Finding Absolute TDC

[ISX]

Here is how to find absolute TDC that you must know in order to set your timing or stuff like that that needs to be tedious.

You will need the cummins barring tool. It is not absolutely necessary but it does make it a lot easier as you will need to be barring then engine over very slowly. You will also need a dial indicator, doesn't have to be insanely fancy.

Now the actual steps.

[*]Bar the engine over until you are at TDC compression/power #1. To do this, you watch the valves on #6 and stop when they overlap. If you are going clockwise the exhaust will shut and the intake will open, when that intake moves that is when you stop. If you are barring it counterclockwise (alternator cranks this way) you would watch the intake close and stop when the exhaust starts to open.

[*]Bar the engine backwards a little bit, maybe 2-3" going by the top of the damper, or at least 25* rotation, remember that this is backwards so it would be counterclockwise rotation.

[*]You should now be at 25* or so before TDC. Put the dial indicator on the top of the valve and loosen the adjustment nut on the pushrod and measure about 0.150 down from where the rocker contacts the valve. DO NOT FORCE IT! It should turn easily all the way to that point, otherwise you may be hitting the piston and must not have cranked the engine back far enough.

[*]Now turn the engine very slowly clockwise so that the piston will start coming up and hit the valve. You must go slow because you only want it to hit it and deflect the gauge about 0.002". So if you zero'ed it out at 0.150", you would want it to drop to 0.148" and stop.

[*]At this point, mark the damper any way you want at any point of accurate reference that you want. I used tape to denote the edge of the speed sensor. If your damper is oily, clean the oil off so the tape/marker/dog hair/whatever you use to mark it will not be rubbed off.

[*]Now, without touching the dial indicator (needs to stay in that exact spot and measurement) back the valve lash back off until it is just contacting the valve, or looser. It must be retracted or the piston will hit.

[*]Rotate the engine clockwise about 3-4" or 35* so we know we are a good way past TDC.

[*]Set the valve lash to the same 0.150".

[*]Rotate the engine counterclockwise slowly until it hits the valve and deflects the dial indicator the same 0.002" (will read as 0.148"). Make another mark at this point.

[*]Now back the lash back off, set it back to the stock lash setting (0.010" Intake/0.020" Exhaust) as now is just as good of time as any to set it back since it is on TDC compression/power, you will be on the inner base circle of the cam.

[*]Now you take those 2 marks, measure them with calipers or something accurate though a tape measure will do if you have good eyes. You will probably get something between 1" and 1.5".

[*]Divide that measurement by 2 and measure from one of your marks towards the other mark (which should be the center of those 2 marks), make a mark at this point. This mark is absolute TDC, so if you want to not have to do this every time, I suggest you make this mark permanent by various means.

Some tips:

[*]The permanent mark can be made with a steel cutting saw blade, cutting a 1/16"-1/8" deep slit into the damper.

[*]You can use a mill to machine a mark into it.

[*]The edge of a file may work as well.

[*]If you think your going to move the dial indicator for some reason, you can zero the dial indicator right at the point where the valve contacts the rocker, then you can do whatever you want and just reference that point each time, going 0.150" from there.

[*]Do NOT forget to back the lash off after making your mark or you risk forgetting and cranking the engine into the valve and doing damage. I can't stress this enough, take your time and read each step carefully.

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