Cylinder head

Cylinder head

Info

In the tables N=new, and SL=service limit

4 stroke engine:
A stroke is a complete movement of the piston in one direction.
When the piston is at it's highest point it is said to be at top dead center (TDC).
When the piston is at it's lowest point it is said to be at bottom dead center (BDC).
There are four piston strokes in one complete cycle of the engine.
The crankshaft completes two revolutions in one complete engine cycle.
The cam shaft completes one revolution in one complete engine cycle.
The four strokes are as follows, starting at TDC on the exhaust stroke:

  1. Intake stroke, piston moves from TDC on exhaust stroke to BDC on intake stroke,
    only the inlet valves are open.
  2. Compression stroke, piston moves from BDC on intake stroke to TDC on compression
    stroke, all valves are closed.
  3. Power stroke, Piston moves from TDC on compression stroke to BDC on power stroke,
    all valves are closed.
  4. Exhaust stroke, piston moves from BDC on power stroke to TDC on exhaust stroke,
    only the exhaust valve is open

When setting the valve clearances the piston should be at TDC on the compression stroke.
Rotate the crankshaft counter clockwise and watch the valves opening and closing to find this point.
It should be 90° after the inlet valves close, use notch on flywheel to find exact position.
Also the index line on the cam sprocket should be flush with the top of the cylinder head at this point.
Also all the lobes on the camshaft should be facing down at this point.

Common faults:

  1. Oil leaking from perished rubber tube between top of rear rocker cover and air box (Crankcase breather tube).
  2. Burnt exhaust valve.
  3. Worn exhaust valve seat.
  4. Excessive carbon deposits on valve or valve seat.
  5. Leaking valve stem oil seals.
  6. Various: worn valve, worn valve guide, bent valve, incorrect valve clearance, valve timing, weak springs.
  7. Leaking head gasket.
  8. Warpage across the top of cylinder or bottom of cylinder head (after over heating).
  9. Exhaust stud seized in cylinder head.


Common mistakes made during cylinder head work:



Tools:



Parts you definitely require:

Parts you might require:



Job

  1. Remove tank
  2. Remove air box (Use correct screwdriver carefully)
  3. Remove carbs but don't disconnect fuel line, rest carbs on battery box and cover with cloth.
  4. Remove inlet rubbers
  5. Disconnect spark plug caps and loosen spark plugs.
  6. Drain coolant and remove water pipes from cylinder heads
  7. Remove camshaft oil feed pipe (chrome)(M7 banjo bolts on heads & M8 on Right crankcase cover).
  8. Remove exhaust pipes, completely remove rear Exhaust pipe, release front cylinder exhaust
    pipe from cylinder head and just move it out of the way.

Rear cylinder:

  1. Remove rocker cover (10mm socket)
  2. Release the tension of the cam chain tensioner (CCT) by
    pulling up wedge A while pushing down wedge B and
    inserting a 2mm pin to secure.
    Note: Do not allow the 2mm pin to fall down into the crankcase
    Note: If the carbs are in place it will be difficult to work
    on the front CCT.
  3. Rotate crankshaft so that the piston is at TDC on exhaust stroke
  4. Remove small cam holder (8mm socket)
    Note: Do not allow dowel pins to fall down into crankcase.
  5. Remove one sprocket bolt (10mm socket)
    Note: These sprocket bolts may have thread locker on them.
    Note: Don't drop bolt into crankcase.
  6. Rotate crankshaft so that piston is at TDC on compression stroke
  7. Remove second sprocket bolt
  8. Remove rocker arm assembly, loosen the 3 bolts and one nut
    in a criss cross pattern in 2 or 3 steps.
    Note: Do not allow dowel pins to fall down into crankcase.
  9. Remove camshaft and sprocket
    Note: Do not confuse the front and rear camshaft and other
    components.
    Note: Secure the cam chain with a cable tie or something to
    prevent it falling down into the crankcase.
  10. Loosen but don't remove both CCT bolts
  11. Remove rubber pad at top rear of the CCT
    Note: Do not allow the rubber pad to fall down into the crankcase
  12. Remove CCT
  13. To remove the cylinder head loosen the 6mm bolt with a long
    reach 5mm hex key, then the 8mm bolts, 8mm nut and 10mm
    nuts in a criss cross pattern in 2 or 3 steps, then remove them
  14. Remove the cylinder head
    Note: Do not allow dowel pins to fall down into crankcase.
    Note: Lightly tap with a rubber hammer to loosen.
  15. Remove the dowel pins, cam chain guide and gasket.

Front cylinder head:

  1. Same as rear except:
  2. Remove the large coolant hose to the right of the thermostat housing
  3. Release radiator from mounts, do not hang the rad on the one remaining hose.
  4. Pull the rocker cover out the space left by the dropped rad and then pull the thermostat
    housing out towards the rear.
  5. Remove upper exhaust stud for frame clearance.
  6. Remove the two frame mount bolts at front of cylinder head.
  7. If you rotate the crankshaft 45° beyond TDC you can reach the sprocket bolts with a ratchet extension.

Cylinder head inspection:

Valves:

Camshaft:

Cylinder head block

  1. Check camshaft journals for scoring or wear or signs of insufficient lubrication.
    Note: Some say that reducing scores with 800-1200 sandpaper and then polishing can improve the oil film.
  2. Clean gasket surfaces with 400/800/1200 grit paper on a sanding block
    Note: Start with 240 paper if there are deep dents.
  3. Check gasket surface for warpage, 0.1mm service limit (Use a steel ruler and feeler gauge).
  4. Clean off carbon with a scotch pad, clean exhaust manifold, clean coolant exit and inspect seal.
  5. Clean valves, valve seats and manifolds with WD40 and compressed air.
  6. Replace all valve stem seals (apply molygrease to seals and guides).
  7. Apply molygrease to valve stems and motor oil to the valve seats and valve faces then install the valves

Cylinder inspection

  1. Inspect cylinder/piston bore for scoring, corrosion or abnormal wear.
  2. Using a bore gauge measure the piston bore
    Take six measurements as in picture=>.
  3. Cylinder Service limits
400cc 650cc
Cylinder ID, new 64.000mm - 64.015mm 79.000mm - 79.015mm
Cylinder ID, service limit 64.1mm 79.1mm
Piston to cylinder clearance, new 0.010mm - 0.055mm 0.010mm - 0.055mm
Piston to cylinder clearance, service limit 0.1mm 0.1mm
Taper service limit 0.05mm 0.05mm
Out of round, service limit 0.05mm 0.05mm


Note: When a cylinder is first bored, the boring leaves uneven lines
on the cylinder wall, to remove these the cylinder is honed with
special grinding stones. To hone a cylinder properly requires very
expensive equipment, even most engine rebuild shop don't have the correct
equipment. The roughness of the cylinder wall is very important, too smooth
will cause faster wear on the rings and cylinder wall and many other effects,
too rough and excessive break-in wear will occur and many other effects.
The roughness left by 220 grit vitrified grinding stones is usually the best.
Also the marks left by the grinding stone should be 35°-45°.
These criss crossing marks are sometimes referred to as cross hatch.
To produce the 45° cross hatch the grinding stones are pulled in and out
of the cylinder as they turn.

Note: Before fitting new piston rings many recommend de-glazing the
cylinder. The surface of the cylinder wall has a particular roughness,
over time the scratches (roughness) will fill with glaze causing less
roughness. De-glazing a cylinder with a hand held drill type honer is
a fine art.

For more info on honing read this text I stole from another website: click here

Piston and rings inspection
Notes:

N = New
SL = Service limit
400cc 650cc
Piston OD, N 63.96mm - 63.99mm 78.96mm - 78.99mm
Piston OD, SL 63.9mm 78.9mm
Piston pin OD, N 14.994mm - 15.000mm 19.994mm - 20.000mm
Piston pin OD, SL 14.98mm 19.98mm
Bore for piston pin, N 15.002mm - 15.008mm 20.002mm - 20.008mm
Bore for piston pin, SL 15.05mm 20.05mm
"Piston pin bore" to "piston pin" clearance, N 0.002mm - 0.014mm 0.002mm - 0.014mm
"Piston pin bore" to "piston pin" clearance, SL 0.04mm 0.04mm
"Piston pin" to "connecting rod" clearance, N 0.016mm - 0.040mm 0.016mm - 0.040mm
"Piston pin" to "connecting rod" clearance, SL 0.06mm 0.06mm

Piston rings
400cc 650cc
"Ring" to "ring groove" clearance, top, N 0.025mm - 0.060mm 0.025mm - 0.055mm
"Ring" to "ring groove" clearance, top, SL 0.08mm 0.08mm
"Ring" to "ring groove" clearance, second, N 0.015mm - 0.045mm 0.015mm - 0.045mm
"Ring" to "ring groove" clearance, second, SL 0.06mm 0.06mm
Ring end gap, top, N 0.2mm - 0.35mm 0.2mm - 0.35mm
Ring end gap, top, SL 0.5mm 0.5mm
Ring end gap, second, N 0.35mm - 0.50mm 0.35mm - 0.50mm
Ring end gap, second, SL 0.7mm 0.7mm
Ring end gap, oil, N 0.2mm - 0.7mm 0.2mm - 0.8mm
Ring end gap, oil, SL 0.9mm 1mm

Cam oil feed pipe (chrome)

  1. Spray WD40 inside the pipe and blow thru with compressed air,
    Note: Make sure the pipe is not clogged before installing.
  2. Replace or anneal all copper washers

Re-assembly
Re-assembly is essentially the reverse order of disassembly.

  1. Your new cylinder base gasket should have a blob of sealant on it, this
    is to seal the seam in the crankcase.
  2. Check that all gasket surfaces are clean, smooth and straight. Using
    gasket sealant won't do any harm but some say it's unnecessary if the job
    is done right.
  3. Install both cylinder dowel pin followed by the gasket and the cylinder.
  4. Install cam chain guide.
    Note: the cam chain guide fits to the front of both cylinders.
  5. Install cylinder head dowel pins followed by the gasket and then the
    cylinder head.
  6. Tap the cylinder and cylinder head home with a rubber hammer, ensure that
    they have fully seated before tightening any bolts.
  7. Tighten all nuts and bolts in 3-4 steps, don't forget the 6mm bolt.

    6mm bolt (1) : 10 Nm
    8mm bolts (2) : 23 Nm
    8mm nut (1) : 23 Nm
    10mm nuts (4) : 48 Nm

  8. Install the two frame mount bolts into the front cylinder head (Very difficult).

  9. ________________________________________________________________________________

    Camshaft installation, valve timing.

    The front and rear cam shafts are different, do not
    confuse them.

    There is an index line on the cam sprockets, when the
    flywheel FT or RT mark is aligned, the index line on the
    appropriate sprocket should be flush with the top of the
    cylinder head.

    Usually when all the cam lobes are facing down, all
    the rocker arms will have a small amount of jog in them.

    ______________________________________________________

    Front camshaft only.

    Turn the crankshaft counter clockwise and align the RT
    mark on the flywheel with the index mark on the timing hole.
    Make sure the REAR cam lobes are all facing UP. If they are
    not, turn the crankshaft counter clockwise one revolution so
    that the REAR cam lobes are all facing UP. (NOTE: This is the
    valve overlap position.)

    Continue turning the crankshaft counter clockwise (128) until
    the FT mark on the flywheel aligns with the index mark on
    the timing hole (approx. 3/8 turn).

    Install any necessary dowel pins.

    Grease cam lobes and journals.
    Install the camshaft in the cylinder head through the cam chain
    and install the cam sprocket on the camshaft.

    With the cam lobes all facing down, align the timing marks
    (index lines) on the cam sprocket with the top of the cylinder
    head.

    Place the cam chain on the sprocket.
    Install the cam sprocket on the camshaft flange and recheck
    that the timing marks (index lines) align with the top of the
    cylinder head.

    Inspect valve clearance.

    Align the cam sprocket bolt holes in the cam sprocket and
    camshaft, install and tighten the cam sprocket bolt.

    TORQUE: 23 Nm.

    Install the holder, oil plate, 8 mm bolts, 8 mm nut and 6 mm
    bolts.

    TORQUE:
    8 mm bolt: 23 Nm
    8 mm nut: 23 Nm
    6 mm bolt: 10 Nm

    Turn the crankshaft counter clockwise 360 and install the
    other sprocket bolt.
    Install the end holder.

    CAUTION
    Note the direction of the camshaft end holder, install the flat
    surface on the holder facing in.

    ______________________________________________________

    Rear camshaft only

    Turn the crankshaft counter clockwise and align the FT
    mark on the flywheel with the index mark on the timing hole.
    Make sure the FRONT cam lobes are all facing down. If they
    are not, turn the crankshaft counterclockwise one revolution
    so that the FRONT cam lobes are all facing down.

    Continue turning the crankshaft counter clockwise (232) until
    the RT mark on the flywheel aligns with the index mark on
    the timing hole (approx. 5/8 turn).

    Place the camshaft into the correct position with the cam lobes all
    facing down.
    Install the cam sprocket and camshaft holders using the same
    procedure as for the front cylinder.
    Inspect valve clearance.

    ______________________________________________________

    Front and rear cam shafts

    Turn the crankshaft counter clockwise and align the FT
    mark on the flywheel with the index mark on the timing hole.

    Install the front camshaft with all the cam lobes facing DOWN.
    Align the index marks on the cam sprocket with the top of the
    cylinder head.

    Continue turning the crankshaft counter clockwise (232) until
    the RT mark of the flywheel aligns with the index mark on
    the timing hole (approx. 5/8 turn).

    Install the rear camshaft with all the cam lobes facing DOWN.
    Align the index lines on the cam sprocket with the top of the
    cylinder head

    Remove the 2mm pins holding the tensioner wedges.

    Note:
    Be careful not to let the 2mm pin fall into the crankcase.
    Do not forget to remove the 2mm pin.

    Inspect valve clearance.
    Double check valve timing and valve clearance.

    ________________________________________________________________________________

  10. Glue the new rocker cover gasket into the rocker cover, clean and oil the
    bolt rubbers.
    Tighten bolts to 10Nm.
  11. Assemble the cooling system and fill.
  12. Re-attach exhaust pipe, Note: The front and rear exhaust manifold gasket are
    different.