GM Metro/Sprint 1985-1993 Repair Guide

Engine Overhaul Tips


Most engine overhaul procedures are fairly standard. In addition to specific parts replacement procedures and complete specifications for your individual engine, this information also is a guide to acceptable rebuilding procedures. Examples of standard rebuilding practice are shown and should be used along with specific details concerning your particular engine.

The choice of a competent, reliable machinist is one of the most important decisions faced by the home mechanic. In most instances it is more profitable for the home mechanic to remove, clean and inspect components, buy the necessary parts and deliver these to a shop for actual machine work. Competent and accurate machine shop services will ensure maximum performance, reliability and engine life. Choose your machinist carefully. If the engine is not machined properly, engine failure will result within a short time period after installation.

Disassembly, assembly, and the majority of inspection procedures presented in this information are well within the scope of the home mechanic. By following the procedures outlined, the home mechanic will be able to save money, while providing him/her self with the gratification of a job well done.


The tools required for an engine overhaul or parts replacement will depend on the depth of your involvement. With a few exceptions, they will be the tools found in a mechanic's tool kit General Information & Maintenance . More in-depth work will require any or all of the following:

A dial indicator (reading in thousandths) mounted on a universal base
Micrometers and telescope gauges
Jaw and screw-type pullers
Valve spring compressor
Ring groove cleaner
Piston ring expander and compressor
Ridge reamer
Cylinder hone or glaze breaker
Engine stand

Use of most of these tools is illustrated in this information. Many can be rented for a one-time use from a local parts jobber or tool supply house specializing in automotive work. However, the purchase of quality tools is never a bad decision.

Occasionally, the use of special tools is called for. See the information on Special Tools and Safety Notice in the front of this book before substituting another tool.


Procedures and specifications are given in this information for inspecting, cleaning and assessing the wear limits of most major components. Other procedures such as Magnaflux® and Zyglo® can be used to locate material flaws and stress cracks.

Magnaflux® is a magnetic process applicable only to ferrous materials. The component to be inspected is magnetized. Fine dust is sprinkled over the component and collects along the crack line. Careful inspection and an eye for problem areas are necessary qualities for a Magnaflux® technician.

The Zyglo® process coats the material with a fluorescent dye penetrant and can be used on any material. The dye is developed and then the component is placed under a black light. Cracks will show as bright lines. Careful inspection and an eye for problem areas are also necessary for this type of inspection.


Never hot tank aluminum parts (the caustic hot tank solution will eat the aluminum.
Remove all aluminum parts (identification tag, etc.) from engine parts prior to the tanking.
Always coat threads lightly with engine oil or anti-seize compounds before installation, to prevent seizure.
Never over-torque bolts or spark plugs especially in aluminum for you may strip the threads.
When assembling the engine, any parts that will have frictional contact must be prelubed to provide lubrication at initial start-up. Any product specifically formulated for this purpose can be used, but engine oil is not recommended as a prelube.
When semi-permanent (locked, but removable) installation of bolts or nuts is desired, threads should be cleaned and coated with Loctite® or equivalent non-hardening sealant.


See Figures 1 through 5

Several methods of repairing damaged threads are available. Heli-Coil® (shown here), Keenserts® and Microdot® are among the most widely used. All involve basically the same principle - drilling out stripped threads, tapping the hole and installing a pre-wound insert - making welding, plugging and oversize fasteners unnecessary.

Two types of thread repair inserts are usually supplied - a standard type for most Inch Coarse, Inch Fine, Metric Course and Metric Fine thread sizes and a spark lug type to fit most spark plug port sizes. Consult the individual manufacturer's catalog to determine exact applications.

Typical thread repair kits will contain a selection of pre-wound threaded inserts, a tap (corresponding to the outside diameter threads of the insert) and an installation tool. Spark plug inserts usually differ because they require a tap equipped with pilot threads and a combined reamer/tap section. Most manufacturers also supply blister-packed thread repair inserts separately in addition to a master kit containing a variety of taps and inserts plus installation tools.

Before effecting a repair to a threaded hole, remove any snapped, broken or damaged bolts or studs. Penetrating oil can be used to free frozen threads; the offending item can be removed with locking pliers or with a screw or stud extractor. After the hole is clear, the thread can be repaired, as follows:

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Fig. Fig. 1: Using a thread insert to repair a damaged hole is a quick and easy fix to a difficult problem

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Fig. Fig. 2: Standard thread repair insert (left) and the spark plug repair insert (right) are identified by size and thread count

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Fig. Fig. 3: Using the specified drill bit, enlarge the damaged hole to accept the insert. Drill completely through the hole or to the bottom of a blind hole.

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Fig. Fig. 4: With the supplied tapping bit, tap the hole to receive the thread insert. Keep the tap well oiled and back it out frequently to avoid clogging the threads

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Fig. Fig. 5: Screw the threaded insert onto the installation tool until the tang engages the slot. Screw the insert into the tapped hole until it is 1/4-1/2 turn below the top surface. After installation, break off the tang with a hammer and punch


A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored or worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats are all possible culprits here. A check of each cylinder's compression will help you locate the problems.

As mentioned in Routine Maintenance , a screw-in type compression gauge is more accurate than the type you simply hold against the spark plug hole. Although the screw in type gauge takes slightly longer to use, it is worth the accuracy you gain.

  1. Start the engine and allow it to reach normal operating temperature.
  3. Stop the engine and remove all spark plugs.
  5. Disconnect the high tension lead from the ignition coil.
  7. Fully open the throttle, either by operating the throttle linkage by hand or by having an assistant hold the accelerator pedal to the floor.
  9. Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.

Be careful not to crossthread the plug hole. On aluminum cylinder heads use extra care, as the threads in these heads are easily damaged.

  1. Crank the engine through 4-5 compression strokes (complete revolutions) recording the highest reading on the compression gauge.
  3. Repeat this procedure for each of the engine's cylinders. Compare the highest reading of each cylinder against the other cylinders. Note difference between each cylinder. The variance should be no more than 12-14 pounds.
  5. If a cylinder is unusually low, pour a tablespoon of clean engine oil (30W) into the cylinder through the spark plug hole and repeat the compression test. If the compression rises after adding the oil, worn or damaged piston rings or cylinder bore should be suspected.
  7. If the pressure remains low, the valves may not be seating properly (a valve job is needed), or the head gasket may be blown near that cylinder.
  9. If compression in any two adjacent cylinders is low and if the addition of oil doesn't help the compression, there is leakage past the head gasket. Oil and coolant in the combustion chamber can result from this problem. There may be evidence of water droplets (sometimes seen as a milky white substance) on the engine dipstick when a head gasket has blown.