Technical Engine overbore heating

I hear you and maybe I was too specific, but my question was “ does over boring cause heating? On any engine....






Bones

 

That depends on the make of engine. Typically Ford thin wall castings can lead to overheating with outrageous over bores. Other makes depends on the amount of overbore and cylinder wall thickness. Obviously if you go too far on any engine with a deficient cooling system you will overheat. Which is why it is imperative to clean the water jacket areas to induce effective cooling as much as you can. Have a good quality water pump and clean radiator with airflow directed through it. Several Shoebox Fords are missing the lower pan below the frame that directed air flow through the radiator. Others are missing the upper radiator surround which causes the same effect. Engine pans on a banger Model A contribute to air flow through the radiator and out the hood sides. Modern engines depend on fan shrouds and other devices to effectively cool them. I have bored several Ford thin walled casting blocks to .40 over with no issues as long as the cooling effectiveness is addressed.

 

The thin cylinder walls on an aggressively overbored engine leads to cavitation of the coolant, not necessarily overheating. The cavitation is caused by the very small motion (vibration) of the cylinder wall during regular operation. The cylinder wall flexes as the pressures inside change. As the cylinder wall pulls away from the coolant, any bubbles caused by boiling of the coolant implode within the fluid's boundary layer. This implosion can generate very high localized stresses and begin to erode material. Diesel engines with wet cylinder liners are especially sensitive to cavitation as the cylinder liners can vibrate and move around a bit more, but any engine is susceptible if the right harmonic conditions are present.

If any of you have ever owned a 7.3 IDI in either a Ford or International truck, you are familiar with the problem. In the late 80's, in an effort to increase the power of the 6.9 IDI V8, International decided to bore it out to 7.3 L (444 ci). Unfortunately the cylinder walls got a bit thin and the engines where notorious for cavitating, especially if regular coolant maintenance was not performed. IH and Ford offered all sorts of remedies, including coolant additives, high flow pumps, and on, but they never fully cured the problem until the redesigned 7.3 PowerStroke came out in late '94. Eventually the cavitation would erode the cylinder wall to the point that it would crack or you would get a pin hole and start burning coolant. This could lead to overheating as combustion gasses get worked directly into the coolant system.

 

As a journeyman level heavy duty diesel mechanic for a number of years I am very familiar with the cavitation erosion that occurs in wet sleeve diesel engines (as well as in the water pump). The formation of the bubbles occurs due to the pressure inside the coolant right next to the cylinder liner falls below the vapor point as the liner vibrates in, or away from the coolant. The bubble forms, and then as the liner vibrates back out, or toward the coolant, the pressure rises very quickly causing the bubble to collapse & implode on itself, as you described; and that implosion results in a very small amount of metal being etched away from the liner. And yes, eventually, if this is left to continue pits form that will bore all the way through the liner, allowing coolant into the crankcase. The most common symptom that comes up is coolant in the oil. When you remove the oil pan and pressurize the cooling system, then slide under the engine with your flashlight you can see the coolant running down from in between a piston and it's liner.

When I started in the business (which I eventually left about 22 years ago) our equipment used old IH diesel engines that would stand any old coolant, they weren't finicky about it. Hell, you could fill 'em up with pond water off a job site if you had to, they didn't care. But the company switched to Cummins engines because IH had sold the business, and the use of the engines wasn't part of the deal. So with the change to Cummins engines our customers continued to not give much care to the cooling systems of their equipment, and we never trained them they had to, because none of us knew what we were in for. Holy crap! All of a sudden we had engines failing with cavitation erosion of the liners, still under warranty, and Cummins said tuff titty, that erosion happened because you didn't use the right coolant, no warranty coverage! Wow, that was a hard lesson. So that's when we learned about Supplemental Coolant Additives (SCA). That was in the 80's. Proper coolant additives coat the surface of the liners, and when the cavitation happens, because it will, the implosions etch away the additive coating, which is them replenished by more additives in the coolant. And this prevents the erosion of the liners. Eventually the additive level in the coolant dissipates, and you have to add some more. Mechanics test the additive strength with test strips on a periodic basis. They also use coolant filters with time release additives to maintain the coolant strength. I've rebuilt a lot of engines over the years due to cavitation erosion pitted liners.

 

My personal experience with SBF's has been that they just run hot... had a 302 that was bored .030 and cooling that one was always marginal no matter what I did. Also had a 289 bored .060 that finally cooled down when I put a 4-row core in the OT car.
Then there was the Y-block in my '56 wagon that had been bored .125 with lots of compression; never ran hotter than the thermostat no matter what I did with it...

 

Can’t always bring the text book to the car! I learned in Physics, back about the middle of the previous century, that area had nothing to do with friction! Just the amount of weight placed on the object to drag it! In other words a skinny tire would get the same traction as a wide tire! Sometimes other factors bend the rules!








Bones

 

Boneyard, another subject, but I also recall my physics professor saying that tire width has nothing to do with traction.

My computer lost the source, but performance tire traction has four factors. Three are intuitive and I can't remember the fourth.
Traction equals:
1. friction (normal force x coefficientof friction)
2.+ adhesion (where contact patch makes a difference)
3.+ surface compliance of tire where it conforms to micro variations of the track surface
4. + I can't remember the fourth

 

#4 is track surface/contact surface the motion and force of the tire reacts against....in this case could be gravel road, could be asphalt, could be concrete, could be either asphalt or cement with rubber laid down on it, could be glued with track prep or rosen, could be packed clay, could be ice, etc.

 

Bones , I have Buddies that punched 302s a ton . I have not researched the 427 cube from a 351W block . The increase in cubes to me would require increase in cooling capacity and good to to go .

 

I don't necessarily think an overbore will cause overheating. But a combination of things probably could. Like a thin cyl wall and high compression with good fuel could. Combustion heat is the killer. Low octane, no compression and thin cyl walls, I see no problem. I do know thin cyl walls and the rings lose stability, causing ring seal problems at high cyl temps and pressure. But a sbf with thin cyl walls and 8 to 1 compression on pump ethanol could probably get by with thin cyl walls. No longer than they last anyway. lol Lippy

 

If you hang out on a lot of Ford Forums and early Mustang sites most will not go over a .040" bore because of overheating beware of rebuilders that are selling .060" crate motors ATK has sold a lot that way as well as your local chain store outlets. The 302 block is almost 100 lbs lighter than the SBC Gen 1. Also it is not uncommon to install SBF head gaskets wrong and that will cause overheating ASAP

 

So regarding “sonic testing”. How thin is too thin? I/e if you got the results, what would you use as a baseline? Next, does sonic testing show any casting oddities? I/e porosity?
Or do you just rely on what a shop that can do it tells you go/no go?

Next thoughts would the results of the testing be dependent on the build of the engine? It’s use, etc. or is it an all inclusive result?

 

Many moons ago my brother had an .060" over bored 289 in a 65 Mustang that wouldn't stay cool. Mustangs have a tiny 17 x 19 radiator. He found a JY crossflow radiator that was huge and adapted it. It worked.

As far as 427 from an SBF the only example I've seen are 4.125" bore aftermarket blocks. In theory you could do one from a Ford 351W block by boring it .040" over and dialing up a 4.16" stroke combo with a 6.125" rod, a piston with a 1.305" comp height and you get 426.6 cid (a 427 in my book) but the crank would be a custom grind. It's possible.

I built a 422 SBF Windsor with that rod and piston but .030" over and a 4.14" stroke offset ground Ford 400 crank. Mighty close to a 427. The rods are Mopar 340 rods and the pistons are KB hypers. I cooked up that formula spinning off a 377 stroker I read about in a magazine and going a couple steps further. That was many years ago before stroker kits were plentiful. Steve Moody at AMS here in Fresno did the machine work.

 

Well , I heard it again today, attributing heating to thin cylinder walls!








Bones

 

the bore on my 302 is .040 , no trouble have walker radiator ,elec. puller fan ..................

 

The shorter the deck height the thinner the wall can be. So compareing different engines that should be kept in mind Numbers from different builders are all over the place. In 1953 Ed Iskendarian went with .060 on a flat head now most flathead builders go with .100 . I see other builders for most engines go with as much .200 on the thrust side. As far as radiator I go with 2 square inches of radiator per cubic inch of engine. I swaped a 350 for a 4.3 in a Astro van and it would not stay cool on the Hiway with out more radiator.

 

Lots of interesting comments here.
I cannot specifically comment on the 302 Ford as requested, but do have a SBC 283 that is bored 60 over to 292 with a pretty healthy cam, as well as other stuff.
It was Sonic checked ahead of time.
Love this little engine, and pure pleasure to drive.
I have a Chevy 396 radiator in it, plus an older good Lincoln MK electric fan and a 180* thermostat.
Once over ~2500 rpms in most any weather it will run at ~195-200* but when back in town, or idling drops right back to ~180*
While at first I was a bit uncomfortable with it, have learned that this is nothing to worry about... but definitely due to the boring situation...

 

I may have misspoke on the initial post. I was just using the 302 Ford as an example. My real question is. “ does overbore, thin cylinder walls cause overheating? “ If so ,how much?

I have my opinion on this, just trying to get others.






Bones

 

I would say "shouldn't", given there's less metal for the heat to have to conduct through before it can dissipate into the coolant.

However, in the same vein as the sidetracks: A question. Those who have experienced an overbore causing cooling issues... Was that the only change made to the engine? A lot of the comments seem to come from "I overbored the engine, and then a new cam and then headers and a different carburetor, and...", any of which could be a factor and not particularly boring out.

Phil

 

As with anything that is done by different individuals, you often get different results. When "they" talk about overheating, is it simply a rise in guage reading, or might it be that water actually boiled over? I'm not knowledgeable specifically about the Ford, but let me tell you about another engine. The little Buick/Olds 215 Aluminum V8s made in the 60s. They were a pretty decent engine. Due to casting problems, GM sold the manufacturing rights to Rover in England. Rover upgraded the casting process along with a few other refinements. As time went by and more power was needed, they bored and stroked the engine. They eventually got it to displace 4.6 liters.......about 280 cubes. Thats basically adding a whole liter to a small engine.
The problem was that as they kept increasing the bore, the liners got thinner and the largest version of the engine began to experience "overheating" . Many of these vehicles got scrapped and the engines were sold to enthusiasts for rebuilding. The general thing was that it probably was a head gasket that had blown due to overheating. Further inspection often revealed that a certain cylinder had developed a hairline crack to the water jacket. It was prevalent enough that Darton made replacement liners specifically for that engine and a specialty company in England is (as far as I know) the only source for them. My suspicion is that maybe some of these thinwall Fords suffered the same afliction. Thats total speculation on my part, but I don't see how an engine that maintains its integrity would suffer any major change in temp just by enlarging bore size. But I been wrong before.........

 

I like the way you think Phil, that is my opinion. Almost every overbore is a new engine, making a lot more horsepower than the old wore out engine! To me that and the friction of a new engine is more of a factor in the over heating than the thinner cylinder walls! The new more efficient engine is just making more heat than the old cooling system can handle! Just my thoughts!






Bones

 

Speaking specifically about the ‘60s SBF, I would have to agree. My (stock V8) Mustang always ran hotter than the other cars I had owned and when I pulled the radiator out to have it serviced I was surprised how small it was. I don’t think Ford provided much headroom as far as radiator area and coolant capacity. I’m sure many guys rebuilt or hopped up their SBF and then dropped it back in without giving much thought to the wimpy, dirty old radiator. And there lies the cause of the proliferation of the overbore fever stories. A better quality radiator made my Mustang run twenty degrees cooler.

 

Yeah, the Buick design was good until they upped the CR and the blocks started to flex too much and crack; that was resolved by adding significant amounts of material in the lifter gallery, the introduction of larger valves increased the torque so then the bottom end gained cross-bolts; taking it from 3.5 liters to 3.9 saw a higher deck- those were ok but once they bored it to 4.6 liters that's when they would overheat because of cracking between center bores and failed head gaskets.
Like you say, it sounds a bit like they found the limit... In normal running they were just fine but pushed hard the smaller contact patches for the gaskets and stresses in the block with additional power output would cause non-immediate failures, and once it's overheated one time that warp sets in and it gets worse from there on in.

Cast iron blocks like the 302 should suffer from that less, particularly with just an overbore, but a lot of the time it's done for performance so it's likely the engine's going to be worked hard. That inherently attracts trouble, I've found.

Phil

 

The OP specifically asked about boring a 302 Ford .060. Why are you sharing your experiences about boring other engine blocks up to .125? I would limit a 302 over-bore to .040. If it won't clean up at .040, he might want to look for another block before going to the trouble (and expense) of doing a rebuild.

 

my experience was a sbc, 30 over. Had a Walker radiator in it, stock it had no problems. After the bore, it ran hot. I called Walker, he said any overbore created probs. for his rads, and of course he sold a "heavy duty" for overbores. I figured out how to make mine cool, wasn't about to buy another one.
So yeah, i'd think that any bore would create a heat problem, how much is anyone's guess.

 

Well, I'll throw my .02 worth in....

Years ago I bought a 428 SCJ motor that had been 'freshly rebuilt'. After installing it, I immediately had overheating issues with it, and after applying the recommended fixes it still wanted to run hot and would quickly overheat if run hard. It was finally determined that the motor had been blown up, damaging the block. Apparently, the shop was unable to source another 428 block and punched a 390 block .080" over (being young and dumb, the standard bore size should have been a tip-off but I was so thrilled at the 'deal' I got I missed it...). So I'm of the belief that thin cylinder walls can definitely contribute to overheating issues.

Remember that these motors shed excess heat in multiple ways. A big chunk (roughly 50%) goes out the exhaust, about 25% into the cooling system, 20% is converted into motion, and the remainder is either radiated into attached parts or transferred into the oil. That's why you find oil coolers on some motors. Don't hold me to these percentages, I'm sure there's variations depending on what you have.

Anyway, a given chunk of metal will 'transfer' heat due to its properties, but size definitely enters into it. Think of it as like sizing wire; a bigger wire can carry more amps, a bigger piece of metal can transfer more heat. So thin walls can develop hot spots because they can't radiate the heat away quickly enough, and while changing the cooling fluid composition, increasing cooling system pressure or adding an oil cooler can compensate for that up to a point, too many hot spots can overwhelm the system eventually.

Also remember that adding more power adds more heat. A .060" over stock rebuild could run just fine, but bump compression, bigger cam/carb, etc even a .040" over combo could become marginal...

The 428? A swap to a .030" over center-oiler 427 block and custom pistons fixed it... although the extra 27 cubic inches (and other changes) did require another radiator upgrade.

 

Thanks, for looking out for me! The question originaly was about a 302 Ford, but I was wondering about overbore in general being the cause of overheating!






Bones

 

In my lifetime I have managed to overheat quite a few engines.

Only one was a Ford Windsor, and it was not over bored.

 

To address the question of insufficient cooling of a slightly larger engine with the original radiator, I replaced a stock 260 V8 in my 1964 Fairlane with a stock 1969 351W. No problems with overheating with the original radiator.
Manual transmission.