How to: Finding TDC (Top Dead Center)

[ExplorerDMB]

If you are doing anything timing related or doing a leak down test; it's always important to have certain cylinders at TDC at certain times with certain valves open/closed. Now, a tip for find TDC is to remove the spark plug and stick a long screw driver in the spark plug hole and start to turn the motor over manually. Do NOT rotate via starter/electrically. Once the screw driver is coming up and starts to move slowly down again, you are around TDC. It's ok to spin/turn the engine in a normal operation condition direction, but do not rotate in opposite direction of normal op - unless it's a small amount. May cause valvetrain to bind.

-Drew

Ps: you could also do it the dial indicator way for TDC:

 

[Glacier991]

On leakdown testing, a whistle that screws into the spark plug hole can tell you when you are on the compression stroke. I actually never really worry about TDC on leakdown testing, since it happens twice in each full cycle (so one of those times I won't have closed valves,) but just want to be sure I have all the valves closed, so once I start to hear a whistle, I'm good to go. I know the piston is somewhere in the bore on the compression upstroke at that point.

 

[rookieshooter]

One other thing about finding TDC. Like on my 302 that I'm going to make sure the cam is 100% degreed in. I want to make sure all the specs are right according to the manufacturer in regards to when the valves open and closes. Also I'm going to run this cam straight up. Meaning no advancing or retarding. Since this is a low end torque cam I surly don't want it advanced. But what I'm getting at is when I use the degree wheel to find TDC. And this will be done by using a stop plate across the a cylinder with a bolt that protrudes down into the cylinder. I'll spin the crank untill the piston hits the bolt. The dergree wheel will be adjusted so the make shift pointer is on zero. Then I'll turn in oppesite direction untill piston hits bolt. Then I'll divide the degrees from stop to stop in half and that will be true TDC. Then I'll use my dial indicator on the cam lobes and see if in fact the cam is opening or closeing so many degrees before TDC and after TDC as per specs. Now for the last step that really makes valve adjusting a snap and finding TDC on each firing order. I'll turn the crank exactly 90 degress via degree wheel and marke the harmonic balancer according to the timing marker thats on the engine. I've seen some afermarket balancers have degree marks all the way around also. Since this engine fires every 90 degrees just turn the crank to the next mark on the balancer and your at true TDC. I hope I said this right.

 

[Glacier991]

No I think you said it exactly right. And your method is the easiest, but really only useful if you have the engine torn down, as in a rebuild. Usually in a rebuild once you get the crank and pistons in, one of the first things to do is to put a degree wheel on (and leaving it on until you finally need to install the harmonic balancer,) and using your method, align TDC. The ease of doing it then as compared to trying to on an installed engine is huge. Also once you have your geartrain in, as Drew notes, turning backwards is not always easy nor recommended.

I was trying to think, other than cam timing and leak down testing, I am trying to think of any other events that have ever made me want to find TDC or some approximation thereof.

When you degree in your cam, you might take a bunch of pics and either do a photo post of that, or we can help you put one together. That would be a useful thread.

(I'd do it but don't have any engine rebuilds on my event horizon - and ain't looking for any )

 

[rookieshooter]

When you degree in your cam, you might take a bunch of pics and either do a photo post of that, or we can help you put one together. That would be a useful thread.

(I'd do it but don't have any engine rebuilds on my event horizon - and ain't looking for any )

Sounds like a plan

 

[Glacier991]

Ahh.. HUGE wave of nostalgia here. You will forgive an old man.... but back in the dark ages, when I was building engines.... and as a student also worked in the Astrogeophysics machine shop at the Univ of Colo. while I was in school (some of you have already been bored privately with my Mars Viking Lander bracket story)... I had a degree wheel and pointer setup.... reading this thread made me go dig them out of the bottom of one of my tool boxes.... wanted to share... or more honestly, wanted to reminiscence.

Here was the set up I used...

The degree wheel was a commecial one, but the pointer was something I made that fit what I was doing at the time...

Materials were cheap, and the equipment time was free.... I was in a learning mode... I wish I still had access to those tools... another view

I do not remember at all what led to this design, but the base fit several engines I was building at the time for a friend who raced...

The post was interference fit and pressed. Anyway here is one last view... maybe this will explain what we are talking about when we say "degree wheel"

(wiping small tear from my eye... reliving days long gone. Thanks for letting me)

 

[old mechanic]

If you are doing anything timing related or doing a leak down test; it's always important to have certain cylinders at TDC at certain times with certain valves open/closed. Now, a tip for find TDC is to remove the spark plug and stick a long screw driver in the spark plug hole and start to turn the motor over manually. Do NOT rotate via starter/electrically. Once the screw driver is coming up and starts to move slowly down again, you are around TDC. It's ok to spin/turn the engine in a normal operation condition direction, but do not rotate in opposite direction of normal op - unless it's a small amount. May cause valvetrain to bind.

-Drew

Ps: you could also do it the dial indicator way for TDC:

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Its great to see that some things haven't changed since the old days. LOL!

 

[rookieshooter]

For some people that might be thinking why don't you just watch the piston reach its highest point. I'am talking rebuilding an engine and the heads are off. The reason is because the piston kinda just hangs there for a while as the crank is still turning. Dwell time I think they call it. Degreeing in a cam is so important in the racing world. Like making sure both valves the intake and exhaust are off their seats at the same time for a few degrees. They call this the split overlap profile of the cam. That's what gives you that allmight lope sound to a wild grind cam. That's when your getting the nth degree (no pun intended) out of your cam. And that is another story.

 

[Creager]

this is exactly what i need to know. Ive never done this, and will be doing it soon. As soon as my engine is back from the machine shop.

Is the engine supposed to turn clockwise or counter-clockwise (from infront of harmonic balancer)? When i had mine torn down, clockwise was easier to turn if i remember correctly.

 

[ExplorerDMB]

Rotating clockwise is the correct way!

-Drew