Helpful Technical Tips for Scientific Mechanics

Discussion in 'General Motoring' started by Nomen Nescio, Feb 13, 2004.

  1. Nomen Nescio

    Nomen Nescio Guest

    1. Always start with a compression test - A vehicle which suffers from
    lack of power, poor economy, rough idle, backfiring, among other symptoms
    needs scientific troubleshooting. And that means a compression test. No
    engine with compression problems can be successfully tuned. Here's the
    criteria for compression evaluation: (C.R. x 15)-0.10(C.R. x 15) = Target
    Compression Pressure, psi. Take your compression readings, average them
    and if less than 10% below Target Compression Pressure your cylinders are
    leaking excessively. Now take the highest and lowest pressure. If the
    highest is more than 20% over the lowest, compression is excessively
    uneven. Those are your two criteria: average and variation. If out of
    bounds, you need to differentially diagnosis piston/rings/cylinder trouble
    and valve/valve spring/valve seat/head gasket trouble. You can do that by
    injecting oil into the cylinder. If the compression rises to normal, your
    problem is cylinder; if not the problem is valves. Another differential
    test is the cylinder leak down test, using compressed air injected via the
    spark plug hole. Interestingly, the compression test is probably the
    original and oldest test of engine condition. It is just as valid now as
    it was in 1918! In those days, mechanics would just stick their thumb in
    the spark plug hole and have their assistant crank the engine; if there was
    good compression the thumb would pop just like a cork out of a champagne

    2. Valve jobs - When you do a valve job, in almost all cases you will need
    to grind only the valves, not the seats. When you diassemble the head,
    keep the valves in order of removal so they can be replaced in the same
    guides. Make a simple wood holder for the purpose. Today's aluminum heads
    with hardened steel seat inserts mean seats do not errode away like the old
    cast iron ground seats used to. Take a good look at the seats; if they
    look unworn, leave them alone. After grinding the valves, check their
    margins. Do not grind away the valve to a knife edge or they will overheat
    and warp. Very important: Lap the ground valves into each respective seat
    with valve grinding paste and a suction cup hand lapper. After lapping the
    valves, pour solvent into the inverted head (with spark plug in place) and
    check for solvent leakage past the valves. If the valves hold solvent they
    will hold gas pressure. Do not fail to do this check because you don't
    want to discover bad seating once the engine has been reassembled and have
    to do the whole job over again.

    3. Use generic specifications - It is not practical to buy factory shop
    manuals for every car you work on, particularly if you run a general shop.
    Motor Manuals used to be all a pro mechanic needed. Today, the Internet
    will provide all the specifics needed like for instance head torques. But
    the pro mechanic knows to tighten head bolts in a circular pattern, little
    by little in about four stages. He knows wheel stud nuts are 60 ft lbs.,
    etc. If you need a schematic, just go to the net. There's a lot of leeway
    in specs. After a while you get used to tightening without a torque wrench
    except for critical bolts like head bolts. You can find a general torque
    spec sheet on the net and post it in your workplace for reference. Most
    teardown jobs are similar from make and model so you can work you way
    through most jobs without reference to manuals.

    4. Bolts and nuts - Clean threads on a wire wheel buffer. Always apply
    anti-seize compound on any bolt going into aluminum. Always apply Loc-Tite
    242 on bolts and nuts onto steel or iron. Avoid overtightening; that is
    the wrong way to prevent loosening due to vibration. Car makers have been
    guilty of not safetying their fasteners except for wheel bearing adjusting

    5. Use sealants - Pure silicone rubber makes an excellent sealing
    material, particularly for applications like valve cover gaskets. Coat all
    gaskets, except the head gasket with this material and your engine will
    remain oil, water, and gas tight. Permatex #2 is basically obsolete now.
    For tapered pipe thread like plugs in the block, use plumber's teflon tape
    wrapped three or four times around the threads.

    6. Use quality tools - But the tools should match the work and workplace.
    Snap-On is the Rolls-Royce of tools and remains the crown jewel of American
    manufacture. But it is too expensive for use in a Blackboard Jungle car
    repair shop; fine for $60/hr Beverly Hills automotive service center. Very
    good alternatives are Craftsman, Proto, and New Britain (Stanley). For
    amateurs on a budget, think Taiwanese brands and even Chinese made tools
    which are strong and serviceable if you don't mind buying Communist goods.
    Important: use 6 point sockets whenever possible for high torque
    applications. A thin, 12 pointer can crack and let go and you can wind up
    with a busted hand. Try to pull, not push on a wrench to avoid busted
    knuckles. Consider using a robust impact socket with your long breaker
    bars; the bolt will break before the socket. Avoid power impact tools
    except for disassembly. Doing assembly work with air impact tools is
    asking for trouble. Keep your tools clean and in order.

    7. Mark your parts - Mated assemblies like rod and main caps need to be
    kept in order. Use a number punch set to mark the parts. Make work
    holders to keep parts in order for reassembly in the same locations.

    8. Cylinder work - A simple ring job can be done with the engine in place.
    Be sure to ream the ridge and "bust the glaze." Mike the cylinder for
    taper and out of round and pay particular attention to the upper one-half
    inch of cylinder. On worn cylinders, use an special ring set specifed for
    worn cylinders. Ask for cast iron rings, not chrome because cast iron will
    wear in and conform to irregularites where chrome will not. These rings
    also have extra tension in them. Do NOT install rings with an expander:
    Instead, simply place the open ends of the ring in the piston groove and
    "drag" the ring over the piston head until it seats in the groove. Easy
    done than said. Believe me it works and will not overstretch or break a
    ring. Set your gaps 120 degrees apart. When assembling an engine use SAE
    80 gear oil on everything; it is a perfect intial lubricant and will mix
    harmlessly with the regular motor oil once the engine starts.

    9. Check bearing clearances with Plastigage. After you do one with
    Plastigage, you can check the others with a wiggle test if you like. A
    general spec is 0.001 in. clearance per inch of journal diameter. You
    will find other general specs that work in any engine.

    10. Oil consumption - When the head is off, if a piston looks around its
    circumference, the rings are pumping oil. If the intake valve filet is
    caked with carbon, the valve guide oil seals are shot. Replace the seals
    if the valves are serviced. Oil consumption problems are solved by
    mechanical repair not selecting different makers and viscosities. When oil
    is at operating temperature, they all run like kerosene and if you have
    sealing problems, oil will leak out or burn in the combustion chamber.

    11. Education - It is advisable to take at least an introductory course in
    Automotives at the Jr. College level. The learning curve is logarithmic in
    that you learn about 80% of the body of knowlege in the first course, with
    the remaining 20% in the subsequent dozen courses. Get your foundation in
    that first course, then you will find it most efficient to pick up the rest
    on your own unless you really need an associate arts degree. A summer job
    as an apprentice is also an excellent approach to mastering advanced theory
    and technique.
    Nomen Nescio, Feb 13, 2004
  2. Helpful Technical Tips for Scientific Mechanics:

    1) Don't read anything posted by Nomen Nescio, alias Student Mechanic, AKA
    Stupid Meckanik, No-man Nabisco, Student Muckitup, Stupid Moron, and a few
    others I forgot about.

    Ted Mittelstaedt, Feb 14, 2004
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