Back in the 1970s and 1980s, the average life cycle of an engine was about five to seven years. After 60,000 to 80,000 miles of everyday driving, most engines would develop an oil consumption problem and begin to experience other signs of wear (loss of compression, loss of power, increased emissions, lower oil pressure, internal noise, etc.). Carburetors were partly to blame for the wear because rich fuel mixtures wash the lubricating oil off the cylinder walls and dilute the oil in the crankcase. These older engines were also built much "looser" (wider tolerances) than most of today's engines, which also increased blowby. Consequently, the rings, bearings and valve guides all experienced accelerated wear.
Today, the situation is much different. The average service life of an engine today is upwards of 200,000 miles with normal maintenance! Fuel injection has all but eliminated the fuel wash down problem, and much tighter tolerances have greatly reduced blowby and oil dilution in the crankcase. So engines last much longer and fewer engines need to be rebuilt.
By the time an engine has reached the end of its service life today, the cost to rebuild or replaced it exceeds the valve of the vehicle. Consequently, most people simply junk the vehicle and replace it with something else.
When an engine needs major repairs, you are faced with an important choice: you can replace the engine with a new, remanufactured or used engine, or you can repair or rebuild the original engine.
Replacing an engine with a brand new one is usually too expensive for many people's budgets, so the choices come down to a remanufactured engine (or short block), a used engine (and the risks that go with it), or overhauling or repairing the engine yourself. A used engine is a temporary fix at best, and only buys the current owner a little more time. Sooner or later, most used engines experience problems of their own and have to be replaced or rebuilt, too.
Remanufactured engines are a popular option these days because they are readily available at competitive prices, which has caused a decline in the number of engines being custom rebuilt ("repowered") by repair facilities and machine shops. A quality remanufactured engine can provide good value for the investment, and most come with a 90-day to one-year warranty. Even so, there are still valid reasons for doing your own engine work.
Rebuilding an engine can cost less than replacing it. Assuming the original engine is rebuildable (wear is not excessive and there is no serious damage), and the amount of machine work required to restore it is minimal, you may realize 20% to 50% or more savings doing a rebuild versus replacing the engine. Most of the savings comes from the labor you put into tearing down the engine and then reassembling it after any necessary machine work has been done.
The tools required to rebuild an engine are minimal: normal hand tools, some feeler gauges, a torque wrench, a ring expander and ring compressor. Any machine work that is needed can be farmed out to a local machine shop.
If the cylinders are worn, they will have to be bored or honed to accept oversize pistons and rings. If not, you can run a glaze breaker down the bores and do the work yourself. If you don't have valve and seat refacing equipment, you'll have to send that out, too. Worn guides can be reamed out, replaced or relined in-house with a few special tools. But jobs such as head resurfacing, line boring, crank refinishing, etc., will have to be farmed out. Find a reputable local machine shop that you can use for this type of work.
Another reason for doing your own engine work is to control the quality of parts and work that goes into the engine. This is something you cannot control when you buy an engine from an outside source. It may be top quality, or it may not. But you do not want to find out "the hard way." The truth is, some remanufacturers reuse a much higher percentage of parts than others do, obviously for cost savings purposes.
You can also save by buying the parts you need in an engine kit rather than individually. A kit gives you everything you need in one box and reduces the chance of mismatching parts. The parts in a kit usually include bearings, rings, pistons, timing chain and gear set, valve seals, gaskets, oil pump, camshaft, lifters and other miscellaneous parts.
You can usually get OEM or better quality parts in most kits, which may be better than the parts found in some remanufactured engines.
One aftermarket supplier of engine kits now offers a 100,000 warranty (including labor) on all of the parts in its premium engine kits - which is a better deal than you will find on almost any replacement engine, new or remanufactured.
New crankshaft bearings are almost always a must when rebuilding an engine. When you remove the old bearings, inspect them for unusual wear or damage such as scoring, wiping, dirt or debris embedded in the surface of the bearings, pitting or flaking. Anything other than normal wear may indicate an underlying problem that needs to be corrected before the new bearings are installed.
Dirt contamination often causes premature bearing failure. The underlying cause may have been a missing air filter, air leaks into the crankcase (missing oil filler cap, PCV valve, etc.), or not changing the oil and filter often enough.
If the engine has a "spun" bearing, it is likely the bearings were starved for oil - possibly as a result of a failed or badly worn oil pump, an obstruction in the oil pump pickup screen, or too low an oil level in the crankcase (leaky gaskets or seals).
Excessive heat can be another cause of bearing failure. Bearings are primarily cooled by oil flow between the bearing and journal. Anything that disrupts or reduces the flow of oil not only raises bearing temperatures but also increases the risk of scoring or wiping the bearing. Conditions that can reduce oil flow and cause the bearings to run hot include a worn oil pump, restricted oil pickup screen, internal oil leaks, a low oil level in the crankcase, aerated oil (oil level too high), fuel-diluted oil from excessive blowby or coolant-contaminated oil from internal coolant leaks.
Misalignment is another condition that may indicate the need for additional work. If the center main bearings are worn more than the ones toward either end of the crankshaft, the crankshaft may be bent or the main bores may be out of alignment. The straightness of the crank can be checked by placing it on V-blocks, positioning a dial indicator on the center journal and watching the indicator as the crank is turned one complete revolution. If runout exceeds limits, the crank must be straightened or replaced.
Main bore alignment can be checked by inserting a bar about .001 inch smaller in diameter than the main bores through the block with the main caps installed and torqued. If the bar does not turn easily, the block needs to be align bored. Alignment can also be checked with a straight edge and feeler gauge. A deviation of more than .0015 inch in any bore calls for align boring. Line boring must also be done if a main cap is replaced.
The concentricity of the main bores is also important, and should usually be within .0015 inch If not, reboring will be necessary to install bearings with oversized outside diameters.
Connecting rods with elongated big end bores can cause similar problems. If the rod bearings show a diagonal or uneven wear pattern, it usually means the rod is twisted. Rods with elongated crank journal bores or twist must be reconditioned or replaced.
Uneven bearing wear may also be seen if the crankshaft journals are not true. To check the roundness of the crank journals, measure each journal's diameter at either bottom or top dead center and again at 90 degrees either way. Rod journals typically experience the most wear at top dead center.
Comparing diameters at the two different positions should reveal any out-of-roundness if it exists. Though the traditional rule of thumb says up to .001 inch of journal variation is acceptable, many engines can't tolerate more than .0002 to .0005 inch of out-of-roundness (always refer to the specs).
To check for taper wear on the crankshaft journals (one end worn more than the other), barrel wear (ends worn more than the center) or hourglass wear (center worn more than the ends), measure the journal diameter at the center and both ends. Again, the generally accepted limit for taper wear has usually been up to .001 inch, but nowadays it ranges from .0003 to .0005 inch for journals 2 inches or larger in diameter.
The journal diameter itself should be within .001 inch of its original dimensions, or within .001 inch of standard regrind dimensions for proper oil clearances with a replacement bearing. If a journal has been previously reground, there is usually a machinist mark stamped by the journal. A 10, 20 or 30 would indicate the crank has already been ground to undersize, and that further regrinding may be out of the question depending on how badly the crank is worn.
When you install new bearings, make sure you have the correct size (standard size for a standard crank, or oversized bearings for an undersize crank), that you have checked the installed bearing clearances, that the bearings are prelubed to protect them against a dry start, that the oil holes and tangs on the bearings are all properly located, and that the rod and main cap bolts are torqued to specifications.
Another component that should also be replaced along with the bearings is the oil pump. Oil pumps wear with age, and may cause a loss of oil pressure that can be very damaging to the bearings. See Oil Pump Diagnosis for more information about troubleshooting oil pumps.
Low compression and oil burning are usually a sign of worn rings and/or cylinders. Replacing the piston rings can restore compression if the cylinders do not exceed service specifications. But if the cylinders are worn or damaged, reboring the cylinders to oversize will be necessary to restore proper clearances and compression.
Replacement rings come in various materials and sizes. Most compression rings are cast iron, though many import engines have steel rings. Rings may be plain faced, chrome-plated, inlaid with molybdenum ("moly") or nitrided for added durability. Replacement rings should generally be the same types as the original.
Ring sizes can be confusing because ring thickness and width may change from one model year to the next. You may have to refer to the VIN number to determine the correct rings for the engine. Oversize rings and pistons of the corresponding size will obviously be needed if the cylinders need to be bored or honed to oversize.
Some shops "plateau" the cylinders after honing. This can be done various ways, but one way to do this yourself is to give each cylinder a few strokes with a flexible brush-type "Flex-Hone" in a drill. This helps remove surface debris and knocks the sharp peaks off the ridges left in the bores by honing.
Cylinders must always be cleaned before new rings and pistons are installed. This means scrubbing the bores with warm soapy water and a brush to remove all traces of honing residue and metal.
Always use a ring expander to install new rings on pistons, and a ring compressor to install the piston assemblies in the block. Cylinder walls must also be lubed to protect the rings and pistons against scuffing when the engine is first started.
Camshaft wear in high mileage engines is a common problem, so inspect the cam carefully to see if it is worn or bent. If the engine needs a new cam, you can install a stock replacement cam or a performance camshaft. Performance cams provide increased lift and duration for more power. If you opt for a hotter cam, be sure to follow the camshaft supplier application recommendations for lift and duration. A common mistake is overcamming a street engine. Too much lift and duration will move the engine's power curve too far up the rpm scale, and may require other extensive modifications such as larger valves, stiffer valve springs, performance manifolds and modifications to the carburetor or fuel injection system to optimize performance.
When a cam is being replaced, new lifters and valve springs should also be installed. Reusing old lifters with a new cam can damage the cam lobes.
Roller lifters can generally be reused, but not on a camshaft designed for flat bottom (actually slightly convex) lifters (and vice versa). Apply plenty of assemby lube to the camshaft lobes and the bottoms of the lifters when these parts are installed to protect against a dry start when the engine is first cranked.
In addition to a new cam, the engine may also need a new timing belt or chain and gear set. The recommended replacement interval for timing belts on most older engines (those made before 1993) is 60,000 miles. The replacement interval for many newer belts has been increased to 100,000 miles. Timing chains have no specified replacement interval, but do stretch with age. This has an adverse effect on valve timing as well as ignition timing, so the chain and gears should be replaced if wear exceeds specifications.
The valve components that may have to be replaced will depend on the age and condition of your engine. New exhaust valves are often needed because they run much hotter than intake valves and often "burn" or fail because of erosion and heat cracking. Exhaust valves also stretch with age, which increases the danger of valve breakage. For this reason, you might want to replace all the exhaust valves with new ones regardless of their condition.
Intake valves can generally be reused unless bent or worn. Replacement is required if stem wear exceeds specifications.
If guides are worn, which they usually are, the engine can suck a lot of oil. Evidence of this is usually heavy black carbon deposits on the backs of the intake valves and heavy carbon deposits on the pistons and in the combustion chambers. Minor guide wear can be reduced somewhat by knurling. If integral guides are worn, they may be drilled out to accept thin wall bronze or cast iron guide liners, or reamed to oversize (which requires new valves with oversize stems). Worn guides in aluminum heads can also be lined, or reamed to oversize or pressed out and replaced with new guides.
Unless valve seats are cracked or badly worn, they can usually be reconditioned by cutting or grinding. Damaged or badly worn seats in aluminum heads will have to be replaced. Bad seats in cast iron heads can sometimes be repaired by machining out the old seat to accept an insert.
The number one mistake to avoid when replacing a blown head gasket is to simply install a new gasket without checking or repairing anything else. In many instances, a blown head gasket is not the real problem but a symptom of some other underlying condition such as a hot spot, overheating or detonation. If the underlying problem is not identified and corrected, the new gasket will likely suffer the same fate as its predecessor.
Always inspect the cylinder head for cracks or other problems when it is removed, especially if the engine overheated. Aluminum overhead cam heads are much more likely to warp and crack than cast iron heads when an engine gets too hot. If an OHC will not turn once the followers have been removed, the head is probably warped and will have to be straightened and/or align bored.
Cracks are not always visible to the naked eye. Porosity leaks in aluminum heads may not show up unless the cooling system is under pressure. To minimize the risk of a repeat gasket failure, cast iron heads should be Magnafluxed (magnetic crack detection) to check for cracks. Penetrating dye will reveal cracks in aluminum. Pressure testing is also an excellent method of detecting internal cracks and porosity leaks in both cast iron and aluminum.
The cylinder head and block should also be checked for flatness before the new head gasket is installed. Flatness specs vary depending on the application, but on most pushrod engines with cast iron heads, up to .003 inch (0.076 mm) out-of-flat lengthwise in V6 heads, .004 inch (0.102 mm) in four cylinder or V8 heads, and .006 inch (0.152 mm) in straight six cylinder heads is considered acceptable. Most aluminum heads, on the other hand, should have no more than .002 inch (.05 mm) out-of-flat in any direction.
Aluminum OHC heads should be checked for flatness in two places: across the face of the head with a straight edge, and down the OHC cam bores with a straightedge or bar.
If an OHC aluminum head requires resurfacing, the amount of metal that can be safely removed is usually quite limited. If a head has been resurfaced and the installed height is too short, cam timing can be adversely affected. Too much compression may also create detonation problems. To compensate, a copper or steel shim may be used with the head gasket to raise the head and restore proper head height (if available). Otherwise, the head may have to be replaced.
Surface finish is also very important. As a rule, most push rod engines with cast iron heads can handle a surface finish of anything between 54 to 113 microinches RA (60 to 125 RMS). But many aluminum OHC heads require a smoother finish to seal properly. Many late model Japanese engines have "multi-layered steel" (MLS) head gaskets that require a very smooth finish of 7 to 15 RA! Such heads should not be resurfaced unless the head is warped or the surface is damaged.
Finally, if the engine has torque-to-yield (TTY) head bolts, replace them. Reusing TTY bolts is risky because you have no way of knowing how far they are been stretched. Also, make sure you have the latest head bolt torque specs. Vehicle manufacturers often revise their original head bolt torque specs to correct problems that have arisen in the field. The new specs can be found in technical service bulletins (TSBs) from the manufacturers.
Priming A Newly Rebuilt Engine
After your engine has been assembled and installed in your vehicle, you need to prime the oil system before you first start it up. The assembly lube and oil you used to lubricate the parts when you assembled the engine will provide some protection when the engine is first started. But it may take 15 to 30 seconds for the engine to build oil pressure if the oil pump and oil galleries have not been primed.
On engines were a pan-mounted oil pump is driven by a distributor, you can use a priming tool or a long screwdriver bit in an electric drill to spin the pump and prime the engine. If the oil pump is mounted inside the front cover and is driven by the crankshaft, you can use a pressurized oil canister to feed oil into the oil system through the port for the oil pressure sending unit.