Big, thick cast iron V8 head bolted to a big, thick block. Or a straight engine. Thin gasket material in between. How can they warp? Lots of torque on the bolts. Apparently they do not straighten out when tightened or we would not need to mill them. So how do they warp?? I just can't wrap my head around that. Exhaust manifolds I get...
High heat can actually change the grain structure and even the orientation or direction of the grain or cells of the metal which especially after cooling can impart great stress. That is, stresses different than it had originally.
Even when solidified, I think of metal as being sort of like a liquid. Just takes more to get it to move.
Every time the motor heats up and cools down, it changes a little. Some places on the heads are thicker then other places on the heads, so the thin places heat up and cool faster then the thick places do. Every time it heats up and cool down, those thin and thick places move different amounts. Since the head is all one casting, when one part of the head is moving, and a different part of the head is not moving the same amount, something has to happen, its called a warped head.
Here's a little story about cast iron I wouldn't have believed if I did not experience it. I was building a sandwich head from two cast iron heads slab cut. I sent the pieces to a noted cylinder head expert to get the ports welded up in areas where I wanted more metal. Much to my dismay when I got the pieces back the head surface had warped by .150". I was so disappointed that to my mind the head was ruined as it was going to take in excess of .150" to get the head surface flat again. I tossed the pieces on a shelf for ten years. Sometime later I was telling a friend about my unfortunate experience and I pulled the head off the shelf to show him how much welding on the head had warped it. I laid a Starret straight edge on the head and to my astonishment the head had relaxed and was perfectly flat! In another instance I removed the head from a race motor that had been completely blueprinted, included milling the block and head surfaces. After having been run all the tapped bolt holes in the block had puckered upwards. The block needed resurfacing again to get it smooth again. My point is, heads are subject to mechanical forces. Heads are subject to thermal forces. When the combined thermal and mechanical forces overcome the integral strength of the head casting things begin to move on a microscopic molecuar crystaline level. The more times heads get cycled through thermal and clamping forces the more likely there will be creep in the casting as evidenced by warping.
See... like liquid. Or at least, putty. Just takes more to get it moving. More heat. More pressure. More time. We avoid allowing a crankshaft or camshaft to lay on its side for very long.
It's called "stress relief". All materials go through change because of outside factors and forces in order to "normalize" the stresses inherent to the manufacturing processes involved in the original construction. Heat cycles, pressure cycles, vibrations cycles all have an effect towards stress relief. It may not become apparent while two components are firmly bolted together, but once separated, the stress relief and subsequent "warp" becomes evident. It was once very common for manufacturers of high-precision components to age raw castings (of any material) for long periods of time (years in some cases) so they could go through ambient heat cycles to help reduce stress. As an example of stress relief, take a 12" long piece of 3/16" X 2" cold rolled steel and mill about 1/32" off of one side (the 2" side). Removing the stresses on that side (from the cold rolling process) will cause it to relieve and turn that once straight flat bar into a banana.
Not completely right for heads in normal use, but one way metal can alter shape due to heat applied in various places. Quite interesting (and useful).
Internal stresses in the casting process can work their way out over time Racers used to use "seasoned" blocks for racing engines where the cast iron has settled into a much more stable structure.
The girl at bottom left is looking at me the way our one cat does. The teacher is looking at me the way our other cat does.
amazing how metal has a memory. Manufacturers who deal with thick materials use heat treat ovens to even that out. At Babcock & Wilcox we would slide 1-5 ton weldments in those gigantic ovens to even out the stresses.
I never really thought about the “why”, but if I had heads in to be rebuilt I always had them surfaced. I figure with the cost to rebuild a set of V8 heads, what’s another 90 bucks.
Hello, I may not have been the first mechanic in our household, but at least my brother gave me some tips that have lasted a lifetime. One of those was to make sure our cars/motors ran cool and kept that way with water/antifreeze and other items. He also told me that when the steam comes out of the radiator, stop the motor and pull over. Do not continue to drive to prevent any further damage. No water or less water is a sign that something is not right. Finally, as assembly of anything goes, even torque specs one each bolt is necessary to spread the pressure evenly over the surface being tightened down. The term “Gorilla Grip” on each nut/bolt is not the answer. He showed me the criss-cross way to tighten each row or rows of nuts and bolts. That at least gives the resemblance of even pressure over the surface being attached. The specific pressure on each bolt was dialed in with the torque wrench and when the original wrench had the arrow pointing to the number, that was it. But, even then, the eye on the arrow could miss a mark or two. So, when the click torque wrenches came on the market, that allowed sound to tell anyone the final tightening is done. But, the even spread of the criss-cross pattern still applied to all nuts and bolts. Jnaki But, in any case, lack of water certainly speeds up distortion, the lack of oil at any temperature also does the same to metal on metal surfaces. So, the upkeep on any motor, surface or inner doings, is always a concern, as well as the even spread of the tightening of specific nuts and bolts. In a running motor, the flow of air also helps keep the metal parts relatively cool or normal running temperatures. YRMV
Different thickness of metal contain different potential to move. Different temperatures change the amount of movement. When looking at a cylinder head, how many different thicknesses are directly adjacent to each other? And again how many different temperature zones are directly adjacent to each other? Actually it’s wonder how the heads can remain relatively flat and straight at all. Some of that cylinder head never gets above the coolant temperature, some of that cylinder head reaches temperatures far above the coolant temperature. The exhaust ports get quite a bit hotter, the bottom of the combustion chamber gets really hot. The metal surface directly in the cylinder has a given thickness. The combustion surface gets really hot on the other side of that surface is coolant temperature.
There is a reason that automotive machine shops have a head facing tool. Heads warp. I think that it is a combination of heat and bolt stretch. I am probably wrong. The next question is how to decks get out of square. I have faced more decks in my life than I want to think of. Both heads and decks come from the factory moderately straight. They both seem to shift around when they season.
Uneven torque. ---and never lay one flat on a hard surface-go to a quality machine shop and see how they store their heads.
From the moment the molten iron is poured into the mold, it is cooling different areas at different temperatures. If one has machined enough iron, he'll learn the same head is harder in some places than in others because of the way it cooled out of the mold. And no, block decks weren't square and flat when they came out of the broaching mill. Watch old videos and GM was shoving blocks through a broaching mill with hydraulic rams at very high speeds. So fast and so loose is why a new 1955 Chevy V8 cost $56 to make. jack vines
I've always heard to let an engine cool before taking the heads off because they will warp. But the top fuelers take them apart hot all the time.
And top fuelers rebuild the motors ever few miles, most probably much less the 5 miles on them, 1/4 mile at a time. Do you want to have to rebuild the motor in your car every 5 miles?
All this and more . Ive done many many heads on propane units with low compression , low torque head bolts ( 2 step 35 ftlbs , 65ftlbs and done ) With a ton of usage and the heads twisted like a pretzel and the bolts are no longer usable . tons of force and heat put on ‘em to really figure it out it’s a bunch of math and sciency stuff that’s wayy above my pay grade !
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