Finding top dead center, or TDC is a crucial step when installing a distributor, timing set, or valvetrain components and is also helpful for many other procedures.
Jeremy Stoermer demonstrates several methods for finding TDC each based on the completeness of the engine you are working on…
Tools Used to Find Top Dead Center
Finding Super Accurate TDC with a Dial Indicator and Deck Bridge
Steve Dulcich shows you how to find Top Dead Center or TDC with a Dial Indicator and Deck Bridge — This is one of the most important reference points to know when you are putting an engine together.
The time to use this method is when the engine is torn down — the cylinder heads are off and the only tools you really need are a deck bridge and dial indicator and something to turn the engine over with. Steve uses a long breaker bar because that gives him the smoothest action and the most accurate results.
Begin by placing the deck bridge. You want the anvil of the dial indicator along the centerline of the piston pin. The reason for this is that if you set an anvil toward the edge of the piston, the piston can rock and it will greatly affect your reading. In the video, Steve is able to rock the piston seven or eight thousandths in either direction — that won’t happen along the centerline.
Start turning the crank until the piston reaches the top of the bore. You will see it in the dial indicator because the needle will reach a peak and then turn around and start going the other way. Zero the indicator at this point.
Now the exact precise point of TDC is a little hard to find because the piston will dwell at top dead center. You should see that your timing marks are close, but it is not going to be super accurate.
You could say at this point you verified the TDC marking and walk away.
How To Get A Super Accurate Measurement Of TDC
If you want to get more precise, Steve shows you how to get a super accurate measurement of TDC.
To get a precise measurement of TDC, you must measure away from TDC. That is because of that dwell — when the piston is at the top of the bore. You do that by using a checking height.
Using a Checking Height
What you are doing when you are using a checking height is referencing the piston position in degrees at the crankshaft. The actual checking height number that you use is arbitrary — it does not matter, it does not alter the results.
Steve uses two different checking heights in the video to illustrate that the result is will come out exactly the same. First, he uses .050 (fifty thousandths) and then does it again at .025.
Always take your measurements with the crankshaft turning in the normal direction of rotation.
so I am going to back this up a little more than 50 000 and then I will record what I have got on the damper at 50 before and 50 after.
In Steve’s example at .050, he measures 13° before TDC, and 11° after, or an error of 2°. He split the difference to determine that the timing mark is 1° off.
In Steve’s example at .025, he measures 10° before TDC, and 8° after, again, an error of 2°.
What if there are no timing marks?
Say you don’t have any timing marks at all. That problem was addressed briefly in the previous video, but Steve explains that you can use the same system to come up with an accurate pointer and mark your damper…
