What Is TDC Of a Piston? 7 Ways To Find The TDC Of an Auxiliary Engine

            TDC (Top Dead Center) means the upper most position of piston in the cylinder of an engine. It is the point from which ignition system measurements are made and the firing order is determined. Engineers are using different methods to find out the TDC of an Auxiliary engine. Here  you can have a look at some of the  methods to find out the TDC.

1. Flywheel Method
It's the simplest and common method ever used to find the TDC. In flywheel,  unit numbers are marked which ever come on TDC. If flywheel shows two units, both the units will be at TDC. One unit will be firing ie, both the inlet and exhaust valve will be closed and hence both the push roads will become loose. Other unit will have the rocker arm at different levels

2. Fuel Pump Method
There is a cut marking on both fuel pump body and plunger of the fuel pump.  when these marking matches ; fuel injection takes place. At this point, piston ignition starts . This will be 4-6 degree before TDC and the inlet and exhaust valves are closed.

3. Cam shaft method
  TDC can be found out by the position of  follower in the  fuel cam and exhaust valve cam. Open the cram shaft door check the position of follower in fuel cam and exhaust cam. If the follower is in the Raise position in fuel cam; at that point fuel injection is going on, this time both the inlet and exhaust valves are closed and the piston is at TDC. At this point exhaust follower is in dwell position.

4. Push Road Method
Check if the push roads are free to turn. The unit at the TDC has the push road free to turn because of loose spring. At this point both the inlet and exhaust valves are closed.

5. Valve Spring Method
This is not an independent method but is used in conjunction with the flywheel method. In this method if the flywheel is indicating 2 units you can check the springs of both the units. The unit where the springs are loose is , the one that is at TDC. The question is that if this method is useful for an engine in use. If you have removed the rocker arms during the overhaul and thereafter you want to use this method then it may cause errors.

6. Crank case method
This is the simplest method . In this method the crankcase doors are opened up and the piston is visually checked weather it is going up or down

7. Dial gauge method
In this method the fuel injector is taken out and from the opening, the dial gauge is put inside; then the turning gear is engaged and the engine is turned over. The pointer of the dial gauge will move in one direction and then stop and start in opposite direction. The moment the pointer of the dial gauge stops and changes its direction of moment ; it is the TDC of the unit. This method is not normally used in day to day practice, but may be used in the calibration of the flywheel if it is not calibrated or after repairs


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  1. L.S,
    What a lot - most very complicated possibilities - to find the TDC of a piston in an engine?
    Just look at the marks on the flywheel

    1. Thanks for reading.. complicated but in some extend it is useful.... Wt will you do if the markings are not clear...?

    2. I can line can pulley / cam lock but not fly wheel the only mark I can see is like an inverted slot(raised)I am wandering if someone has put flywheel on out of alignment

  2. When the inlet push rod is moving downwards,turn quarter turn more the flywheel. piston will be at TDC

  3. Is it ok to forget flywheel if cam lock is right ,I have turned engine a few times and can not see any mark only a raised piece that looks like an inverted slot about 3mm wide which dose not line up ,help please

    1. If the marking are not clear in the flywheel try to find out the TDC by above methods... At least 2 method should satisfy....

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