain applications i've actually seen a larger pulley cool better on the highway, while the smaller pulleys are more efficient at low speed and idle, pumping water and spinning the fan faster.. regardless, in custom applications, i always find the smaller pulleys to work better overall. buick never released the very small 3 groove style 60s pulleys on cars that were not fan clutched with a shroud. non ac always ran a bigger pulley and fixed fan. 7 bladed flex fan typically is a good answer with the tiny pulleys on tight clearance applications, but theres no doubt theres a significant loss in HP with all fan set ups on the smaller pulleys vs the latter.. if you can run the larger pulleys for your custom application, then have at it.
this list of other problems are often added up and also in combination of the other issues listed above. wherever theres one issue, theres often many. check for excessive water pump impellor to timing cover face clearance. theres often extreme corrosion from cavitation and or the water pump bearing failed at one point and the impellor milled out the face of the time cover. this is typically only an issue with the aluminum 59 to 66 covers. people ask about the flow cooler water pump design. i havnt seen evidence of them making a difference, plus the hooked impellor blades are said to create more cavitation, which is why buick made the blades straight. 5 blade AC impellors are standard replacement for all application. stay away from over seas made 53 to 55 322 water pumps with plastic impellors. the impellors spin on the shaft. all our rebuild pump use american made bearins. check water pump section on site.
lean fuel mixture can and will cause excessive combustion chamber temps, causing burned valves, holes burned in pistons, galled wrist pins and destroy bearings, and of course hotter, coolant temps. plugged exhaust from a frozen shuyt heat risor valve doesn't help the situation and can also cause mechanical damage and hot running conditions. incorrect/ especially retarded ignition timing will also encourage hot combustion chamber and coolant temps. the other issue is rusted out head gaskets. this is extremely common. even though thicker, composite head gaskets last much longer than steel shim OG style. ive seen steel head gaskets rust out in as little as 7 years from cavitation. the cooling system route is from timing cover through block, to back of the cylinder head, then up and forward to the water manifold. the hottest temp the coolant gets before making it to the radiator is in the water manifold. the head gasket is designed to block off all the holes on the deck of the block and heads. if the block offs are rusted out, then the water will short cut through the whole engine, and not cool it. ive had customers actually take the new gaskets and punch holes in the block offs because thats how the old rusted out ones were. do not do that!.