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Top Dead Centre Question

JPSmit

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So, I need to static time the car. I'm pretty sure I have the concept clear in my head except that I want to make sure I'm not 180 deg out.

I understand the issue but I'm not quite clear how I can tell.

any advice gratefully received
 

ChrisS

Jedi Knight
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First I'll assume that you can determine the TDC position but not which TDC. With the rocker / valve cover off look at the valves for the #1 cylinder. It will be at TDC between the compression and power stroke when neither valve is moving. If they are both moving then you are between the exhaust and intake stroke. You want neither valve moving.
 

BugEyeBear

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Combine this with a visual look into the #1 spark plug hole to observe the piston at the top of its stroke. Or use a probe (I use a plastic 1 piece probe with a handle large enuf to prevent the whole thing from falling into the combustion chamber) in the #1 spark plug hole so that you can "feel" that the piston is at the top of the stroke as you rotate the engine by hand, & you'll be good to go!

As "ChrisS" mentioned, the piston will be at TDC twice during the 4-stroke cycle. Once between the compression & power strokes, and again between the exhaust & intake strokes. You want the TDC when the spark plug fires. That's the one between compression & power...

"SUCK - SQUASH - (Spark) - BURN - BLOW"

intake - compression - (spark) - power - exhaust

Hope that helps!
(...pretty basic if you think about it!)

"Cheers!" :cheers:

-Bear-
 

BugEyeBear

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Good Videos!

I've also seen a similar setup where a manometer is used...

A similar tube device is threaded into the #1 sparkplug hole, but this tube travels up, then down, then up again. In the "down then up again" section some oil is captured, & using just air pressure from the cylinder you can watch this oil travel up as the cylinder is advanced towards TDC.

The down side of this setup is that any small air leak (in the device, or past valve seats) will cause inaccurate results.

I like John's method.

Worth noting is his mention that you'll need to get most of the oil out of the cylinder prior to starting the car. IF you don't you'll be trying to compress this oil and you could damage the engine. I'd use a suction device to get most of it out, then maybe blow out the remainder with compressed air.
(I've had more than one occasion off-roading where I've attempted to cross a stream that was a bit deeper than expected and LITERALLY Flooded the engine (with water). After winching my Jeep out I would remove all the spark plugs, put the trans in gear and push the car a few feet to force most of the water out of the cylinders. OTHERWISE you'd be trying to compress this water and you could damage the engine.)

"Cheers!" :cheers:

-Bear-
 

jlaird

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Use a number 2 yellow pencil to check when the piston is up. A nice fresh one so it is long enough not to fall in.
 
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JPSmit

JPSmit

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jlaird said:
Use a number 2 yellow pencil to check when the piston is up. A nice fresh one so it is long enough not to fall in.

does it have to be yellow - dang back to the parts store again.

:laugh:
 
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Jp
if you use any other color but yellow, be very very careful that you dont get any of the paint in the hole. What happens is when you the plugs back in and start it up a chemical reaction occurs that will weld the plugs to the head.
























j/k :devilgrin:


mark
 

BugEyeBear

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JPSmit said:
jlaird said:
Use a number 2 yellow pencil to check when the piston is up. A nice fresh one so it is long enough not to fall in.

does it have to be yellow - dang back to the parts store again.

:laugh:

I KNEW it had to be Yellow, but I DIDN'T KNOW it had to be a #2...
Must be one of those SCANTRON compliant motors....

:crazy:

-Bear-
 
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JPSmit

JPSmit

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still haven't got at the car - too cold - too many storms and too much Christmas stuff - bah Humbug :madder: (at least to the cold and storms)

Thanks for everyone's input - one more question. I understand that each piston cycles twice per (4 stroke). When the timing marks line up the first time, the piston is at TDC (or whatever the mark is, 2 deg BTDC etc.) The second time those timing marks marks line up (one revolution of the crank) is it at TDC again? (without the spark) or is it at the bottom of the cylinder? (does this make sense?)

thanks again all
 

bill_young

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TDC is the point where the piston is at it's highest travel in the cylinder. This occurs once with every rotation of the crankshaft. Every time the mark on the balancer and the pointer line up the piston should be at the top of it's movement. For timing purposes we're usually talking of TDC when both the intake and exhaust valve are closed and the mixture would be under compression, that's where we reference the igniton timing from.
 

aeronca65t

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Yes, as Bill says, the piston is at TDC when it's at it's highest point.

That happens twice during the 4-cycles:

Once, when both valves are closed and the spark plug fires (to ignite the fuel mix). You could call this "ignition TDC".

A second time when the piston pushes the spent exhaust out of the cylinder (the exhaust valve is ususally *just* closing when the exhaust TDC occurs).

Obviously, you want the spark to go off around ignition TDC and not exhaust TDC

See below:

otto.gif
 

Jim_Gruber

Yoda
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A great graphic to keep it simple, thanks for the input.
 

jlaird

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Very nice graphic.
 
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Magical intake valve. :smile:
 

bill_young

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Common Trevor, haven't you ever heard of those early turn of the century engines such as used by Benz that used a spring operated intake valve, the piston going down in the cylinder created enough vacuum to open the intake valve against the spring pressure. Considering that this is a single cylinder engine, the points are driven off a lobe on the cam it really looks like it was based on the early Benz engine. :laugh:
 

BugEyeBear

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For what its worth, I THINK the graphic is GREAT!

THANKS Nial for the contribution!
(I saved a copy for future use...)

(BTW: I looked at your webpage, did you repair all of the Watkins Glen "Blue Wall" damage?? Hopefully it was mostly limited to sheetmetal... Is #909 the rebuilt car??)

"Cheers!" :cheers:

-Bear-
 
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