BMW Coupes and Sedans 1970-1988 Repair Guide

Engine Overhaul Tips

Print

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

Competent and accurate machine shop services will ensure maximum performance, reliability and engine life.

In most instances it is more profitable for the do-it-yourself mechanic to remove, clean and inspect the component, buy the necessary parts and deliver these to a shop for actual machine work.

TOOLS



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 (see Routine 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
 
Scraper
 
Valve spring compressor
 
Ring groove cleaner
 
Piston ring expander and compressor
 
Ridge reamer
 
Cylinder hone or glaze breaker
 
Plastigage®
 
Engine stand
 

The use of most of these tools is illustrated in this section. Many can be rented for a one-time use from a local parts jobber or tool supply house specializing in automotive work.

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

INSPECTION TECHNIQUES



Procedures and specifications are given in this section 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 Zyglo® process coats the material with a fluorescent dye penetrant and can be used on any material. Checks for suspected surface cracks can be more readily made using spot check dye. The dye is sprayed onto the suspected area, wiped off and area sprayed with a developer. Cracks will show up brightly.

OVERHAUL TIPS



Aluminum has become extremely popular for use in engines, due to its low weight. Observe the following precautions when handling aluminum parts:

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 hot-tanking.

Always coat threads lightly with engine oil or anti-seize compounds before installation, to prevent seizure.

Never over-tighten bolts or spark plugs, especially in aluminum threads. Stripped threads can usually be repaired using any of several commercial repair kits (Heli-Coil®, Microdot®, Keenserts®, etc.)

When assembling the engine, any parts that will be in frictional contact must be prelubed to provide lubrication at initial start-up.

When semi-permanent (locked, but removable) installation of bolts or nuts is desired, threads should be cleaned and coated with Loctite® or other similar, commercial non-hardening sealant.

To determine whether a particular engine or component is constructed of aluminium, use a magnet. If the magnet attaches to the item, it is not made of aluminium, but a metal with a high iron content. If the magnet does not attach to the item, it is most likely made of aluminium.

REPAIRING DAMAGED THREADS



See Figures 1, 2, 3, 4 and 5

Several methods of repairing damaged threads are available. Heli-Coil®, Keenserts® and Microdot® are among the most widely used. All involve basically the same principle-drilling out the stripped threads, tapping the hole and installing a prewound 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 Coarse and Metric Fine thread sizes and a spark plug 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 prewound 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 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 repairing 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 shown in the figures.



Click image to see an enlarged view

Fig. Fig. 1: Damaged bolt hole threads can be replaced with thread repair inserts



Click image to see an enlarged view

Fig. Fig. 2: Standard thread repair insert (left), and spark plug thread insert



Click image to see an enlarged view

Fig. Fig. 3: Drill out the damaged threads with the specified drill. Drill completely through the hole or to the bottom of a blind hole



Click image to see an enlarged view

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



Click image to see an enlarged view

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

CHECKING ENGINE COMPRESSION



See Figure 6

A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period of time 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 earlier, a screw-in type compression gauge is more accurate than the type you simply hold against the spark plug hole, although it takes slightly longer to use. It's worth it to obtain a more accurate reading. Check engine compression as follows:



Click image to see an enlarged view

Fig. Fig. 6: A screw-in type compression gauge is more accurate and easier to use

Gasoline Engines
  1. Warm up the engine to normal operating temperature.
  2.  
  3. Remove all spark plugs.
  4.  
  5. Disconnect the high tension lead from the ignition coil.
  6.  
  7. Fully open the throttle either by operating the carburetor throttle linkage by hand or by having an assistant floor the accelerator pedal.
  8.  
  9. Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.
  10.  

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

  1. Ask an assistant to depress the accelerator pedal fully on both carbureted and fuel injected models. Then, while you read the compression gauge, ask the assistant to crank the engine two or three times in short bursts.
  2.  
  3. Read the compression gauge at the end of each series of cranks, and record the highest of these readings. Repeat this procedure for each of the engine's cylinders. Compare the highest reading of each cylinder to the compression pressure specification in the Tune-Up Specifications chart in (see Engine Performance and Tune-Up ). The specs in this chart are maximum values.
  4.  

A cylinder's compression pressure is usually acceptable if it is not less than 80% of maximum. The difference between each cylinder should be no more than 12-14 pounds.

  1. If a cylinder is unusually low, pour a tablespoon of clean engine oil into the cylinder through the spark plug hole and repeat the compression test. If the compression comes up after adding the oil, it appears that the cylinder's piston rings or bore are damaged or worn. 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. 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 on the engine dipstick when a head gasket has blown.
  2.  

Diesel Engines

Checking cylinder compression on diesel engines is basically the same procedure as on gasoline engines except for the following:

  1. A special compression gauge adaptor suitable for diesel engines (because these engines have much greater compression pressures) must be used.
  2.  
  3. Remove the injector tubes and remove the injectors from each cylinder.
  4.  

Don't forget to remove the washer underneath each injector; otherwise, it may get lost when the engine is cranked.

  1. When fitting the compression gauge adaptor to the cylinder head, make sure the bleeder of the gauge (if equipped) is closed.
  2.  
  3. When reinstalling the injector assemblies, install new washers underneath each injector.
  4.  

 
label.common.footer.alt.autozoneLogo