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When an engine has accumulated a lot of miles, the internal parts inside the engine may be worn or damaged and have to be replaced. Which engine parts you replace when you rebuild your engine will depend on their condition and whether you are overhauling the engine or just fixing something that is wrong with it.
The first step in rebuilding an engine is to correctly identify the engine. Auto makers are making constant changes from one model year to the next, and may even make mid-year changes, so to find the correct replacement parts for your engine you will need the year, make and model of your vehicle, and probably its VIN number. The VIN number can be found on a metal plate at the base of the windshield on the driver's side, or on the vehicle title or registration.
The digit or number in the VIN code that identifies the engine depends on the year and make. For 1981 and newer vehicles, the engine code is usually the 8th digit (Chrysler, GM & Ford), but may be the 4th digit (Jeep 1981-89). On 1980-81 Chrysler, the engine code is the 6th digit. On 1967-80 Fords, it is the 5th digit. On 1972-80 GM and 1974-80 Dodge & Plymouth models, it is the 6th digit. Once you know which engine you have, you can order the correct replacement parts or an engine overhaul kit.
There are two approaches to rebuilding an engine repair: you can replace individual parts piecemeal on an "as needed" basis to save money, or you can do a complete engine rebuild. The piecemeal approach is to replace only the parts that are worn or broken and leave everything else alone. The underlying motivation here is primarily economic. The vehicle and/or engine may not be worth spending much money on, so only those parts that absolutely have to be replaced to keep the engine running are replaced. Other repairs can wait for the next owner or the junkyard. This approach may skimp on needed machine work and attempt to "make do" with standard sized replacement parts such as bearings, rings, pistons and valves.
The other approach is to do a complete overhaul when the engine needs major repairs. This is obviously the more expensive approach, but one that usually provides much longer-lasting results. By replacing major wear components such as the rings, pistons, bearings, oil pump, valve guides, exhaust valves, valve springs, timing chain or belt, etc., the engine is restored to "like-new" condition. This approach may involve the purchase of an engine kit or buying the parts individually. It usually also requires having machine work done such as turning the crankshaft, overboring the cylinders, refacing the valves and seats, resurfacing the cylinder head(s), and maybe align boring or honing an overhead cam head and/or the main bores in the block.
The question is, how far do you want to go and how much money do you want to spend on your engine? An engine may need a valve job but may also has some wear in the cylinders. If you just pull the heads and have the valves redone, the improved compression may increase blowby into the crankcase if the rings and cylinders are worn.
The important point here is to decide what engine parts may be needed to fix a particular engine problem, and then to replace any additional parts that might also be needed.
Worn crankshaft main and rod bearings can cause a loss of oil pressure.
REPLACE ENGINE BEARINGS
Let's start with bearings. Common reasons for replacement include noise and low oil pressure. If a bearing has seized or spun, it will take more than a new set of bearings to fix the problem. The crankshaft journal will likely have suffered damage so turning the crank to undersize will probably be necessary. The big end of the connecting road may also be damaged or out-of-round which will require reconditioning the rod or replacing it.
Replacement bearings come in standard as well as various undersizes, so rule number one is to make sure the bearing size matches the crank journals. Typical undersizes include .010.020 and .030 inch., which are usually marked on the back of the bearing shell. To determine the correct size if the crank has not been turned, the diameters of the rod and main journals on the crank must be measured with a micrometer. Machinists will usually mark the journal size on the crank if the crank has been reground. Bearing clearances should always be measured when the bearings are installed to make sure they have the correct fit.
The old bearings will often tell a story as to why they failed. If the bearings are full of scratches and abrasion, dirt and/or a lack of adequate lubrication probably played a role in their demise. Lack of proper maintenance (not changing the oil and filter often enough) may be the real culprit. A worn oil pump and/or clogged oil pump pickup screen may also have been factors, so these parts should also be inspected and replaced if necessary.
Use assembly lube when installing new bearings to protect the bearings and journals until the engine can be fired up. Dry starts ruin a lot of newly installed bearings, so priming the oil system prior to cranking the engine (if possible) can reduce this risk.
REPLACE PISTONS & PISTON RINGS
Pistons and rings may have to be replaced if a compression test reveals the rings are not holding a good seal. Taper, distortion or damage in the cylinder bores can be factors, too. Worn rings and/or cylinders will allow a lot of blowby into the crankcase. This reduces power, increases fuel consumption and emissions, and contributes to sludge formation as well as oil dilution (both of which are bad for the bearings).
Broken rings and/or cracked pistons are usually the result of detonation (spark knock), which may have a variety of underlying causes (no EGR, overadvanced ignition timing, a buildup of carbon deposits in the combustion chamber, cheap gas, engine overheating, etc.). Either of these conditions will also increase blowby and oil consumption.
Replacement rings come in various types, styles and sizes. Standard size rings are okay if the cylinders are not worn excessively (which requires measuring taper with a cylinder bore gauge). Oversized rings are required if the cylinders are worn and are bored to oversize.