Beware the ‘rebuilt’ engine

Beware the “rebuilt” engine.

Please consider the following.  When an engine is being assembled at a factory, all the parts are brand new.  That is, they have never been exposed to oil or dirt or what is significantly more important … heat.  Consider how many cycles of heating and cooling an engine will experience over say 100000 miles.  Thousands.  Now, after several hundred thousand miles, each and ever part in that motor has been heated and cooled thousands of times, and has changed slightly.  No, not even slightly; minutely.  The tiniest fraction.  A good example of this is a quite common problem with Chevy exhaust manifolds.  Often times if you remove an old Chevy exhaust manifold, you will find that it has shrunk when you try to put it back on the motor.  You will be able to get most of the bolts in, but several on the end ports will not line up.  So what has happened?  Over time, and after thousands upon thousands of cycles of heating and cooling, the exhaust manifold has shrunk.  It is an unpredictable and inconsistent property of cast metal associated with thermal cycling.  Shrinkage is not always the end result either.  It is more accurate to make the general statement that metal may tend to change shape slightly after extensive thermal cycling.


This behavior can be explained by comparing new metal to a new pair of shoes.  At first there are areas of the show that press against your foot or are too tight.  But after wearing them for a while, they loosen up and your feet adjust to them.  Metal is similar in that there may be slight stresses in the metal parts when they are made, but over time and many heat cycles, the stresses relieve themselves.  Essentially, metal can be thought of as a crystalline structure.  And when metal is being formed into some shape, it is usually done by heating the raw metal until all the crystals let go of each other.  Then they are forced into the desired shape and allowed to cool down.  Upon cooling, the crystals bond back together and restore the strong and durable qualities that we have come to know and love.  However, the strong and durable qualities that we love so much are mostly dependent on how much we heated the raw metal, and how quickly we cooled it down afterwards.  This process is generally known as tempering.  Our problem, or the stresses that I mentioned can occur when the some of the crystals in a piece of metal cool down and solidify in a specific orientation as they are inclined to do, while a section of their immediate neighbors mistakenly solidify in a different orientation.  This situation creates a stress inside a piece of metal.  Now this, as I said before, is an over simplification of several gross generalizations, but I am using them to illustrate a point.  If a metal part has some inherent stresses built right into the metal during manufacturing, over time and thousands of thermal cycles, and stresses and strains and vibrations, the metal will relax and the stresses will correct themselves or lessen to the point of becoming irrelevant.  If the flaw is too extreme, the cycling and vibrations will eventually result in a catastrophic failure of the metal and the part will break.  This is a very general explanation why some engines break down and some seem to run forever.  It is pure luck.  To be thorough, there are many mechanical breakdowns that can be attributed to poor designs, but there are just as many that can be attributed to material failure.  They are purely random and largely uncontrollable material failures.  So why have I explained all about this in so much detail?  Because I wanted to provide the background for one important fact; if it ain’t broke, don’t fix it.  If an engine has a major flaw, it can happen at any time.  Catastrophic failures can happen at any time.  They are not really related to the age of an engine.  If a motor is running and does not give any indications that there are any problems, it will most likely stay that way for the foreseeable future.


Many people use the term ‘rebuilt’ when describing the condition or overall quality of an engine.  However, this term can be misleading.  Simply rebuilding a motor can mean almost anything.  Generally, rebuilding an engine means that the cylinders get bored out, new pistons get installed, and depending on the extent of the rebuild, all other components including bearings get replaced.  So how much more power can be expected from a freshly rebuilt motor?  Almost none.  Sure, if your engine was miled-out and had no compression left, the rebuild is going to be like night and day.  But in terms of raw performance, rebuilding an engine without adding any high performance parts will not add much power.  Given this statement, I have a piece of advice that applies specifically to purchasing a vehicle:


Avoid paying a premium for a vehicle or an engine that has been “rebuilt” unless you are purchasing it from a reputable machine shop that is willing to stand behind the work. 


If a seller tells you that the engine in his car has been rebuilt, ask to see the receipt or paperwork on the engine.  Ask if there is any warrantee on the parts or labor that went into rebuilding the motor.  If he cannot provide anything substantial, refuse to pay more than what the car would be worth with the stock motor still intact.


Here are some facts to consider.  Again I am dealing with gross generalizations, but they are important facts to consider.  Internal combustion engines are in their best functioning state when the original factory components and assemblies are still intact.  Let me elaborate.  It is unlikely that an engine can be assembled more securely than it is when it is first built at a factory.  This is due to many factors, but mostly it is because of the highly controlled environment in which engines are assembled at a factory.  Hardly any engine components are assembled by hand, but if they were assembled by hand, it was in a controlled environment outside of which, similar quality is difficult to duplicate.  Furthermore, assembly machines are built to manufacture and assemble one specific motor.  Why is this important?    Because the tolerances and quality control in a manufacturing environment like this is much higher than it is in a rebuilding shop.  Now this is not to say that engine rebuilders or machinists aren’t very skilled and pay very close attention to their work.  It is simply a statement that engine manufacturers have the benefit of high volume, high quality parts.  They have the benefit of computerized machinery that is specifically designed to ensure that every bolt is tightened to an exact torque specification; that every bearing surface is flat and square to within a phenomenal degree of precision. I guess that the proof is in the pudding here too.  The data shows that rebuilt components rarely ever achieve the same longevity as OEM parts.


Let’s consider the opposite.  When an engine is overhauled, it is completely torn apart and thoroughly cleaned.  Okay, that’s not such a bad thing.  But then the machining takes place.  The cylinders are bored out or honed making them larger, and consequently making the cylinder walls thinner.  The crank may be ground.  This means that the surface where the crank and rod bearings ride is ground down and also made smaller.  In some cases the head mating surface is milled which means a thin layer is shaved off the surface of the block where the heads attach. This milling process leaves the cylinder slightly smaller and might actually increase the compression slightly.  But lets consider what adverse affects might occur with each of these processes.


  1. Cylinder honing makes the cylinder walls smaller.  Over the millions of strokes of the OEM piston, the cylinder walls of an engine are exposed to countless heating/cooling cycles.  Remember the first paragraph in this section?  The heating and cooling cycles actually harden metal over time.  So the cylinder walls have been exposed to millions of heating and cooling cycles and have therefore been significantly hardened.  During a rebuild, the hardened surface is removed and the new pistons end up sealing against a slightly weaker metal surface.  As well, depending on how much metal has been removed from the cylinder walls, the fact that the cylinder walls are thinner can be quite significant.  Boring cylinders 0.60 over is about the maximum that most machinists will recommend.  Even at 0.60, the cylinder walls have been made significantly thinner.  Now the heat generated in the cylinder has less metal mass between it and the water jacket.  This change in cooling capacity within the cylinder can drastically affect how the cylinder functions.
  2. Bearing races.  Similar to the argument in the previous section, grinding a crankshaft will flatten out the bearing surfaces, but it will also remove the hard surface that was there before.  As well, the bearing surfaces after grinding are slightly smaller.  The load bearing capacity of the crank overall is less.  Think about it, if you took a driveshaft and ground a band of metal off the outside of it, do you think it would still be as strong?  Think about the bearings.  After grinding the crank, there will be more room where the bearings need to go.  So during a rebuild, they will put oversized bearings where the original bearings were.  Bearings are made of much softer metal than any other metal in the engine.  So during a rebuild, we effectively increase the size of the bearings.  In very general terms, the oversize bearings reduce the stability of the crankshaft in the block and the stability of the push rods on the crankshaft.
  3. Milling the heads.  Yet again we are removing a layer of hardened metal.  A more significant factor with the milling process is the flatness of milled surface.  This surface plays a pretty important role in the function of the motor.  The head gasket goes on this surface and seals all that lovely compression in the cylinders so that you can eventually use it to chirp your tires in the Safeway parking lot.  The slightest deflection in this surface will stress the heads and may lead to a head gasket leak at the least or a cracked head at the worst.


The basic facts are that machinists, for the most part, are very skilled and very precise in their work.  But rebuilding a motor basically entails removing hardened metal from some very important surfaces inside an engine.  That fact is combined with the possibility that during the machining, some surfaces are not milled or ground quite square.  In short, there is a way higher probability that flaws occur during a rebuild than at the factory.  So when deciding whether or not to rebuild your motor, consider the following:


  1. Are you adding horsepower by inserting performance components to the bottom end of your engine?  If you are, then rebuilding is unavoidable.  If you have a numbers matching block that you want to use, make sure you find a reputable machine shop and GET A QUOTE IN WRITING FIRST.  If you do not feel inclined to use the stock motor, consider purchasing a new motor.  I know, I said it. Buy a new motor?  Check it and see, there are brand new motors available nowadays that are comparable to the cost of rebuilding.  And citing all the facts that I have just laid out, new motors will give you better bang for your buck.
  2. Is your motor miled out,  If this is the case, weight your options.  Do you want more power?  Is the stock motor smoking, running rough, etc.  It may be much cheaper to explore some other options prior to doing a complete rebuild.  Check the timing chain, replace your ignition system and make sure it is tuned properly.  Make sure that your fuel delivery is adequate.  Poor carburetion can cause rough running and power loss and is easy and cheap to fix compared with rebuilding your power plant. We will cover this in more detail later, but a compression check is worth its weight in gold.  If your compression is even (within 10%) between cylinders and is over 100 psi, you motor is not that worn out.  Consider adding a cam to freshen things up.  Bottom line, explore your other options before spending the big bucks on a rebuild
  3. Your motor is seized, shot, missing, inappropriate for what you are trying to rebuild?  Consider 2 options.  First, purchase a new motor.  For all the reasons listed above, a new motor is a way better investment than rebuilding a used one.  Remember, ask about the warrantee!  Buy a good used motor, if you can hear it running or preferably driving beforehand, even better.  Any good, low mileage v-8 engine can be made to adequately power a hot rod.  If you want big power, look for new race motors.  Beware rebuilt high horsepower engines.  At the very least make sure that you get a really good deal.



First, if it ain’t broke, don’t fix it.  If an old engine seems to run all right, doesn’t smoke, and has some power left in it, for god sake leave it alone!!!!!  I have heard so many guys say,” Yeah it runs fine, but I’m going to pull it all apart and have a look at it to make sure it’s good”  Well here is the point of my argument.  Just the act of taking an engine apart can be destructive.  Remember how I went on about stresses and strains within the parts that make up a motor?  Well when you disassemble a motor, you will un-stress all these parts and when you go to put them back together, you are very unlikely to return them to their original state.  And however small the difference may be, it is most often not beneficial in the long run.  There are statistically few internal malfunctions that can occur in an engine that will be visible to the naked eye; malfunctions that you might detect by stripping a motor apart and looking for.  In general, if something significant happens inside a motor, you will be made aware of it.  You will notice a noise, smoke out the tailpipe, or a lack of power, rough running.  Basically I will say it again, if it ain’t broke, don’t fix it.  If you are going to rebuild it, rebuild it.  But don’t take it apart to see what’s inside, as it pertains to engines, this often does more harm than good.


In general, you will not get as much mileage out of a rebuilt motor as you will out of a factory engine.  For all the reasons I explained above and several others, rebuilt engines tend not to last as long as untouched stock power plants.  Therefore, if you are not trying to squeeze more performance out of a stock block and you are lucky enough to have come across a decent stock motor, let it ride.  Don’t rebuild it just for the sake of rebuilding it.  Wait till there is a reason to rebuild.  Most likely you will be able to drive your antique your whole life before the stock motor gives up on you.


I currently own and drive a 1996 Chevy K1500 pickup.  I love this truck.  It has done more work for me than I can possibly put into words.  I would like to say that I treat it right, but that would not be the absolute truth.  When something broke, I fixed it.  I never blatantly mistreated it on the road, but when there was some hard work to be done, I drove as hard as was needed to get the job done.  At the time I wrote this book, my truck had 400,000 kilometers (250,000 miles).  I have replaced nearly every other component but the engine.  I have had to re-gasket a couple of things, replace a couple of peripheral components, but the original engine is still intact and running fine.  So the proof is in the pudding, let your stock motors be.

Let’s summarize regarding engines:


  1. Horsepower and torque are generally the same thing.  Typically, more torque equals more horsepower.  However sometimes horsepower increases can be misleading because they refer to increasing the maximum rpm of an engine, which is usually not in a range that will be beneficial to the average hot rodder.
  2. The most significant power increases are a result of increasing the compression in a motor.  Increasing the compression of an engine is most often accomplished by altering the ‘bottom end’ of an engine. (pistons, crankshaft, ….) These types of upgrades are the most expensive, and usually have to be performed by a machinist or in conjunction with rebuilding a motor.
  3. Most other types of performance components only serve to increase the air flow through an engine.  (intake manifolds, high performance heads, headers)  These components will increase horsepower, but mostly in a higher range of rpm.  In most cases, these types of components are installed for esthetic reasons or for sound effect.

I hope this post helps some of you decide which path to take when it comes to choosing whether to rebuild, replace, or simply refresh your engine.  Look for the some of the great deals our advertisers have on engine products and services.  As well, check out our service directory to find the reputable shops nearest you.

And as always, tell your friends!