four stroke engines

 Four stroke take two times of crank rotation to complete one cycle when compared to two stroke engines.The four stroke has the following stages,

1)Intake 

2)Compression

3)Power and

4)Exhaust.

Removing the cylinder head :
The coolant has to drained out before embarking on dismantling the cylinder head.proper precautions need to be taken for disposing the coolant.
Inspection of cylinder head.cylinder head needs to inspected with a straight edge and a feeler gauge  for warpage.
warpage is usually condition in which the original machined surface is distored due stress vibration, change in temparatures etc which can effect the ceiling of head gasket can lead to over heating and soforth.The maximum allowance for warp is .05mm.

Valve guides . The valve guides are tubular structure pressed into the block of the head   The wear of  valve guides will effect the the valves to locate the valve seats . Failure to seal the combustion chamber properly, the engine start to burn oil and can lose performance , leaking from the top of the cylinder head into the exhaust and intake manifolds. New engines are fitted with valve stem oil seals..
we check the valve guides clearence using ball gauge or telescopic gauge
INSPECTION
Valve mechanism
1. Inspect each valve for following:
a. Damaged or bent valve stem
b. Rough or damaged face and  c. Damaged or unevenly worn stem tip.

Valve stem : Valve stems were more prone to wear till the head seats are coated or made of stellite.The wear in Valve stem can cause  loss in compression and excess oil consumption.Resurface or replace valve if neccesay.

Calculate valve stem-to-valve guide clearance. Subtract outer diameter of
valve stem from inside diameter of respective valve guide.If clearance exceeds specification, replace valve,
Valve seats : The wear in valve seat can cause leak in compresesion and also can affect the performance of the engine heat dessipation also not uniform.
Inspect contact surface of each valve seat and valve face for following:
a. Roughness
b. Damage
c. Pitting
d. Crack


Check for valve sink by measuring protruding length  of
valve stem to lower valve spring seat.

                                                                                     

valve springs : valve springs are crucial in controlling the breathing and exhaust in internal combustion engine.The valves are mechanically operated by camshaft through tappets or valve lifters and closed by valve springs.
valve spring installed height and seat pressure:

Valve Spring Installed Height

The installed height of the valve spring is the distance between the valve pocket and the outer edge of the spring retainer (which is the height of the valve spring) when the valve is closed. To check installed height, by following procedure:

1. Install the valve in the guide.
2. Install the retainer and valve locks.
3. Install all spring cups or valve spring shims 
4. Hold the valve closed by pulling the retainer up tightly against the valve locks.
5. Measure the distance between the outside edge of the valve spring retainer and the spring seat.  a height micrometer should be used.
6. If the installed height is not within 0.020", either machining of the valve pocket, or removal/installation of valve spring shims is necessary.

Inspection of valve spring tension:

cylinder head  /assembly of the valve springs:
cylinder head faults, treads and studs: cylinder head needs to be inspeted for warpage and can be remachined if required
core plug:  the  core plugs can be inspected for any sign of water leakage. When one of the core plugs is leaking, it is safer to replace them all.
camshaft for wear and general condition::
camshift bearings:

* Core plugs tend to corrode over time and this causes them to leak.
* Be aware that after an engine is overhauled and the core plugs replaced, there's a possibility the first time the engine runs at full operating temperature, core plugs may pop out if they are not installed properly.

camshaft bearing oil clearence:
Assembling the cylinder head;
OUT BOARD AND SHORT BLOCK:
Inspection of pistons:
Inspection of cylinder bore:
Pistons and rings:
Piston ring end gap:
Inspecting connecting rods:
Connecting rod and twist bend:
Crankshaft Inspection:
Big end journals:
Inspecting the cylinder block for warpage:
Cylinder block treads and studs.
Inspection of timing gears,sprockets,chains and belts.
Core plugs for leakage and damage:
Inspecting oil pumps:
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