Top Dead Center

[zdude1967]

My question...is top dead center right when the piston reaches the top of its stroke or the slight turn of the crank that you can do before it begins to drop. I use a chop stick and hand turn the crank the piston reaches the top of its stroke but then remains there for the slightest turn of the crank before it begins to drop. So where is TDC

[Mark Maras]

 Halfway between the start of piston movement either direction. TDC should line up with the largest mark on your vibration dampener or timing tab depending on the year. To get it exact, "back in the day" we used a piston stop and a degree wheel. There's probably something electronic that will find it by now.

[zdude1967]

So it is in the middle of when it first reaches the top of the stroke and when it begins to fall again

 

[Mark Maras]

 That's correct.

[Patcon]

When you check this with the engine apart and a dial indicator, it becomes apparent that there isn't really much of a pause it just becomes very hard to physically perceive it when doing it by hand.

[siteunseen]

I did a weird way but it worked. The front two cam lobes towards the front will look like rabbits ears. I used a feeler gauge to get the lash equal on those two rockers. Atlantic tech tips suggest a nano something or the other but I found my way easier. If you google "finding tdc classiczcars.com" there will be tons of info. The chop stick or straw that i used will get you close but this needs to be dead on in my opinion.

Good luck!

[7tooZ]

I did a weird way but it worked. The front two cam lobes towards the front will look like rabbits ears. I used a feeler gauge to get the lash equal on those two rockers. Atlantic tech tips suggest a nano something or the other but I found my way easier. If you google "finding tdc classiczcars.com" there will be tons of info. The chop stick or straw that i used will get you close but this needs to be dead on in my opinion.
Good luck!

@siteunseen help me under why it so important on a stock engine that maybe more than 20 years old. I use a timing light with a dial back and it seems impossible to tell a difference of 2 or 3 degrees Unless it’s a highly tuned motor there so many other things that vary how well it runs. How many timing degrees might there be between when when the piston reaches the high point and when it starts to move down. Just curious.


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

[Mark Maras]

9 minutes ago, 7tooZ said:

 How many timing degrees might there be between when the piston reaches the high point and when it starts to move down. Just curious.


 

 

 I think the answer to that question depends on one's method and the accuracy of measuring the vertical movement of the piston. Anyone had a dial indicator on a piston top lately?

[Captain Obvious]

In theory, the pistons only ever completely come to a stop exactly at TDC and BDC.

However, in reality, it'll stop for the entire time you are taking up the bearing clearances. Hopefully that isn't very long...

[siteunseen]

15 hours ago, 7tooZ said:

@siteunseen help me under why it so important on a stock engine that maybe more than 20 years old. I use a timing light with a dial back and it seems impossible to tell a difference of 2 or 3 degrees Unless it’s a highly tuned motor there so many other things that vary how well it runs. How many timing degrees might there be between when when the piston reaches the high point and when it starts to move down. Just curious.


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

I didn't have a manometer.  Once I figured out the front two lobes were exactly the same distance at TDC it was easy.  Took 5 minutes.  I was building a 2.4 and wanted, and got, it perfect.  Two sets of Vise-Grips to lock the cam in place then I installed the timing kit and front cover.  Easy peasy for me, my method. 

 

 

Finding TDC

A manometer can be used to find TDC.

A manometer is simply a U shaped  tube with a water or oil "indicator"  in it. Air pressure or suction at one end of the U causes the indicator to move.

For our Z application, a drilled out spark plug,  3' length of plastic tube,  and a few ml of coloured water is all that is needed.

Note that a one thousandth inch change in the piston is nearly 1/2 and inch of water change.

 

[zdude1967]

So when I marked the timing wheel etc. I used the chop stick brought the piston to the top of the stroke and then turned it a bit more till it started to drop them marked it between the two. Soooooooo I should be at a good TDC at this point from what I am reading ....right?

[siteunseen]

Yes!  Your right on the money for for all practical purposes.  I apologize for muddying the waters but I was rebuilding mine and have more time to do silly "crap", it wouldn't let me use s h i t. 

Actually you want that exact spot where the piston "floats".  Neither raising or falling.

 

 

 

Edited February 20, 2019 by siteunseen