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Smokedtires 11-12-2023 05:20 PM

Timing gears and top dead center
 

3 Attachment(s)
Hey gang, I want to run something by you all. With my cylinder head off, I brought piston #1 up to top dead center (TDC). I noticed the timing gears were not aligned as when I matched the cam gear to the crank gear timing mark. The crank timing mark looks positioned correctly to the gear keyway slot.

So my question is, should #1 piston be at TDC when the timing marks are aligned? Or is the piston at TDC when using the timing pin to locate the timing divet through the timing cover? I don't see how it is possible to change the relationship of the gear to TDC unless the crankgear is mounted backwards, I don't believe that is the situation though.

BRENT in 10-uh-C 11-12-2023 06:18 PM

Re: Timing gears and top dead center
 

This is often an area of confusion. The marks are only designed to place the Camshaft into proper timing orientation to the Crankshaft. It has nothing to do with location of the Piston(s).

Big hammer 11-12-2023 06:20 PM

Re: Timing gears and top dead center
 

Timing pin the the divit and the piston at TDC before the power stroke after the end of the compression stroke. The piston comes to TDC twice in a 4 stroke engine

Cam timing when the gears lineup ( your good ) , ignition timing when the timing pin drops into the divit and the piston at TDC , are completely different using the divit for both

Smokedtires 11-12-2023 07:32 PM

Re: Timing gears and top dead center
 

Thank you gentlemen for the explanation. I did a search and didn't find anything. I will continue my journey towards putting this back together. ��

nkaminar 11-13-2023 08:13 AM

Re: Timing gears and top dead center
 

Smoked, While you have your head off, turn the engine over by hand and observe the valve action. At the top of the exhaust stroke, when the piston is at the top, the exhaust valve should just be closing and the intake valve should just be opening.

The exact top dead center is hard to hit because the piston is not moving very much. You can use a pointer and some marks on the pulley to exactly hit top dead center. Move the piston down an inch or so in the counter clockwise direction. Put a mark on the pulley where the pointer points. Measure the piston location with a caliper. Position the piston the exact distance down in the clockwise direction and put a mark on the pulley at this position. Top dead center is half way between these two marks. This technique was used to time steam locomotives many many years ago. You can use a degree wheel instead of the pulley if you have one.

The exact timing of the overlap between the two valves will vary a little depending on the cam grind.

ModelA29 11-14-2023 07:03 PM

Re: Timing gears and top dead center
 

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I think you will find one of the valves would be open with the dimple where it's at - RED. Take another 360 on the crank and the dimple will line up at TDC for timing - YELLOW.

Smokedtires 11-15-2023 09:59 AM

Re: Timing gears and top dead center
 

Thanks Nkaminar, that makes a lot of sense! I wondered how that was done with piston dwell at the top and bottom of the rotational cycle.

Thank you ModelA29, after realizing the timing gears were more for crankshaft to camshaft timing that helped me understand the piston may not be at TDC when the timing gear marks are aligned. Your illustration is perfect for where the dimple is located when at TDC and using the locator pin through the timing cover.

ModelA29 11-15-2023 05:00 PM

Re: Timing gears and top dead center
 

Here is a little trick to get around the vagueness of dwell.


https://www.motortrend.com/how-to/ac...ne-piston-tdc/

Smokedtires 11-16-2023 08:21 PM

Re: Timing gears and top dead center
 

Quote:

Originally Posted by ModelA29 (Post 2270027)
Here is a little trick to get around the vagueness of dwell.


https://www.motortrend.com/how-to/ac...ne-piston-tdc/

That is a great article, thanks for sharing.


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