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.

 

Most 402 blocks will go at least 0.060 over, which would correlated to the same as 0.090 over for a 396.

 

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.

 

I agree. I had a .060 over 350 that never came close to running hot.

 

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.

 

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

What's the OP's point?

 

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.

 

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.

 

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...

 

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!

 

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.

 

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?

 

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

 

I have found that throughly cleaning out the crankcase, lower cylinder walls etc or whatever you want to call them that the engine runs cool as it should.
All the gunk falls down that way and cleaning it out restores the original cooling efficiency.
Remove the pipeplugs just above the oil pan ans rod them out, blow them out boil the m out, wwhatever it takes.

 

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

This is the same as having a faulty radiator cap , no sealing....drop in pressure.
This effect can often be seen by idling a engine with no heating issues for a long time with no cap on and the coolant temp goes abnormally high.

 

I have a motor that is a 396 punched out to a 427 standard bore size. It is also filled up with hard block to the bottom of the w/pump holes . Pistons are the orig 435 hp dome pistons. It is in my 67 L78 camaro with a factory radiator and shroud. You can't get it to overheat. Doesn't matter if its in 95 degree traffic jam or not. It just doesn't overheat. Its about a 10 1/2 to 1 compression

 

Any overheating issues as mentioned are likely related to the cooling system, and not cylinder bore.

 

But..... We're not talking about random chunks of metal with a varied heat source. We're talking about a specific block dimension, and specific heat ranges, which we have literally tens of thousands of examples for. Mine being only a few. I too am a degreed engineer. But I, nor would I guess any good tech, need to pull out the old formulas to diagnose this phenomenon.

Resume debate...

 

I'm curious how the same amount of energy will heat a larger piece of metal quicker than it will a smaller piece of metal. Doesn't it require more energy to heat a larger object than a smaller object of the same material?

OK think it like this...
Heat disapates thru the medium, in this case metal, bascally in the same manner as a gas in a room... the heat is the gas the atmosphere in the room the metal and the size of the room the size of the metal
Now shoot a CO2 fireextingisher into a big room... the condensation hangs for a couple seconds and disapears.....now fire it into a very small room and it hangs around for much longer before disapating
now take

 

Heat doesn't rely on surface area as much as other aspects... I too do not remember exactly what goes on, it has been a while since I have actually had to apply these things. I mostly do statistics now... which is why I first entered the debate. LOL