This guide is loosely based around the 2.0L 420A inline four, but can be used as a guide for other engines.

There are many steps and aspects to performing a good engine overhaul or buildup. Some say that engine rebuilding should be left to the pros and I’m here to show you that it’s not true. A majority of the work can be performed by the average home mechanic. This is assuming you have the proper tools and patience to get the job done. Other steps and procedures do require a machinist or shop to get the work completed.

I’ve broken this down into topics, from teardown to break-in to keep it as organized as possible. - Jaded Silver

75% of all engine rebuilds fail within the first 20k miles because the mechanic did not take the extra steps to clean everything.

Tiny abrasives from the sandpaper you used to sand your headlights last week can destroy a brand new engine minutes into break-in. One piece of that abrasive in your engine oil supply can scour a crankshaft journal, carve a nice ring into the cam bearing surface in your head, and then possibly make its way into the cylinder between the piston skirt and cylinder wall.

What happens?

The crankshaft journal heats and bearing spins, the cam bearing race runs out of round, and the cylinder wall gets scored. Basically you blew the motor in about 1000 revolutions of the crank. Maybe not right away, but 10k miles down the road when your car is eating a quart of oil per day you’ll be kicking yourself in the ass.

Take the time to clean every tool you have even the slightest thought of using. Everything from wrenches to that micrometer you say you clean every day. The work area must be free of dust, blowing wind/air, moisture, etc. Keep a supply of WD-40 for cleaning tools off and a couple rolls of BluMol towels to wipe them down.

Get the engine in an engine stand. If you failed to drain the fluids do it now.

During disassembly sort out the main parts like valves, the block, steel brackets, various steel bolts, etc. Take these to a local machine shop and have them hot tanked.

Hot tanking is a caustic solution bath that dissolves rust, paint, grease/oil/dirt, and leaves everything clean.

Aluminum parts cannot be hot tanked as the solution will literally dissolve the part. These instead can be cleaned at home is a bucket using kerosene (not gasoline you idiots!!!). Get all the aluminum parts in the solution and allow them to soak overnight. Scrub and rinse them off with water, dry them completely with compressed air, then wrap the parts up in plastic trash bags to avoid contaminating the parts with foreign materials until they are assembled.

Any small parts like rocker arms, lash adjusters, etc can be cleaned in the same manner, but once you dry them off spray the parts with a light coating of WD-40 to repel moisture and prevent rust.

When disassembling the engine, remember organization and order will save you time.

Start with outer brackets and manifolds. Sensors should be cleaned and tested as in the appropriate forum section I wrote. Replace sensors as necessary.

Start up top with the valve cover, work down to the valve train components and head, then work on the crankshaft and piston/rod removal.

There are a few parts that must be kept in the same order you remove them, as they have broken-in to their respectful places.

They are as follows:

A fairly common way to do this is to label a piece of cardboard and place the parts in labeled fashion. This is terrible, as these parts you cleaned earlier (you did.....didn’t you??) are now contaminated with cardboard fibers. It’s better to use cheap plastic dishes and tubs to separate and organize the parts. Keep them covered until the parts are used.

However, if you plan on using new head bolts, pistons/rods, rocker arms, and/or lash adjusters, there is not need to keep the originals parts in any order.

The bearing caps MUST be kept in order. Label them by using a set of numbered dies to punch numbers into the crank cap and cam cap tops BEFORE you remove them.

Continuing disassembly, remove and toss all old seals and gasket material. Scrapers and picks can be used but be careful. A nick or gouge in a sealing surface can cost you a surface machining at the least, a new part at the worst.

Knock out the freeze plugs from the engine block, as they will be replaced with new ones.

Remove the oil pump AND THROW IT ASIDE.

No joke here, buy a new oil pump and save yourself the hassle of rebuilding your old one just to have it fail on you.

Using a ring expander, remove and toss the piston rings, being careful not to gouge the pistons if they’re being reused. Use a ring groove cleaning tool and remove any carbon deposits on and between the ring lands.

Toss the crankshaft main and rod journal bearings.

Separate the parts and clean as listed above.

Once the entire engine is disassembled AND cleaned, it’s time to start measuring components for wear and get any necessary machining completed.

Let’s start with the head and related components.

The head itself should be assembled by a professional machine shop. I say this for a lot of reasons, the most important reason being there’s alot of work involved. Valves must be measured for wear along their stems and be check for stem straightness. Valve springs must be checked that their natural released height is within limits and not compressed from age. Valve guides must be check for wear and replaced if needed. Camshaft bearing races should be checked for wear and out of roundness. Camshaft bearing surfaces need to be checked for the same cases. The head itself needs to be magnafluxed for cracks and checked for sealing surface flatness. If the head surface is found to be uneven, the camshaft bearing races must also be align machined. It all requires thousands of dollars in machinery that most at-home mechanics just don’t have. If you do have the machinery, more than likely you run your own machine shop and don’t need instruction on using them.

Overall the following procedures should be performed by the shop, regardless of part condition:

This only ensures that the head will not cause any problem in the near future.

The crankshaft needs a few measurements too.

You must measure, or mic, each one of the main bearing races and the connecting rod journals for wear, taper, and out of roundness. You can do this if you have a proper sized micrometer.

Using the micrometer, take a measurement of one of the main bearing races in the center of the bearing surface. Write that measurement down, rotate the micrometer 90 degrees around on the same bearing race and take another measurement. The difference (if any) between these two measurements is the out of roundness of that bearing race.

Now measure the same bearing journal off to the side of the bearing surface and write that down. Measure the same bearing race on the other side of the bearing surface. The difference between these two measurements is the taper of that surface.

Complete these measurements for all the mains and journals.

If any of these measurements fall outside the OEM factory specs(listed in the manuals) the crank must be machined for undersize bearings. Either way, the crankshaft bearing surfaces should be polished smooth by a machine shop. Normally an engine that has been taken care of will show no wear on the crankshaft journals. If this is the case, you can get away with just polishing the crankshaft journals and using new crankshaft bearings.

Plastiguage is something I like to avoid in rebuilds. It’s a material used to measure bearing clearances in engines. It is effective, but I feel that if a clearance must be checked with Plastiguage after using a micrometer, it’s already far enough out of spec that machining should be done anyway and undersize bearings used. The crankshaft should also be x-rayed for cracks, just to be on the safe side and avoid installing a bad crankshaft.

If you plan on using the stock pistons in your rebuild they must be checked as well.

Measure the diameter of each piston with a micrometer, 90 degrees to the wrist pin. If the piston falls outside factory specs it must be replaced. If two or more pistons need replacing, replace them all. You must also check that ring land clearances are not too large, or the rings will taper and not seat properly. You do this with a piece of old ring material and a feeler gauge. Put the ring in the piston ring groove and measure the clearance between the top of the piston ring and the bottom of the piston ring groove. This is the ring gap and if it’s too excessive, your new rings will not seat and wear in properly and will cause major oil consumption in the future.

The block is the next item.

The first thing is to make sure the gasket surface is smooth and flat, like with the head. Any deviance outside factory specs means the block surface must be resurfaced. The same thing applies to the block as the head and if the head surface is machined, the block bearing races must be align machined. Once these are checked, the cylinders must be measured.

Using a dial bore gauge, measure the taper, out of roundness, and bore diameter of each cylinder. Take two measurements for each cylinder, 90 degrees to each other, and for the full length of the cylinder. Any cylinder that falls outside specs needs to be honed and/or bored out.

My recommendations are that if 2 or more cylinders need to be bored, that all the cylinders get bored/honed and oversize pistons/rings are used. If any piston/cylinder combo is right on the edge of falling out of spec, spend the money and bore the cylinders and install new pistons. Cylinder boring can be completed at home if you rent a boring bar, but I highly recommend letting shop take care of this. However, if both the pistons and cylinders fall within specs you can deridge/hone the block at home and save some money.

De-ridging removes the upper cylinder ridge that normally occurs in worn engines. As the pistons move up and down, the rings wear into the cylinder walls and eventually create a very slight ridge at the top of the rings upward travel. Although this ridge is extremely small, it’s enough that it can break new piston rings and the piston ring lands on the pistons themselves. You can actually feel the ridge inside the cylinder with your finger. When you mic the cylinders for wear and you notcice a ridge that in excess of .004" you should have the cylinder bored out. If the ridge is less than that, you can de-ridge the cylinder(s) with a rotary drill stone hone, really quick with LIGHT pressure. You have to do this just enough to knock the ridge down. Usually two passes does the trick well enough. The rest will be taken care of in the next step.

If you plan on honing out the cylinders yourself to use stock bore pistons, use a multi-ball flex hone for best results, attached to a variable speed electric drill. With the block securely mounted, hone the cylinders out a bit. Oil up the cylinders first and keep a good supply of fresh motor oil flowing to the cylinders being honed at all times. Pour it on if you have to. Keep the rpm slow and steady and move the hone up and down the length of the cylinder. A good rule to follow is to move in an down then up motion in the cylinder once every second, for fifteen seconds. This rate gives you a nice 60 degree crosshatch in the cylinder walls and allows great oil retention.

When honing is complete, re-mic the cylinders and check that clearances are within spec. Wash and clean the block out completely. Blast out any filings and grit from using the hone. Dry the block out. Spray the cylinders with a mild soap solution and wipe them clean with soft clean white rags. Keep doing this until every cylinder leaves no trace of grime on the rags. Once that’s done, dry the block with compressed air and spray the cylinder walls, head surface, and block underside with WD-40.

One other thing alot of people overlook is bolt stretch. Over time the head, crankshaft, and connecting rod cap bolts stretch under load. This causes excessive play and wear or bad sealing, but also can lead to failure. Be sure to check the bolts against the factories maximum stretch limit. If they exceed replace them. I recommend replacing the head bolts with new ones regardless of condition but it’s entirely up to you in deciding to go this route.

Once you’re certain that all the parts are cleaned thoroughly and all necessary machining is completed, you may begin the assembly process. Before you begin you’ll want to make a list of everything you will need to complete the job and check it off as you get the items, supplies, part, or tools. There are a few things commonly overlooked you will definitely need before assembling the engine.

They are:

The factory manual lists the required procedures and specs on rebuilding the engine properly. When you assemble the engine, ANY friction part (other than piston rings and seals) MUST get prelubed using a good quality engine prelube. Motor oil is NOT a substitute by any means and should never be used as a prelube. I recommend BluMol or Sealed Power engine assembly prelubes. All crank/rod bearings, cam lobes and bearings, rocker arm pivots and rollers, lash adjusters/pushrods, piston pins(if new pistons are used), balancer shaft bearings, and valve stem tips should have prelube applied to the prior to assembly. Valve stems and pistons/rings/cylinders should be prelubed with fresh, proper weight motor oil.

Bolt torque lube is used on all high stress bolts. Head bolts, crank cap or bedplate bolts and connecting rod bolts or nuts must be lubricated with torque lube before the bolt/nut is threaded into place for fastening. This lube prevents thread friction and allows proper torque specs to be reached. ARP makes Moly Lube and it’s the best bolt torque lube around, so why use anything else. The torque wrench is on this list because I’ve seen all to many people guesstimate a torque and let it slide. Next thing they know a rod is through their engine block because they didn’t torque a rod cap properly. Don’t have a torque wrench, get one now. Be sure it reads on a decent ftlb/Nm scale up to 180ftlbs. I also keep another, light duty, inchlbs torque wrench for fasteners like the valve cover or bolts that affix to the aluminum head(i.e. thermostat housing).

With all that I mind, start the assembly by fitting the block to an engine stand with full 360 rotating capabilities. Install new freeze out plugs into the proper block holes using a hammer and a brass dowel. Seat the plugs recessed about 2-3mm past the face of their respectful holes. Next is the crankshaft. Rotate the block so the bottom faces up and install the upper half of the crankshaft main journal bearings into the bearing races in the block. The bearings have notches that align with notches in the bearing races and caps. Install the thrust bearing on the face of the crankshaft, lubricating all sides of the bearing with a bead of engine prelube. Prelube each of the upper crankshaft main bearing halves with engine prelube. Put two small beads of prelube on the bearing face that touches the crankshaft, one bead on both sides of the central oil channel in the bearing.

Very carefully and slowly lower the crankshaft into the block. Align the main bearings to their respectful main races and set the crank in. Make sure none of the bearings shifted in their races. Now take the main crankshaft bearing caps (or bedplate for 420A) and install the other halves of the bearings into them, aligning the notches. Put a drop or two of engine prelube on the back face of each of these bearing halves to keep them in the caps or bedplate as you flip it over onto the block. Prelube these bearing halves in the same manner as the other ones listed above. Place the caps in their proper places (you labeled them right!!!!) or lower the bedplate on if so equipped. Take the cap bolts or bedplate bolts and lubricate the threads and shanks of each one with torque lube. Install the bolts and hand-tighten them.

The bolts must be torqued in a specific, three-stage sequence. Refer to the manual for specific details. Once the bolts are torqued properly, rotate the crankshaft slowly. It will be a bit tight to turn, but should not bind at all. This is also a good time to check crankshaft end play using a dial indicator. Take and clamp the dial indicator to the front face of the block so it’s reference tip touches the end face of the crankshaft. Push the crankshaft all the way back in its cradle and set the dial indicator to zero. Now pull the crankshaft forward in the cradle and read the play, if any, and compare it to factory specs. If the play is excessive you may need to either have the crankshaft welded and then machined, or you may need shim stock or an oversize thrust bearing. What you need will depend on your engine and the actual cause of the play.

Connecting rods/piston assembly is next. If you purchased aftermarket piston/rod assemblies, you should have them assembled and balanced by a machine shop. This balancing procedure ensures that the piston/rod assemblies are of equal weight. Assemblies that are not balanced can produce destructive harmonic frequencies throughout the crankshaft that can result in a catastrophic failure of your bottom end. It’s an inexpensive way to prevent damage. Your harmonic balancer is there to dampen these frequencies, but it can only do so much. Perfectly balanced engines do not need harmonic balancers, but since most engines are mass produced, balance perfection is not possible and the harmonic damper keeps everything in check so to say.

Either way, once your assemblies are ready to go in, you need to install the rings using a ring expander. Depending on your piston design and rings used, some styles use direction specific rings so be sure you have the right ones in the right place. Be careful, rings can break easy if opened too far. Once the rings are on we need to put the assemblies in the block. First, rotate the block in the engine stand so the head gasket surface faces up. Wipe out clean and oil up the cylinder you’re going to work on first with fresh motor oil. Take small 3" long pieces of rubber hose and slip them over the ends of the connecting rod bolts to prevent scratching the cylinder walls. Install the upper half of the connecting rod bearing into the rod. Use the same alignment and lubing procedures as used on the crankshaft bearings. Rotate the crankshaft so that the connecting rod bearing journal underneath the cylinder you’re working on is at BDC or Bottom Dead Center. Lightly oil the piston skirts and compress the piston rings but don’t cover the whole piston skirt with the compressor and DO NOT OVERTIGHTEN the compressor itself. Use a scissor type ring compressor and not a band compressor to prevent ring breakage.

Line up and lower the assembly into the cylinder and make sure you have the piston direction correct. There is usually a mark on the piston dome, like an arrow or line, denoting a direction that the piston faces (like towards the intake manifold). The piston manufacturers provide this info for you. With the piston assembly properly aligned into the cylinder and oiled, lightly tap the piston from the top (through the ring compressor) into the cylinder using a wood dowel. Do this slowly and watch to see that the connecting rod doesn’t scratch the crankshaft journal as you attempt to align the rod bearing up to it. Once the piston/rod assembly is seated against the crankshaft, rotate the block back over and install the rod lower bearing cap. Make sure the cap goes back on the way it came off. They can go on one of two ways, but only one is right. Rods are manufactured using a fracture process to form the caps from the rods themselves. Mounting the caps backwards will result in uneven bearing load displacement and engine failure. Use the lubing and torque techniques as before. If you have or have access to a bolt stretch gauge this is the time to use it. Measuring bolt stretch is a more accurate way to fasten connecting rod cap bolts. Bolt manufacturers know that a specific bolt, tightened to a specific length, has a specific fastened torque. With this technique, you can be assured that the rod caps bolts are all of equal and proper torque.

Once this is done, flip the block back over, rotate the crank so the next journal is down (you will need to install the harmonic balancer bolt in the crankshaft end and turn it with a wrench), and install the next piston assembly. Do this for all the assemblies. Take your time with these steps, as they are areas where you need precision. It only gets easier from here on in so be patient and do not rush.

The bottom end is for the most part finished and now the head must be installed. Rotate the crankshaft until cylinder number one is at TDC. Now rotate the crankshaft counterclockwise until all the piston domes are NOT at TDC. Remove the bolt from the end of the crankshaft. FROM HERE ON IN, DO NOT ROTATE THE CRANKSHAFT AT ALL UNTIL TOLD TO DO SO!!! Follow the head gasket manufacturers recommendations on installation procedures. With the head gasket prepped and in place, lower the pre-shop-assembled head onto the block CAREFULLY as not to gouge the gasket. Lube and install the head bolts and washers hand tight. Torque the head down in a three-stage torque sequence specified in the manual. For the 420A owners using the ARP headstuds, you must follow ARP’s listed torques settings and not the OEM settings.

At this time, take clean lint-less shop towels or rags and plug up the spark plug wells. The shop that rebuilt your head should have capped or plugged off the intake and exhaust ports, but if not go ahead and do so now. Now that the top end is sealed up, flip the block over again. First, install the rear crankshaft main seal and install it using the correct installation tool(large sockets and pieces of pipe are not proper installation tools, GO RENT ONE!) You don’t have to oil the crank seals like most people think. Oiling the seals just attracts more crap and dirt particles to them, causing increased wear on the lip of the seal. Next, install the oil pump to the front of the motor, or where ever it’s affixed to. Be sure to seal the pump to the block in the proper places(according to your manual) with silicon sealant. Some engines, like the 420a, also require that the oil pump be primed before installation. You simply need to fill the pump cavity with fresh oil to prime the pump.

Install the oil sump pickup tube into the block, and if equipped, install the oil filter standoff to the block as well. Seal and install the oil pan gasket with silicon sealant. With this gasket, lightly coat the gasket with the silicon, hang it somewhere for about fifteen minutes, allow the silicon to "skin over’, then place it on the block or bedplate surface. This skinning technique gives you a good seal and still allows the gasket to be removed without destroying it, if you ever needed to remove the pan in the future. Place the oil pan over the block and torque down the bolts. Most engines have a torque sequence for the oil pan. If not just, work from the center out to the ends of the block to avoid pinching the gasket. Now install the crankshaft main front seal, using the correct tool. Install the crankshaft timing sprocket and keyway and check to see that the timing marks are NOT at TDC. Flip the block back over and continue with the head assembly.

It’s easier to complete the next few steps if you work with one camshaft, and related parts, at a time. First, lube/oil the lash adjusters and install them into their respectfully labeled places if you used the original ones. Place a drop of prelube in the pocket on each rocker arm. Put a good size drop of prelube on the valve stem tips. Now put the rocker arms into their respectful places along the head. Apply beads of prelube to the camshaft bearing races and the cam lobe areas in contact with the rocker arms. If the camshaft bearing races were machined due to excessive bearing wear you’ll need to install the undersize bearing now using the same procedures as for the crankshaft bearings. Install the camshaft into its proper place. Install the camshaft upper bearing retainers, apply torque lube to the bolts, and torque down the caps in the proper torque sequence listed in your manual. Repeat these steps for the other camshaft.

Install the rear camshaft halfmoon seal on the intake camshaft, and cam sensor signal magnet on the exhaust camshaft. Be sure to use silicon on the halfmoon seal. Install the camshaft position sensor in its proper place. Take your valve cover and install the valve cover gasket and spark plug well seals. It is not necessary to use silicon sealant on the valve cover gasket and spark plug well seals. Instead, just put a bead of silicon on the top of the halfmoon seal and at the edge corners of the camshaft end caps. Remove the plugs you put in the spark plug wells and install the valve cover. Install the valve cover bolts with their seals and tighten down the valve cover. Torque the valve cover bolts to 12 Nm in the proper sequence so you do not kink the gaskets. If you kink the gasket, it results in a leak and if you have luck like mine, the spark plug wells fill up with oil. Install your oil cap. Put the spark plug well plugs back in again.

Install the cam seals using the proper tool. Next, install your new water pump and gasket and use silicon sealant on this gasket. Install the gasket and water pump while the silicon is still wet and do not over tighten the bolts and crank the pump housing. Install the rear timing belt cover and install the camshaft timing sprockets and torque the retaining bolts. Use a bit of Loctite on the camshaft sprocket retaining bolts Take the timing belt tensioner pulley assembly and timing belt idler pulley and install them onto the block. Align the camshaft sprocket timing marks as specified in your manual. SLOWLY rotate the crankshaft clockwise until the crankshaft timing sprocket mark aligns on the oil pump housing. DO NOT GO PAST THE MARK! Install the timing belt and hydraulic tensioner unit and secure as specified in your manual. Slowly rotate the crankshaft and check to see the timing is correct and that marks stay aligned. If you encounter a major force that stops the crankshaft from rotating, back up to TDC and recheck the timing belt installation.

Once the timing is set properly, rotate the crankshaft so the marks are aligned and install the crankshaft position sensor in the engine block with a new paper spacer. Install the front timing belt cover and inspection cover. Install the front engine mount plate and use blue locktite on the bolt threads. Remove the spark plug well plugs and install new spark plugs, gapped to the proper setting. Install a new thermostat and gasket and install the thermostat housing to the head. A new radiator cap (spout cap 420A) is a good idea as well. Install the exhaust manifold and lower intake runners. Use new gaskets for these parts to reduce the risks of vacuum and exhaust leaks. When you install the fuel injectors into the manifold, be sure to use new o-ring seals and lubricate them with fresh motor oil. Install the fuel rail(s) to the lower manifold. Install the upper plenum and throttle body, using new gaskets with them as well. Mount the A/C, alternator, and P/ S pump brackets/tensioners (if applicable), engine mount brackets, and mount the alternator to the block. Also install any sensors like the knock sensor or TPS. Depending on the engine model, you can install the ignition coil(s), distributor (if equipped), and spark plug wires. If your engine is not a DIS ignition equipped engine, be sure to set the ignition timing a specified by the manufacturer. With all this done, your engine is now ready to be installed in the car.

Crank the engine over until it starts. Where do your eyes go? RIGHT TO THE OIL PRESSURE GAUGE!!! If you do not see ADEQUATE oil pressure within ten seconds of the engine starting, SHUT THE ENIGNE OFF!!!! Do not start the engine again until the source of the oil pressure loss has been traced and fixed. It may mean pulling the motor apart again, but at least you won’t have to buy and ENTIRELY new engine when your internals seize together. If the oil pressure rises to normal levels right away, let the engine idle and come up to its normal operating temperature. Keep an eye on the oil pressure and let the engine run for about fifteen minute after the operating temperature is reached. Check that everything is operational, no CEL codes appear, and no leaks are present. Operate the throttle at the TB VERY slightly just to vary the rpms a tiny bit. After fifteen minutes shut off the engine and do an oil/filter change. Do the same process again and end it with another oil/filter change. This process flushes out the engine of any contaminants, the WD-40 and prelube you used, and internal part break-in material. Now the car is ready to drive.

Here are a few things to remember while driving during the next 3K miles:

Do not accelerate hard or free-rev the engine.
Do not lug the engine at really low RPMS.
Avoid sustained high-speed use.
Keep an eye on oil pressure and level

The engine will be using oil for awhile, as the parts need to do so in order to break themselves in to each other. After 1500 miles, do another oil/filter change. After that you can resume the normal 3K mile interval for oil changes. When you do the first 3K mile interval oil change, change out the spark plugs to new ones. A new engine sometimes fouls out its first set of spark plugs because of oil consumption during break-in. After that, you’re set to do whatever.