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  #1  
Old Dec 23rd, 12, 06:01 PM
D94281 D94281 is offline
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Jason
 
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Default Overbore overheat myth

Take this for what you will. I had a very warm running 396 which i spent countless hours trying to get to run cooler. Even ended up with an expensive aluminum radiator.

The motor dropped a cam lobe and I rebuilt the whole thing. I agonized over wether to bore it .090 over because of all the talk of overheating with that much over bore. I wanted to end up with an 11:1 static compression ratio and the only pistons I found to get me there needed a 90 over bore. I did it and when I got the motor back in the car was still borderline overheating after a while on the highway.

I eventually suspected a microscopic leak in the heater core because of some moisture in the gauges and bypassed it with a short length of heater hose.

Wala - car now runs cool as a cucumber in 90 degree heat. Will need to put a 180 deg thermostat in as it won't warm up past the 165 I put in it during my efforts to fix the problem. I suspect it would run just as cool if I put the stock three core radiator back in it.

I would bet a lot of these "overheating overbores" out there have an undiagnosed cooling system problem as well...
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  #2  
Old Dec 23rd, 12, 07:33 PM
Eleanor's Nemesis Eleanor's Nemesis is offline
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Michael Gekko
 
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Default Re: Overbore overheat myth

Some 396-402 blocks are a good deal thicker than others, and have enough cylinder wall to take the overbore without any other problems. Typically the thick blocks are '65-'66 vintage iirc.

Glad to read you found the culprit that caused your overheating issues.
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  #3  
Old Dec 23rd, 12, 07:54 PM
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Default Re: Overbore overheat myth

Most 402 blocks will go at least 0.060 over, which would correlated to the same as 0.090 over for a 396.
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  #4  
Old Dec 24th, 12, 03:51 AM
Busted Knuckles Busted Knuckles is offline
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Default Re: Overbore overheat myth

I agree about the overbore/overheating issue. Thinner cylinder walls may cause flexing and problems with ring seal, but not overheating. I think that's a circle track myth.
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  #5  
Old Dec 24th, 12, 01:39 PM
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Default Re: Overbore overheat myth

I agree. I had a .060 over 350 that never came close to running hot.
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  #6  
Old Dec 24th, 12, 02:03 PM
Steptoe Steptoe is offline
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Default Re: Overbore overheat myth

Quote:
Wala - car now runs cool as a cucumber in 90 degree heat. Will need to put a 180 deg thermostat in as it won't warm up past the 165 I put in it during my efforts to fix the problem. I suspect it would run just as cool if I put the stock three core radiator back in it.
I think u hit a nail on the head there...
A comment often make is we spend a lot of time money on bodies, engines, even replacing good oil pumps, but come to the cooling system, very really conside pulling the radiator header tanks and manually cleaning bores...ie over hual the cooling system properly... yet rebores involve are rebuild.

"it won't warm up past the 165 I put in it during my efforts to fix the problem"
An another comment I often make... mine will go above 5 degs... If Im towing one of our vintage cars up a long steep grade.....or traveling from sea level up into the snow line...both rather extreme.
I bet if u dropped a 125 thermostat it still would not go over +5 degs of that....a 140 will not either.
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  #7  
Old Dec 24th, 12, 08:06 PM
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Default Re: Overbore overheat myth

In real life, one experience out of the millions of engines bored is mathematically meaningless.

What's the OP's point?
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  #8  
Old Dec 25th, 12, 05:08 PM
ace's68 ace's68 is offline
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Default Re: Overbore overheat myth

It is probably a myth.
There are thousands of engines out there that are overbored and the owner may not even know it.
How is this mathematically meaningless if this information is part of statistical data? Get me enough recorded temperatures, and engines that are bored/unbored and I will gladly compute the data.
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  #9  
Old Dec 25th, 12, 06:40 PM
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Default Re: Overbore overheat myth

I believe the point of the thread was that if you have a cooling problem, it probably isn't because of the engine itself. It may be a problem with the heater core and/or radiator.
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  #10  
Old Dec 26th, 12, 11:29 AM
Steptoe Steptoe is offline
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Steps
 
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Default Re: Overbore overheat myth

Quote:
I believe the point of the thread was that if you have a cooling problem, it probably isn't because of the engine itself. It may be a problem with the heater core and/or radiator
Exactly...
And given most people will overhaul engine and not deal to, overlook the cooling system...or at most a quick radiator flush...
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  #11  
Old Dec 26th, 12, 11:35 PM
puff puff puff puff is offline
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Default Re: Overbore overheat myth

i would think that an overbore would lower the overall temperature---because thinner metal would dissipate heat quicker/better?

???
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  #12  
Old Dec 27th, 12, 08:24 AM
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Default Re: Overbore overheat myth

Quote:
Originally Posted by puff puff View Post
i would think that an overbore would lower the overall temperature---because thinner metal would dissipate heat quicker/better?

???
The more you over bore the thinner the walls between the cylinders becomes. The thinner the walls between cylinders is the less metal there is to dissipate the heat... Remember you have heat from both sides of the wall!

An exaggerated example is if there was 2" between each cylinder the heat mid way ( 1" away from each cylinder) between them would be lower than if you had 1/4" of metal between the cylinders where mid way between them would only be 1/8"...

When you start pushing the limits of an overbore another issue is the strength of the metal between the cylinders and just how long will the engine will last and how much abuse it will take!
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  #13  
Old Dec 28th, 12, 04:10 AM
Busted Knuckles Busted Knuckles is offline
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Default Re: Overbore overheat myth

DjD, how does heat come into the outer part of the cylinder wall from the coolant??? It's dissipated by the coolant, not the other way around - the outer surface of the cylinder is a good bit hotter than the coolant so heat travels away from the cylinder, not into it.
How does having a thicker cylinder wall help dissipate heat? I think it would retain it since it's denser than the surrounding water. Heat travels through the cylinder walls from the cylinder, it should cool faster if it has less distance to travel. The only area of a cylinder that can actually dissipate heat is the very outer surface of the cylinder barrel where it's in contact with coolant, the rest of it either retains it or transfers it.

Big blocks RARELY overheat with a decent cooling system, even if they're punched .100 over or more. That's because folks that build big blocks generally pay closer attention to cooling needs than folks that build small blocks, IMHO.

If there was anything to this myth, aftermarket block manufacturers would include something about it in their machining instructions, but they don't. Cylinder wall thickness for rigidity/ring seal is the only thing they're concerned about.

I think the whole wive's tale got started by folks removing smogger 305's and 307's and replacing them with much more powerful engines including the engine that got this whole myth started, the lowly 400.

The internal combustion engine generates heat. The more power it makes, the more heat is generated and the better the cooling system needs to be.
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  #14  
Old Dec 28th, 12, 07:53 AM
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Default Re: Overbore overheat myth

Quote:
Originally Posted by Busted Knuckles View Post
DjD, how does heat come into the outer part of the cylinder wall from the coolant??? It's dissipated by the coolant, not the other way around - the outer surface of the cylinder is a good bit hotter than the coolant so heat travels away from the cylinder, not into it.
How does having a thicker cylinder wall help dissipate heat? I think it would retain it since it's denser than the surrounding water. Heat travels through the cylinder walls from the cylinder, it should cool faster if it has less distance to travel. The only area of a cylinder that can actually dissipate heat is the very outer surface of the cylinder barrel where it's in contact with coolant, the rest of it either retains it or transfers it.
I should have said absorb not dissipate... In even simpler terms take a 1" thick sheet of metal and a 1/4" thick sheet and put a torch on one side of the metal and your hand on the other. Which one are you going to feel first?
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  #15  
Old Dec 28th, 12, 01:00 PM
alanrw alanrw is offline
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Alan
 
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Default Re: Overbore overheat myth

Doesn't that just imply the coolant will get warmer faster at startup? I have never seen a figure for heat soak in a block as a cooling system consideration. Isn't it pretty much the transfer of heat via the coolant to the radiator?

As to the OP, why would a pin hole leak which doesn't result in substantial coolant loss cause the car to run hot?

alan
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