Cylinder head

Info

• Both cylinder heads can be removed with the engine in the frame.
• The front cylinder cannot be removed while the engine is in the frame.
• O.25mm and 0.5mm oversize pistons and rings are available for the bros 650cc however
  no oversize is available for the 400cc

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:

• Dropping something down into the crankcase:
  • Dowel pins, nuts and bolts.
  • Rubber pad behind cam chain tensioner.
  • Cam chain.
  • Cam sprocket bolts.
  • Many iron based items can be picked up using a magnetic pick up tool.
• Forgetting to install cam chain guide before installing head.
• Setting incorrect valve timing resulting in incorrect valve clearance.
• All bolts / nuts should be loosened in 2 or 3 steps or else the casting might crack.
• It is absolutely essential to tighten all fasteners with an accurate torque wrench.
• Make sure the cam chain is mounted correctly on the drive sprocket before mounting it
  on the driven sprocket.
• The driven sprocket has "in" stamped on it, this side should face towards the cam lobes.
• Apply molybdenum disulfide grease to all moving parts before re-assembly.
• Pour fresh motor oil over camshaft and chain before installing rocker cover (avoid
  spillage into spark plug hole).

Tools:

• Screw drivers, spanners etc
• High quality sockets and breaker bar, preferably 3/8" drive for working in tight spaces
  (8mm, 12mm, 14mm and 5mm long reach hex key on socket).
• Torque wrench (preferably 3/8" drive) 10 Nm - 50 Nm.
• Rubber hammer.
• Valve clearance adjusting tools, feeler gauge etc.
• Cable ties.
• Contact adhesive.
• Valve spring compressor.
• Scotch pads, fine valve lapping compound and lapping stick.
• Spark plug tool.
• Heat source for annealing copper washers.
• Motor oil.
• Coolant.
• 2mm pin for cam chain tensioner.
• Some way to testing the quality of the valve-valve seat seal.
• Vernier caliper to inspect length of valve spring etc.
• Lots of clean cloths
• Several clean containers marked so that front and rear cylinder components don't get mixed up.
• Molybdenum disulfide grease (Molygrease)(Molgrease).
• High tensile M8 nuts for removing exhaust stud.
• A bore gauge would be handy to inspect the cylinders.
• A cylinder honing tool to de-glaze the cylinder before installing new rings.

Parts you definitely require:

• Head gasket
• Valve stem seals

Parts you might require:

• Gasket sealant
• Rocker cover gasket
• Exhaust gaskets
• Exhaust studs
• Copper washers
• Pistons, piston rings, piston clips, piston pins, cylinders etc

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:

• The valves are closed ¾ of the time and while closed are cooled by contact
  with the valve seat, the exhaust valve relies heavily on valve seat contact for cooling, thus if the
  valve does not close fully or there is poor contact then the valve will over heat and burn.
• Exhaust valves and their seats wear/burn much fast than inlet valves.
• A new exhaust valve should have clean fresh steel on it's contact area.
  When the valve starts to wear, corrode or burn this area will start to turn
  black, when this happens the valve should be replaced and the valve seat
  should be re-cut, however light lapping can improve the contact area.
• The quality of the seal between the valve and it's seat is important for good
  compression, you can fill the combustion chamber with liquid and to see if it leaks
  past the valve or you can blow thru the manifold to check for a bad leak.
• Use a valve spring compressor to compress the valve springs just enough so that the
  cotters (collets) can be removed.
  Note: Do not compress valve springs more than is required to remove cotters.
  Note: Place the cylinder head in a tray or something so that when the cotters fall
  out they won't be lost, or you could use a magnet however be careful not to magnetize anything
  
• Remove the Exhaust valve and check if it's burnt Check valve seat for excessive denting or wear.
  
• Check the quality of the seal between the valve-valve seat.
  
• If the valve seat is pitted then it should be recut. If the valve is burnt it should be
  replaced. However, if this is not possible lightly lapping the valve with fine lapping compound
  for 1 minute might improve the quality of the seal and increase the contact surface area.
  Using a hand lapping stick is desirable as powered tools may be too aggressive.
  Note: When lapping, lapping compound is pushed out of the way in a few seconds, re-spread the
  lapping compound ever few seconds
  Note: Do not allow lapping compound to enter the valve guides.
  Note: Clean off all lapping compound with WD40 and compressed air afterwards
  Note: Excessive lapping may reduce the durability of the valve seal because
  it does not produce a perfect 45° surface on the valve and valve seat
  Note: If a valve and its seat look good then don't lap them at all.
  
• The service manual has instructions for recutting valve seats.
  Valve dimensions and service limits:
  
• The service limit for the width of the valve contact area is 1.5mm.

  	400cc	650cc
  IN Valve guide ID new	5.500mm - 5.512mm	5.500mm - 5.512mm
  Ex Valve guide ID new	5.500mm - 5.512mm	6.6000mm - 6.615mm
  IN Valve guide ID service limit	5.54mm	5.54mm
  EX Valve guide ID service limit	5.55mm	6.65mm
  IN Valve stem OD new	5.475mm - 5.490mm	5.475mm - 5.490mm
  EX Valve stem OD new	5.455mm - 5.470mm	6.555mm - 6.570mm
  IN Valve stem OD service limit	5.47mm	5.47mm
  EX Valve stem OD service limit	5.44mm	6.54mm
  IN Stem to guide clearance new	0.010mm - 0.037mm	0.010mm - 0.037mm
  EX Stem to guide clearance new	0.030mm - 0.057mm	0.030mm - 0.06mm
  IN Stem to guide clearance servicel limit	0.07mm	0.07mm
  EX Stem to guide clearance servicel limit	0.09mm	0.09mm
  IN/EX Valve seat width new	0.9mm - 1.1mm	0.9mm - 1.1mm
  IN/EX Valve seat width service limit.	1.5mm	1.5mm

  
  
  
• Service limits for valve spring free lengths:
  

  	Inlet inner spring	Exhaust inner spring	Inlet outer spring	Exhaust outer spring
  400cc	35.7mm	38.7mm	44.5mm	42.5mm
  650cc	36.8mm	37.5mm	40.6mm	41.3mm

Camshaft:

• The journals of the camshaft should have an OD of at least 21.949mm.
• The oil clearance between the camshaft journals and the cylinder head journals
  should not exceed 0.113mm.
• Check cam lobes for flat spots and check the height of each lobe.
  Cam lobe heights:
  

  	Inlet new	Inlet servicel limit	Exhaust new	Exhaust service limit
  400	37.268mm	37.19mm	37.685mm	37.60mm
  600	38.060mm	37.98mm	38.082mm	38.00mm

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

Piston and rings inspection
Notes:

• When measuring the piston measure 10mm from the
  bottom and at 90° to the piston pin hole.
• To inspect the piston rings remove them from the
  piston and insert them into the bottom of the cylinder,
  which should have little wear, and measure the ring end
  gap with a feeler gauge.
• Clean piston crown, ring lands and ring skirts
  before inspecting.
• When removing the rings from the piston do not confuse the
  different rings, also do not expand the rings more than is
  necessary to remove them.
  

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.