REALLY finding TDC??? - Team Camaro Tech
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post #1 of 15 (permalink) Old Aug 14th, 08, 11:24 PM Thread Starter
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REALLY finding TDC???

I've been doing different things with my recent build to help teach myself things I need to know (with your help). I have, to date, always installed my cams straight up. I though that it would be fun to put my dial indicator on my motor before I slapped the heads on to find TDC other than on my balancer. I was suprised to find that there seems to be a breaking over point where the rod goes from being almost all the way up to just starting down and my dial indicator didn't move. In other words, it seems that I could move my motor over just slightly and not move the dial. If that's the case, how in the heck to you really determine TDC???? Thanks!


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post #2 of 15 (permalink) Old Aug 15th, 08, 12:01 AM
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Re: REALLY finding TDC???

mark the piont where it stops and then the piont where it starts
then find the 1/2 way between..Damn near as, close as u will get TDC
be carefull
The cam turns 2x to the crank, so make sure that the crank is on the fireing stroke and not 180 degs out

My Spelling is not incorrect...it is creative

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post #3 of 15 (permalink) Old Aug 15th, 08, 12:27 AM
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Re: REALLY finding TDC???

Here's a pictorial:

http://www.fourwheeler.com/howto/67578/photo_01.html

Click on the other pictures for the remaining steps.

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
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post #4 of 15 (permalink) Old Aug 15th, 08, 03:03 AM
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Re: REALLY finding TDC???

use a piston stop to find the actual true TDC.
without the heads on, bolt a piece of angle iron with a bolt in the middle across the top of the cylinder so the piston will hit it as it comes up.
gently crank the motor over one way until it hits the stop- and mark the balancer or write down the number on the degree wheel.
then, turn it the other way until it hits the stop, and mark the balancer or write down the number.
true TDC is exactly halfway between these two marks.
if the heads are installed, you do the same thing but with a stop screwed into the #1 spark plug hole.

you don't plan sincerity.
you have to make it up on the spot.

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post #5 of 15 (permalink) Old Aug 15th, 08, 08:46 AM
 
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Re: REALLY finding TDC???

Quote:
Originally Posted by novaderrik View Post
use A Piston Stop To Find The Actual True Tdc.
Without The Heads On, Bolt A Piece Of Angle Iron With A Bolt In The Middle Across The Top Of The Cylinder So The Piston Will Hit It As It Comes Up.
Gently Crank The Motor Over One Way Until It Hits The Stop- And Mark The Balancer Or Write Down The Number On The Degree Wheel.
Then, Turn It The Other Way Until It Hits The Stop, And Mark The Balancer Or Write Down The Number.
True Tdc Is Exactly Halfway Between These Two Marks.
If The Heads Are Installed, You Do The Same Thing But With A Stop Screwed Into The #1 Spark Plug Hole.
+1^
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post #6 of 15 (permalink) Old Aug 15th, 08, 08:55 AM
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Re: REALLY finding TDC???

+1 to the above for using Timing Wheel and Piston stop, zero timing wheel, using piston stop.

Ed

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post #7 of 15 (permalink) Old Aug 15th, 08, 03:00 PM
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Re: REALLY finding TDC???

I'll 2nd the using the piston stop and degree wheel. I highly recommend using comp cams crank socket so you can easily move your degree wheel. Its not necessary, but it sure makes life easier.

http://www.jegs.com/p/Comp+Cams/753185/10002/-1/10783

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post #8 of 15 (permalink) Old Aug 15th, 08, 03:40 PM
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Re: REALLY finding TDC???

You can do it with your dial indicator, halfway inbetween any dimension down coming upwards from both directions. That sounds wierd, what it mean is to have the piston coming up each time you stop at say .050" before Zero on the indicator. The opposite of this would be .050 on the down stroke, and you would get some pin slop and piston slop in your reading.

BTW that dwell time you discovered at TDC is the reason they play with rod lengths. Altering rod length alters how long the piston is at TDC and thereby changes the crank angle when the power stroke begins.

Tim Smith
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post #9 of 15 (permalink) Old Aug 15th, 08, 11:21 PM Thread Starter
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Re: REALLY finding TDC???

Quote:
Originally Posted by 77wolf10.85 View Post
Quote:
Originally Posted by 77wolf10.85 View Post
BTW that dwell time you discovered at TDC is the reason they play with rod lengths. Altering rod length alters how long the piston is at TDC and thereby changes the crank angle when the power stroke begins.

If this is a secret power trick Tim, please switch to stealth mode when giving me this info!!! LOL


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post #10 of 15 (permalink) Old Aug 16th, 08, 08:04 AM
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Re: REALLY finding TDC???

The piston is at TDC exactly the same amount of time regardless of the length of the rod.

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
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post #11 of 15 (permalink) Old Aug 16th, 08, 03:23 PM
 
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Re: REALLY finding TDC???

I used a micrometer dial indicator to find TDC on my 454. True TDC for mine was actually 4 degreees ADV on the factory indicator. I use 4 degrees as "my zero".
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post #12 of 15 (permalink) Old Aug 16th, 08, 03:38 PM
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Re: REALLY finding TDC???

Quote:
Originally Posted by onovakind67 View Post
The piston is at TDC exactly the same amount of time regardless of the length of the rod.
I disagree. The difference is minimal, but it does exist.

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post #13 of 15 (permalink) Old Aug 16th, 08, 04:13 PM
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Re: REALLY finding TDC???

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Originally Posted by ldrisner View Post
I disagree. The difference is minimal, but it does exist.
When is the piston at TDC? Is it at TDC 0.001 BTDC? Is it at TDC 0.001 ATDC? TDC means top dead center, not somewhere around the top of the stroke. The length of the connecting rod will slightly affect the rate at which the piston approaches and leaves TDC, but TDC equals TDC for any rod.

At the same crank angle in a standard 350 SBC, how many degrees from TDC will a 5.7" rod be 0.001" deeper in the cylinder than a 6" rod?

2? 5? 7? 10? 15? >15?

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
Semper Fi! L/Cpl Edwin L. "Tim" Craft, B Co 3rd AT's, Khe Sanh Combat Base, February, 1968
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post #14 of 15 (permalink) Old Aug 16th, 08, 04:52 PM
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Re: REALLY finding TDC???

yeah I misspoke. It is at TDC the same time, it's the arc to the sides that alters.

Tim Smith
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post #15 of 15 (permalink) Old Aug 16th, 08, 05:24 PM
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Re: REALLY finding TDC???

The arc to the side before the piston moves, I should say.

Tim Smith
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