Tuesday, September 20, 2011

Engine Reflections

Cylinder Head

Toyota 1600 4A-FE

The cylinder head has to deal with very high stress and temperatures. Because of this the parts that make up the cylinder head have to be checked for damage, and measured to make sure they are within the manufacturers specifications.

Inspection of the cylinder head

The face of the cylinder head is checked visually for damage like: chips, scratches, corrosion, broken edges, and broken threads. The face of the cylinder head is also checked for warpage by placing a straight edge across lengths of the face, a feeler gauge checks the warpage of the face. This kind of damage could effect how the cylinder head mates with the cylinder block or possibly make the engine lose compression, which can shorten the life of the head gasket.

Checking a cylinder head for warpage

Valve Guides

Valve guides provide support and lubrication. for the valve as it travels up and down. Valve guides are visually inspected for damage like bending or chips. The guides are also measured in diameter at 6 locations with a telescopic gauge and micrometer to determine the amount of wear, taper and ovality. Because of the up and down movement of the valve in the valve guide, the guide can taper at the top and become oval as the valve has a thrust side where more wear happens. if a valve guide is bent then the valve can suffer excess wear.

 The condition of the valves is very important as their operation is vital to engine operation valves operate under very high temperatures and ave to been tough to cope with forces.. The valves are visually inspected and measured for wear, cuts, carbon build up and operation. Valve stems are measured with a micrometer at 3 positions, the top middle and bottom. The top and bottom have the largest amount of wear as it is the largest thrust area. the stem measurement is subtracted from the valve guide measurement to find the valve oil clearance. If this clearance is too small the valve could seize.

Valve Collets and keepers

Valve collets and keepers hold the valve onto the valve spring. if they are damaged or bent then the valve could come off the spring and it would no return to the valve seat.

Valve seats

The valve seats are inspected for damage, wear, seat angle and carbon build up. The valve seat seals the valve with the cylinder head so that no compression is lost. If the seat is damaged or worn the valve may not sit properly. The angle of the seat also determines how well the valve sits. Small carbon deposits that build up from combustion can collect on the valve seat and make the valve not sit properly.

Carbon and damage on a valve seat

Valve Spring

The job of the valve spring is to return the valve to the valve seat after it is opened, and hold it there. The forces on the spring can bend the spring which affects its operation. This is measured with a set square and vernier calliper. if the valve spring is bent by more than 2.0 mm then it should be replaced as the spring may not be returning the valve to its seat properly. The valve springs are also checked for: free length, installed height and spring tension. All of these checks ensure that the spring is working properly. Spring tension is measured on a valve spring pressure tester, the valve spring can lose it's tension and elasticity over its lifetime and its ability to return the valve to its seat.

The camshaft is responsible for the opening and closing of the valves, any amount of damage or wear to the camshaft can affect engine timing and engine operation. The camshaft is visually inspected for cuts, metal imperfections, or scuffing on the cam lobe. The camshaft lobes are measured for lift and wear. If they are worn enough then the timing of opening the valves would be affected greatly.

Lift is measured by subtracting the base Circle from the base circle and lobe lift.

Camshaft journals are visually inspected for wear and damage. Damage to the journals could make the camshaft wobble or have increased wear. Camshaft journals are measured for wear as well. If a camshaft bearing is tapered or has become oval from wear then it's operation will be affected, the journal may not spin true which may affect timing. The Journals are measured at both ends to calculate the taper from one end to the other, A high amount of taper could mean that the journal is running unevenly. Ovality is measured with a micrometer on the y and x axis, ovality is the wearing of the circular journal into an oval shape, this can make the journal and camshaft run poorly. Cylinder head bearings on modern engines are usually integral and cannot be replaced, leading to the entire head being replaced.

Camshaft Run out

Camshaft run out occurs over time as the stresses on the camshaft journals wear and twist until the journal has points of wear. this also contributes to taper and ovality of the journals. Run out is measured with a DTI gauge that is positioned on the journal to record the slight changes in wear.

Camshaft bearing oil clearance

The bearing oil clearance is the space that oil can travel through to lubricate the bearing. If the oil clearance is too large then the camshaft will not rotate smoothly, if the clearance is too small not enough oil can enter for lubrication and the camshaft may seize due to the immense heat the cylinder head works at. Oil clearance is measured with a plasti

Bucket pad thickness

On some DOHC engines the camshaft lobes follow a bucket pad type system which is on top of the valves. These pads which are case hardened to lengthen their life span and protect from the constant wearing of the camshaft lobe acting upon it. The valve clearance and timing can be adjusted by changing the height of these pads. If the pads are severely worn then they may not be opening the valve properly, affecting the timing and performance of the engine. Bucket pads are measured with a micrometer and along with the valve clearance measurement are used to figure out the thickness of the replacement pad.

Simple bucket valve system

More complicated variable valve timing mechanism

Camshaft End float

Camshaft end float is the lateral movement of the camshaft while it is capped down. Every camshaft has a small amount of end float, this amount can be increased as the engines gets older by wearing the thrust bearings that stop lateral movement. These bearings can easily be replaced. Too much end float can make the camshaft wear more aggressively as it may be out of place laterally. End float is measured with a DTI gauge positioned on the end of the camshaft as the camshaft is moved back and forth.


1 comment:

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