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JeepsAndGuns 03-21-2021 07:11 AM

Radiator suggestions to keep a 401 cool
I have been fighting this ever since I installed this 401 many many years ago. Just recently I had my thermostat stick open. While I had the system open I flushed it and changed the coolant when I replaced the thermostat.
This made me start thinking about it again and a desire to want to try and fix it.

I rebuilt the 401 about 17 years ago. It is .040 over. Temps are pretty much never below 200. In the winter it runs at right at 200, even if it is 30 degrees outside. Setting at idle or at any speed. During the summer it usually runs at 205 to 210.
For a long time I thought it was the cheap aftermarket mechanical temp gauge. However a few years ago I swapped to some high quality aftermarket gauges and the temp readings are the same.
The radiator is a copper/brass 4 row I got from a local parts store. I bought it when I built the engine. It still seems fine and there is no damage to the fins or the tubes.
I have used a (new) mechanical clutch fan (yes the shroud was on there) and even a large electric fan (which is currently what is on there) Neither one make any difference.

I was wondering if there is any chance one of those aluminum radiators would do any better? I do not mind spending $200-300, but I can not swing $500 right now.
Anyone have any suggestions on a radiator with good cooling capacity that will not break the bank and will not leak in a week?

letank 03-21-2021 02:06 PM

401 at .040 are know to run hotter or warmer. You have EFI, so your mix is set right. You could try to run a bit richer... some of the old air cool Off Road bikes run hotter... and usually a tad richer does the trick. I have a oil temp gauge on the XR and I know that a steep uphill climb will gets the engine (NOT the OIL) in the 270ish after 30mins... so I stop and enjoy the views. When I tried to play with richness, it did not do anything...

I would check the oil temp with a infra red thermometer otherwise known are non-contact thermometer, the oil will be 10 to 30 degrees hotter.

The easiest could be to run some of the fancy cooling liquid to see if it helps, I have a bottle of water wetter, but never used it. We know that water cools better than coolant, play with the % coolant/water.

Removing the pintle that allows for air purge in the thermostat helps, or drill a small hole.

Otherwise 205 to 210 is not that terrible as long as the temp is staying in that range.

Edit after FSJ junkie post...

I have never really checked the exact temp when the temp gauge ran toward the 3/4... I keep driving, and a few minutes later the temp will go back below the middle and toward the 1/3... It all depends on the driving situation, long uphill, hot day... same hill different time of day, the OEM gauge will respond differently.

Add a trans temp gauge, that is a good indicator of what is happening overall... a few years back driving back from cold weather, 35F at high elevation, the trans oil never managed to go past 100F for 70miles until the outside temp was in the 50F...

TomJ 03-21-2021 02:34 PM


The run a sandwich adapter and cooler lines from oil filter to a transmission cooler (or huge small-fin oil cooler).

FSJunkie 03-21-2021 03:16 PM

A 195 thermostat doesn't even start to open until 195. It's not fully open until 218. Meaning the cooling system isn't at full capacity until the engine warms up to at least 218.

The design dictates that the coolant must be 195 degrees or hotter to create any sort of flow what so ever through the radiator, so the engine will almost always be over 200 degrees.

I don't blink an eye until the coolant temp reaches 220 and keeps climbing without signs of stopping. The coolant boils at around 250, but the gauge is just an average and there are areas in the block where the coolant is hotter than the gauge says, so you tend to get localized boiling in the block at around 230 to 240 on the gauge. That's about when the radiator boils over.

This is all coolant temp. The only reason we watch coolant temp is so it doesn't boil. Otherwise we should only care about the block metal temp. The block will not reach dangerous temperatures so long as the coolant remains liquid. Basically if it's not boiling over you're not in danger of hurting the engine.

That being said you can never have too much cooling system capacity. You should never need more than a three row, 3/8" tube spacing, 20 fins per inch, 17"x24" core radiator with a 17" to 21" fan and full shroud. That's basically the stock cooling system. 401 Matador police cars used that setup to idle with the A/C on in Phoenix all day and sustain 125 mph indefinitely. If it's good enough for them, it's good enough for you.

mwood65 03-23-2021 01:56 PM

Just remove the hood,

always works for the guys on Roadkill. ;) :D

rang-a-stang 03-23-2021 02:11 PM

I have a 401 that is .030 and the huge BJs crossflow aluminum radiator, electric fan, and a 195 degree TStat. You can see my temps in some data logs from TunerPro here:

After my No TStat experiment, I plan to put in a 180 degree TStat just to give me a little more breathing room between normal operating temp and hot.

I have this radiator:

tgreese 03-23-2021 02:38 PM

You guys understand that thinner cylinder walls transfer heat to the coolant faster? No personal experience with this, but there has been plenty of discussion about 401 engines that run hot after they are overbored for a rebuild.

I would point out that the core alignment in the cylinder block varies for each block. You could - hypothetically - have a badly aligned core that can't tolerate 30 over, and a well aligned core that will go 60 over. There is a sonic test for this, which I would recommend for every 401 that's a candidate for an oversize bore. I don't have any idea what that thickness should be - though this has been covered in earlier posts; cylinder+wall+minimum+thickness

conehead 03-23-2021 05:26 PM

I was having running hot issues on my 87 Wagoneer with the 360. I bought a radiator Brice Thomas Radiator in Gadsden AL, 256-546-4613. It was kind of pricey but it solved my problem.. I recently ordered another one for the 401 in my J20. I had to send them my original radiator for measurements that they still have on file.

FSJunkie 03-24-2021 12:51 AM

Thin cylinder walls causing overheating is a common thing that one hears in chat circles at car shows, but the laws of thermodynamics and materials science don't support it, nor does my own experience with plenty of engines that I drove when they were standard bore and drove again with a .030, .040, or even .060 overbore.

What I have noticed is newly rebuilt engines tend to run hotter than they used to when they were worn out. It takes them a few thousand miles to return to their cool running selves again. I've noticed it on almost every engine I've rebuilt, but they've all settled back down after a break-in period.

The OP's engine is thoroughly broken in though and is not doing anything that I would consider overheating....or even running hot. Sounds as normal and healthy as could be to me.

SJTD 03-24-2021 08:43 AM

I could weep with joy. Two different Posters referring to the radiator having 4 rows rather than 4 cores. ;)

Anyhoo, seems to me a thin wall might suffer differential expansion and poor compression in that hole when the engine gets hot? In other words maybe the cylinder gets out of round.

On the overheating theory, I've never given this one much thought. On the surface it makes some sense. Cast iron is not a perfect conductor so thin walls will conduct more heat to the coolant than thin.

More stuff I learned in school and didn't use for 30 years. Meaning heat transfer, not engine building.

babywag 03-24-2021 09:51 AM

I would say it's working normally.
You stated your original post.
With a 195 tsat it'll run 200-210...only an issue if it gets to 220 or higher consistently.

tgreese 03-25-2021 03:30 PM


Originally Posted by FSJunkie
Thin cylinder walls causing overheating is a common thing that one hears in chat circles at car shows, but the laws of thermodynamics and materials science don't support it, nor does my own experience with plenty of engines that I drove when they were standard bore and drove again with a .030, .040, or even .060 overbore.

Perhaps you don't understand that a typical block from an engine like the 360 or the 258 starts off with more iron in the walls than a 401. In the 401, the bore diameter is large compared to the bore spacing, and something had to go. Big block displacement from a small block geometry.

I don't understand how you can appeal to thermodynamics. There is a fixed amount of heat created in combustion, but there is no rule for how much of that heat diffuses through the cylinder wall versus how much leaves with the exhaust gases. It seems clear that the same heat gradient across a thinner cylinder wall would transfer more heat to the coolant.

Sorry - none of this helps the OP with his original problem. I would say add as much additional cooling capacity as you can, to stay ahead of the thermal load. If this does not work, one could sleeve the cylinders to the factory diameter. That would restore the factory cooling performance, at a cost of about $100/hole plus parts.

JeepsAndGuns 03-28-2021 07:40 AM

Well I do know when the thermostat stuck open, it would not reach temp. I can not remember the exact temp, but it was only about halfway to where it usually is.
I got back home, let it cool off for a min and then checked the level. It was low (that is when I found out the lower hose was leaking). I topped it off then fired it back up to make sure there was no air in the system and that is when I saw it was flowing wide open.

Many years ago I tried that water wetter stuff with zero change. I switched back to regular coolant.

From the sounds of all the replies, it sounds like replacing the radiator is not going to make much difference. I know the overbore 401's are known to run hot, but I thought I remember reading on here once that it only becomes a real problem at 60 over.

I have thought about switching to a 180 degree t-stat, and I might, just to see if it makes a difference. However work has started picking up, so it might be a little while before I get a chance to do that.

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