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Old 03-21-2013, 01:46 AM
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TDC location question

OK guys... This one has me stumped, maybe its because im working 12hr nights but I cant get this in my head for some reason.

Long story short, my sbc 364ci bracket car likes a lot of timing, i mean like 40+ total. I was thinking maybe i should verify TDC to make sure my tab and balancer are correct, so I started looking at findind TDC with a piston stop.

Everything I read says with rockers off of #1 rotate engine clockwise on compression stroke until piston hits the stop, then rotate counter clockwise until it hits again, divide the amount of degrese between them by 2 and thats TDC. What im missing is isnt that a full 360 degrese of rotation from where it hits to where it hits again? Which would always be 180 degrese? Also my brain is saying piston is no where near the top if I divide by 2 between the times it hits...

What am I missing, guys? I just want to verify balancer and tab are correct. Balancer has tape. I know im missing something simple, but cant figure it out.

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Old 03-21-2013, 02:52 AM
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I watched a video on how its done, and it makes sense but I cant picture, in my mind, why the piston doesnt hit the stop at the same point both times around. I think the video I saw it was about 24 degrese different, meaning TDC would be "12" Help me understand.

Thanks guys!
chevyguy13
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Old 03-21-2013, 06:18 AM
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Some heads like Vortec heads you can't use a TDC locator through the spark plug hole.
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Old 03-21-2013, 07:40 AM
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Put a degree wheel on the crank. The stop goes into the bore. The piston hits the stop before it reaches the max height. You read the degree wheel and turn the crank around the back way and take the other reading. The middle of the 2 readings will show you TDC. Take the stop out and move the crank to that reading and then the piston will be right on top.
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Old 03-21-2013, 09:35 AM
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homemade manometer

Built one of these yesterday. Works great! Already new that the TDC was true so wanted to check it out. The fluid will move as u turn the crank so turn slowly and use plenty of clear hose. When the fluid changes direction u've went to far. Shot some wd40 into that little hose using the straw that came with the wd40. Had a cheap compression tester that had a quick disconnect hose so didn't have to cut anything.

How to find TDC - Do It Yourself Hot Rod Kustom Website
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Old 03-21-2013, 02:10 PM
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Chevyguy13, I think its those 12 hour night shifts and probably not sleeping during the day that are fogging your brain ... I know the feeling when my busy times of the year come at me !.

Think of it like this in your mind, without any stop in the spark plug hole and you have lined up the mark on the balancer with the corresponding tab on the timing cover, that in "theory" means the piston is now exactly at TDC. Rotating the engine the full 360 degrees brings you right back to the timing mark lined up once again .... 360 degrees and that never changes and the piston would once again be at TDC. My feeling is that you threw in visions of the timing set and cam/valve movement into the equation and throughly confused yourself. Blank out anything that is going on with the valves other then ensuring the tool you use into the spark plug hole doesn't interfere with the movement of the valves.

So now, back to the crank shaft/balancer/timing cover marks/ and the piston relation to them. Visualize doing what you were with your spark plug hole tool and how you were now NOT able to rotate the engine over to reach TDC assuming you had the tool in far enough to hit the piston before it reached the top of its travel. For easy figuring lets say your rotating the engine clockwise from a BTDC spot that put your piston well away from the tool and as you rotate the engine and the piston contacts the tool, you are showing 10 degrees BTDC as per how your balancer is marked. Now turn counter clock wise all the way around until the piston again contacts the tool and it also reads 10 degrees on your timing tape but its now 10 degrees ATDC on your timing tape. What the stop allowed you to do is rotate the engine 20 degrees LESS then a total 360/one rotation turn due to the stop or put another way, it was only allowing to rotate a total of 340 degrees. So yes, you are in essence NOT able to turn the engine over a full turn because of the stop.

You are correct though in that noting where the timing tape stopped relative to your TDC mark on your timing cover, taking those two spots on the timing tape where the balancer stopped at and counting those total degrees of rotation that the engine was NOT allowed to travel through and dividing by two. In theory the timing mark on the balancer should be exactly at the half way point of your two stop points.

If not, then you know for sure the balancer mark relationship to the timing cover marks are not telling you the true story of where TDC is but your test procedure did and can ether correct the situation to make it right or just make notes of it and work off of your "actual" TDC which may be 5 degrees different as an example by working off your timing tape ... until you change the cover or harmonic balancer and do the procedure all over again to know where your at in the future.

I hope that was more helpful then confusing !
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