How to measure tappet clearance

TAPPET CLEARANCE

TAPPET CLEARANCE

This is the clearance available between the rocker arm tip and valve stem when the valves are closed position & engine at the cooled condition at the compression stroke.

Tappet clearances are necessary to allow for thermal expansion of the valve spindle length at working temperature. When this clearance increases or decreases, there is an adverse effect on the timing of opening and closing of the valve.

• If the clearance increases- The valve will open late and close early.
• If the clearance decreases- The valve will open early and close late.

The general clearances are somewhere between .30mm to 1.5 mm depending on the manufacturer and the valve (Air or Exhaust). A clever engineer will always keep an eye on it and will adjust them timely.

How to take the tappet clearance

Bring the piston to TDC of compression stroke, this will ensure by

• The flywheel marking 
• The fuel pump mark 
• The checking the camshaft fuel 
• The valve cam position.

 

On this position fuel cam lobe will touch the fuel pump or valve cams will be on base position(off cam position) that means both valves are closed position, at this time push rod will be free this can be check by rotating push rod. and take the clearance between stem and rocker arm tip by using a feeler gauge and adjusted by loosening or tightening the tappet adjusting screw.

Normally we check it by flywheel marking and pushrod. Taking tappet clearance is a skill, same as the adjusting it. First of all, you need to make sure that the unit you are going to take tappet clearance should be at top dead center. This is clearly marked on the flywheel. You can use turning gear to turn that particular unit to TDC. Usually, there are 6-8 units in four stroke engines. So at the same time, the flywheel will show two units at TDC. To make sure that particular unit is on the compression stroke and not at the power stroke, you can turn the push rods of both units by hands to check. The one with free push rods is the one which is at compression stroke and the one with tight push rods is at power stroke. So avoid the one which is at power stroke.
Now you need good filler gauges of different thickness. Now gently try to push these filler gauges between the clearance of valve and rocker arm. You may have to try different thickness. Just check in the manufacturer manual as for how much standard clearance should be.

Now you know that your clearance is less or more. But now comes the adjustment part. This is quite important though. You need to open the nut on the rocker arm as shown below. After opening, you can adjust the clearance.

Note: Keep the filler gauge all the time between them, to get the perfect clearance.

Tip: When you move filler gauges while adjusting for particular clearance, you should be able to slide it smoothly (not too tight and not too loose). After adjustment, tight the nut with given torque. Keep the record for future use. Hope this will be useful for you.

• EXCESSIVE TAPPET CLEARANCE will cause the valve to open late and close early in the cycle and will reduce maximum lift it will also noise and get damage from the impact of the working surface.
• LESS CLEARANCE will cause the valve to open early and close late with increased maximum lift it may prevent the valve from closing completely as it expands this may cause valve burning and low compression.

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Reference:

1. Marinersgalaxy.com

2. Daihatsu Manual

3. Yanmar Manual

4. MAN B&W MANUAL

How to Measure Crankshaft Deflection

Crankshaft Deflection:
Crankshaft deflection must be taken twice- before starting the d'carb and after completion of d'carb. This is done to ascertain whether the crankshaft journal is deviated from the theoretical axis or not. The deflection is recorded by using dial deflection gauge which is fitted between the webs of the crankshaft in each unit.

The complete shaft is then rotated in the direction of the operational rotation of the generator which may be a clock or anti-clockwise. The reading of the dial gauge is noted while turning the crankshaft which indicates opening and closing of the webs. Smaller the reading better is the crankshaft deflection.

Procedure:

With the running gear in place, the crank to be measured has to be turned towards (before or after) B.D.C until the dial gauge can be fitted next to the connecting rod at
the position indicated. Pre-tension the dial gauge slightly and set it to “0”. Turn the crankshaft with the turning gear, and record the dial gauge reading in the crank positions as shown in the figure. The difference between the indicated values at B.D.C. and T.D.C. shows the amount of crank deflection during one revolution. When measured values lie above the maximum permissible limits, the cause has to be found and the necessary remedial measures taken. (defective main bearing, engine
support altered due to hull deformation, loose holding-down bolts, defective shaft line bearings, etc.).

Tools, Parts and Materials (For Engine Cadets)

Reference: NYK CADET TRAINING NOTES