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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 09-14-2013
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Finding TDC

Hi all--I have a Yanmar 2QM20 installed in my Herreshoff Leeboard Ketch that badly needs a birthday.--The main pulley is quite badly rusted and I can no longer find the TDC mark line on the pulley.Could some kind soul please advise me of an alternative method of finding TDC so I can reinstate the mark line with the pointer-Thanks in advance-WK
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Old 09-14-2013
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Re: Finding TDC

Here's some options:
How to Find Top Dead Center in a Marine Piston Engine - Fuel Pump, Camshaft, and Flywheel inspection Methods
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Old 09-14-2013
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Re: Finding TDC

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Originally Posted by White Knight View Post
Hi all--I have a Yanmar 2QM20 installed in my Herreshoff Leeboard Ketch that badly needs a birthday.--The main pulley is quite badly rusted and I can no longer find the TDC mark line on the pulley.Could some kind soul please advise me of an alternative method of finding TDC so I can reinstate the mark line with the pointer-Thanks in advance-WK
If you are trying to find TDC for adjusting valves I just use a remote starter button and "bump" the engine over, bump, bump, bu, until the valves I am adjusting are at TDC.....
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Re: Finding TDC

Thanks Boz and Maine Sail-Several of the various methods supplied in article are difficult to apply to Yanmar 2QM20 motors-and bumping starter until push rods are loose does not necessarily mean you have true TDC-On looking at my parts book it appears that the keyway on the front of the crankshaft lines up with TDC on the crank and therefore if I turn the motor so the keyway on the main pulley lines up with the pointer,this should be TDC.Can anybody confirm this?
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Old 09-15-2013
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Re: Finding TDC

BLUF: I'm not an expert. With that said, I can't answer your specific question on the keyway, but here's a couple of other things to try.

I was taught to find the stroke where the exhaust rocker returns and the intake just starts to open, mark pulley at the top. Rotate the pulley 360 to give you the tdc between compression and power stroke.

Nigel Calder's book says to "slowly rotate the engine in its normal direction of rotation. Watch the inlet valve's pushrod as it moves up and down. When it is almost all the way down, the engine is at the bottom of its inlet stroke. Mark the crankshaft pulley, and turn the engine another half a revolution. Now the piston will be close to TDC on its compression stroke."

Should give the same spot. You can do this with a large breakover bar and a socket on the front pulley. Be absolutely sure you only turn the engine the direction the engine runs/starter turns.

How about the flywheel? I don't remember if Yanmar has them, but many manufacturers put an observation port in the back and marks on the flywheel to show each cylinder TDC and each cylinder injection timing mark. Those should be very accurate.
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Re: Finding TDC

Remove spark plugs or injectors, put a screwdriver in the hole, with a wrench turn the crank slowly and carefully, not to scratch the cylinder wall with the screwdriver. At a motorcycle shop you can find specific tools that screw into the spark plug hole that are extremely accurate. Find TDC rotating the engine in the running direction.
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Re: Finding TDC

Thanks Boz and DR--Will try your ideas-I could not find inspection cover to view flywheel markings which surprises me--I tried screwdriver down one injector orifice but it appears you cannot access the piston top due to the precompression chamber angle-I may be wrong on this.-Cheers-Chris
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Re: Finding TDC

Porfin can help you out. His engine is a Perkins, but the process to find exact top dead center is the same.
Perkins 4.108M DIY Rebuild -- A Narrative
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Re: Finding TDC

Shade tree strikes again. Don't make any serious adjustments based on this. As the cylinder comes up it is pushing air out the injector hole at TDC it stops and starts sucking air in. Feel the air coming out when it stops you are NEAR TDC + or - .
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