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True TDC location

How to Degree a Cam

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Good Morning. I was seeing your module on how to find the real TDC, and I was left with a doubt. After the actual TDC is found, we move the pointer to the value found. This value is shown before and after turning the crankshaft clockwise and counterclockwise. In this case what is the value that we use, the value that is in clockwise direction or the value counterclockwise ?

The true TDC is the point half-way between the values found when dead-stopping the piston in each direction.

Thank you David Ferguson. So in my case I used a comparator watch and the value of the real TDC I found was 8.5 º, so will I use the value of 4.25 º?

I'm sorry, but I don't know what a "comparator watch" is. Assuming it is a device that you set to zero when the engine is dead-stopped in one direction, then it measures the relative angle when the engine is dead-stopped in the other direction, then yes - half the indicated value would be correct.

I expect it is Google translating it - watch=dial, comparator=measuring.

You need two tools, one is a degree wheel and adjustable pointer - there are other ways, but this is MUCH easier and will be used for the cam'timing - and something that will give the same piston position in the bore both before and after TDC.

A stop is easier as the piston can be brought up against it and you don't have to carefully turn the engine over to bring it up to a DTI point - oh, you need to bring it UP both ways, as the bearing clearances and piston rock may otherwise be throwing it off slightly.

As you found, the initial check will show different degrees before and after, and the correct point will be the mid-point between them, normally one would adjust the pointer for this, and retry the procedure to confirm it.

For me, the next thing is to make clear, and accurate, marks on the front pulley* and cover as this will be useful for setting any required timing offset later. Some engines can be fitted with adjustable pointers, which can be set to the true TDC position.

Then, as you've got the degree wheel zero-ed on TDC, you can then easily read the camshaft(s) timing relative to that mark.

*Some engines aren't that easy to mark on the front pulley, so some you may be able to mark on the flywheel, if you have better access. WAY back in the day, the Bedford trucks we had in the fleet had their TDC indicator on the flywheel - a ball bearing pressed into it was lined up with a pointer cast into an opening in the rear of the block.

The principle to understand is two fold:

1. We can't accurately find TDC by just bringing the piston up to the top of the stroke with a pointer or similar down the bore contacting the piston. This is due to the dwell at TDC, along with piston rock etc.

2. If we use a positive stop as recommended, we will be stopping the piston a little way below TDC. We want to do this on BOTH sides of TDC and then true TDC is exactly in between. If you're using a degree wheel then you take a reading on one side of TDC and the other and TDC is half way between the two points. Let's say you see a reading of -8 deg on one side and +10 deg on the other side, this would mean our initial estimate of TDC was a little off. We can find true TDC by adding the two measurements and dividing by two. In this case (-8+10=2 2/2=1). This means true TDC is at a value of 1 deg on the degree wheel.

I hope that answers your question as I'm not 100% sure on your reference of 8.5 deg.

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