Engine Reflection

Engine Reflection

Cylinder Head

An inspection of a cylinder head begins with a visual examination, searching for signs associated with wear e.g warping, cracks, chips, deformation etc, followed by a number of measuring tasks to determine wear tolerances.

Cylinder Head Warpage is measured using a straight edge and a feeler gauge. The straight edge is held against the machined surface and a feeler gauge is fitted were possible. If a measurement is able to be taken this measurment is compared with manufacturers tolerances. If warpage is too great then the surface will need to be machined again.

Valve guides are checked by feeling for play with the valve in place. Measurements are taken of the bore, two measurements 90' of each other, at three places down the valve guide bore. These measurements are used to determine Taper and Ovality within the guide.

Valve Stems are visualy checked (Stretching, bending, erosion, wear marks) as well as measured. These are measured using the same process as when measuring the guide bore.

Stem to Guide clearance is measured by comparing the two previous measurements. Subtract the valve stem width from the guide measurements and you will be left with the stem to guide clearance.

Valve seats are measured for angle, width and finish. These measurements are compared with manufacturers specifications and if need, reseated.

Valve spring squareness is measured using a surface plate and engineers square. A feeler gauge is used to measure how far off sqaure the spring is. This measurement is compared with manufacturers tolerances and replaced if need be. If a spring is not square this is a sign of wear.

Valve spring Length measured using a vernier. A set of valve springs should be within 1.5mm of each other. If this is not the case then the whole set should be replaced. 

Valve spring installed height is the measurement taken from the spring seat to the retainer. This is done without the spring in place. Remeber to account for the VSI(Valve spring insert). This measurement is used with a spring pressure tester to determine spring tension.

Spring Tension. When using a spring pressure tester the spring is put in place and would down untill it is the same length as when installed (Spring installed height). As the spring is wound down it pushes in the base plate giving a pressure reading. Spring tension can be increased or decreased by adding VSI however remember that ther must always be 1 VSI per spring. All springs should be within 5kg of each other.

Good Practise is to check all nuts, stud, bolts, threads as you work and replace or recondition where needed.

Frost plugs should also be checked for leaking, deformation and corrosion.

Cam Lobes are measured to determine the lift of each lobe. The are measured at the widest point and the narrowest point(where the wear begins). This measurement allows us to work out how much travell the valve will have and when it start to open and close.

Bearing Journals are also measured. From these mesurement we can determine Taper and Ovality.

Runout of the crankshaft is measured usind a DTI gauge(Dail test indicator) and a mircometer. A measurement is taken usinng a DTI gauge then converted to a metric reading using a mircometer. This measurement is then compared with manufacturers tolerances.




Plasti-gauge is used to measure oil clearances in various parts of the engine. For example when measuring the oil clearance of a crank/cam journal a slither of plasti is placed in the journal and then as the bearing cap is tightened it squashes down. Once the cap is removed the thickness of the plasti-gauge is measured to determine clearance.

Short block & Outboard

The Piston is measured along its thrust sides. Aprox 10-15mm from bottom of skirt. This measurement is done with a micrometer and then compared to mannufacturers specs to determine if/how much wear has occured. A visual inspection is also done.

Cylinder Bore is measured the same was as the valve guide, however a bore gauge is used. This measurement will allow us to calculate Taper and Ovality of the bore.

Piston to Bore clearance is determined using measuremennts of the piston and bore. Subtract the width of the piston from the bore and this will give you the clearance.

Piston ring side clearance is measured using a feeler gauge placed just above the ring while in place. This is measured to ensure that the ring has enough clearance after the ring has expanded due to heat. The measurement is then compared with manufacturers specifications.

Piston ring end gap is also measured to ensure clearance upon thermal expansion. The ring is placed within the bore and using a feeler gauge a measuremnnt is taken, thenn compared to manufacturers specification.

Connecting rods are measured for bend and twist. This is done using an engineers square and a feeler guage. if deformation is minor then the rod may be rectified rather than replaced.

Note: Some manufacturers recomend replacement rather than repair.

Note: If diagonal wear marks are present down the piston this could be a sign of a deformed Con Rod.

Main bearing & Big end journals, as with Cam journals, are measured using a micrometer. Multiple measurements are taken of each journal to determine Taper and Ovality. Measurements are compared with manufacturers specification to determine wether replacement is needed.

Crankshaft Deflection/Run out is measured usinng the same tools and processes as when measuring Cam shaft runout.

Cylinder block face is checked for warpage, erosion, cracks, corrosion. Using a stright edge and a feeler gauge the cylinder block face is measured like the cylinder head face.

Timing gear such as belts, chains, gears, sprockets are all checked for damage annd wear.

Water pumps are checked for corrosion, damage, leakage and flow rate.

Oil pumps clearances are measured using a feeler gauge and the reading is then compared to manufacturers specifications to determine condition. Gears are check for damage and wear.

This excersize of dismantling, measuring and reassembling an outboard, short block, 2-stroke and cylinder head has given me practical and measuring experience. Although there is alot of measuring involved a big part of this process is visual and I feel this excersize has given me the ability to identify wear and damage.