Technical Engine overbore heating

Some guys were discussing that boring an engine, specifically a 302 Ford, .060 over would cause it to overheat, but .030 wouldn’t!
I have heard all my life about the “ overbore heating” thing. I know there are engine gurus here on the H.A.M.B., and thought maybe they could weigh in on this issue.
I have my opinion, but would like to hear the opinions of others.










Bones

 

Me too.

Edit: actually, come to think of it, this has been discussed before. The Mexican block came into the discussion.

 

Sonic test the wall thickness before, and see what you have to work with. You can also find out if there was a core shift when it was cast...

 

I had a 3970010 Chevy block bored to 4.060" and it never overheated on me. Temp gauge is nailed at 180 all the time. I´ve only put about 70k miles on it, so it may start to overheat someday.

 

If the cylinder liners get too thin they'll flex and leakage down past the rings will increase. Not sure if that would lead to higher temperatures anywhere else than in the pistons (heat transfer piston-ring-cylinder is reduced). Besides, such issues should mostly show on high power, high rpm, normal everyday driving should still be just fine.

 

Has not bothered my Buick. .125 over.

Ben

 

He asked about the 302 ford witch is a thin wall casting, some have overheating issues at .030 over.

 

So what is the theory of what causes the overheating? The reduced thickness between the wall of the cylinder and the water jacket results in additional heat transfer from the cylinder into the water jacket, enough to cause more heat in the coolant than the radiator can shed to the atmosphere? Or the additional cubic inches results in an increase in heat produced that is greater than the cooling system can dissipate? Are the Ford cooling systems designed with so little cooling system capacity that this is an actual problem? (I know my 2013 Escape was, but that's off topic here). Is that in all models using a 302, or just certain models. Pickups? If not one of those 2, then what?

 

Dad's 455 Pontiac is bored .070" over with the lower half of the block filled with concrete, it runs 180* on the hottest days of the year.

 

i have bored many Ford 302 engines .030" over or more, never a heating problem that was the fault of the overbore. Ford designed their engines to be bored up to .060". Make sure the radiator is new or like new and it will cool fine.

 

Bones,
Years ago, when thin-wall Ford small blocks where still fairly new, I always heard that you should never bore one more than .030, .020 if you could. But I'm not even sure you can get .020 over pistons these days unless custom made or special order. I also see where at least two of the nationally recognized engine rebuilders are advertising their Ford small blocks at .040 overbore. I just have to wonder what has changed?

 

I've never had any problems too work though with cooling but it's a hot topic on some of the Ford forum's. Most 289/302 and 351s are low compression 2v motors so increases in compression ratio might have something to do with it. Most of the 60s unit body cars have small engine compartments so air flow has an effect plus radiator size. Tuning has an effect also, some guys can't tune there way out a music video.

 

Way way back in the day we would bore the Blocks all the way till
you hit the water jacket & Sleve it Way Way Back in the Day.!
and never had a Problem.

Just my 3.5 cents

Live Learn & Die a Fool

 

The discussion was started with the 302 Ford, but I was wondering about over-boring in general causing heat. My opinion is if the cylinder walls are thick enough to seal the rings it will not cause overheating.
Many times I have heard of engines rebuilt, bored out, and then the car overheats. I have always thought it overheated because the new engine was more efficient and the old cooling system was some what under par!

I was just wanting opinions as to if the overbore itself could cause overheating and what the effects of thinner cylinder walls is on heat transfer to the cooling system.
Thanks





Bones

 

The 49 Olds engine in my Roadster is bored to stock 324 bore size. Everybody said you could not do that but it was done before I got it. It runs 10 degrees hotter than the bored out 324 I took out. and works great.

 

I have an over bored flathead. Of all engines.
Naturally I was concerned about it.
This one is .120 over.
No magic here. Just a pressurized system and high velocity pumps.
Stays at 190.

 

Unless you're talking early flathead blocks, .120 is nothing. I've done my last three .125 over (pistons are cheaper), and a lot of guys run 3 3/8 (+.1875), and .030 over that. With the right block, sonic testing, some luck (and maybe a few sleeves), there have been 3 7/16" bore flatheads. There was a guy earlier with a Buick straight eight that he took out to .125 lover. The guys I used to race with would bore a Roadmaster engine 1/4", put in a set of pistons from an International, and run 'em at 356 ci.

Those are old engines. I guess SBF's are a whole different situation.

 

I got a .060 350 for free.
I’ve heard all the overbore heat tales too.
Never had an issue.

 

The over heating claims , most of the time was other part in cooling system where changed , different cams , not correct timing, also it depends on where you take temperature from will be different in different locations,
& what Considered hot??
Most engines make the most power with water temps 220-240

 

Ive heard guys worry when engines run 190.
Lol

 

I read somewhere that pre '74 302 blocks could most likely go to .060 over but it wasn't recommended, core shift being the biggest issue and pistons/rings were expensive.. .040 aimed at engines that are already over .030 that are "proven" so another .005 a side would be good odds on holding up?

 

It occurred to me that as the cylinder walls get thinner, at some point the remaining mass would not be able to transfer the heat effectively into the rest of the block, the iron would heat more than usual in concentrated spots, leading to more heat transferred into the coolant. Basically, the same heat is being produced by combustion, but less of it is transferred into the block and more transferred to the coolant. Just a theory, may not be accurate at all, but it seems more plausible than either of the other 2 scenarios I posted earlier.

 

When i was in the service stationed in California i got the bright idea to bore a 265 out .125 to make a 283. Waste of engine life i found out. Should have just went .030 or .040 and saved a couple more bores. Oh well, we all learn. It ran great, 10 t0 1 compression, solid Duntov cam, 2x4s and 411 rear gears. drove it back to midwest on leave. At 70 i think it tacked 3800 or there abouts. Reved so high i couldnt keep generators on it. ran 210 the whole time. Traded it off so wouldnt have to deal with return trip. Was an awesome little engine though.

 

SBF Windsor blocks are thin-wall castings. Mexican blocks, if you can find one, have the thicker main-bearing caps, like the K-code engines. However, I don't recall their having thicker cylinder walls. I would have any block magnifluxed and sonic tested before spending the money to rebuild it. I would also think that re-sleaving a block would be cost-prohibitive, unless it's a rare, valuable, numbers-matching block going into an equally valuable car. Personally, I wouldn't go beyond .040.

 

Initial post was "specifically a 302 Ford". If we want to discuss other engines lets start another make specific post. While the 302 Ford is a thin wall casting they can be successfully be bored to .40 over without fear of over heating issues. Any more should be sonic checked or you risk the possible scenario. High flow water pump and clean radiator are also essential along with a clean block to start with.

 

If in dought think about block fill. I've seen SBC filled to the water pump holes run on the street with on issues. When you think about it only the top 3 inches on 302 ever sees combustion gases.

 

Having bought,sold,traded over 50 flathead v8 engines over a 58 year period and built over 25 flats for my own use and customers, I have seen 2 engines that were overbored .250 or 1/4 inch. Both were running moving driven vehicles. When I build a big bore(anything .125 & larger) I spend a gr8 amount of time making sure the block is very clean. Even after hot tanking,I take a coat hanger and fab a sharpened j hook and “drag “ it along the pan rail and usually pull out a fair amount of “casting” sand that would normally remain in the block. Another trick is to barrel roll a empty block many times over a thick piece of plywood. The plywood absorbs the rigorous “shoves down” with no damage to the block.What I am trying to say is the “cleaner” the block is,the cooler it should run. I realize your question is concerning a thin wall Ford, but you still need to prep the block and get it clean in the water jackets.

 

From my Chem E days, in general a heat exchanger effectiveness is a function of area and thickness of separation. Specifically boring a cylinder larger benefits both. I currently have two Ford SBF blocks at .030" over and a Dart SHP. One of Ford blocks is mildly modified (9:1) and the other one is 12.6:1.
The 12.6:1 motor undoubtedly puts out more horsepower and a lot more heat. Horsepower and
heat are obviously related such that a built engine will produce more heat and not necessarily from thinner cylinder walls. Early Ford blocks are so thin walled that most sonic test don't even bother to
average the non-thrust thickness. My 12.6 motor has a non-thrust variance .084"-.149", however the
major thrust thickness has an average of .180" and no issues except it would not be a good street motor.
It will be 195-200* after an eigth mile pass. The Dart block has a 4.125 bore and 60 lbs heavier.