bbix_211

Top Dead Center

20 posts in this topic

Year, make, and model of vehicle would be a good place to start.

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If you want to do it the hard way, pull the spark plug and stator cover.  Put a long wooden dowel just smaller than the plug hole itself into the spark plug hole.  Rotate the engine so the dowel drops all the way down into the bore and then mark this spot on the dowel against a reference point on the cylinder head, this is approximately BDC.  Continue to turn the engine until the dowel moves all the way upward.  Mark this spot on the dowel against the same reference point on the head, this is close to TDC.  Then turn the engine so the rod drops exactly half an inch back down into the cylinder and mark this point on the flywheel against a reference point on the engine case.  Then rotate the engine the other direction past TDC until the rod drops down into the engine exactly half an inch and mark the flywheel against the same reference point on the engine case.  True top dead center is in the middle of those two marks.

Just make sure you're using a long enough dowel so it won't actually fall into the engine.  Usually ten or twelve inches long should be fine.  Depends on the engine.  A two stroke with a short stroke might only need a few inches of dowel.

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Look in the manual?

Google it?

One might suggest if you don't know how to find TDC, you should not get involved in any work that requires setting the motor to TDC.

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10 minutes ago, CDNSXV said:

One might suggest if you don't know how to find TDC, you should not get involved in any work that requires setting the motor to TDC.

This is often the answer to a lot of questions on here. 

If you have to ask, you shouldn't be screwing with it.

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2 hours ago, CDNSXV said:

One might suggest if you don't know how to find TDC, you should not get involved in any work that requires setting the motor to TDC. 

How does one learn to do such work? I could be wrong, but asking questions is better than getting out the tools first. Forums exist to help fellow riders w/ questions like this. Not sure why so many are resistant to pass along their experience. Why suggest to "google it"? Maybe he thought that he'd get better info from fellow riders? I don't understand why people are on forums if they don't want to share with one another. Makes no sense. Maybe less newer riders are getting into the sport b/c too many with the knowledge find it too much of a burden to pass what they know along?

I agree on getting in over your head, but everyone starts somewhere and asking questions is exactly what he should be doing.

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There used to be a tool that you could screw into the plug hole that had a rod in the center that would do what was described earlier in here with a wooden dowel but this would measure the distance for you in millimeters  and you could lock it in place then take it out and measure. I don't know if they are still available or not I still have one that I haven't used in probably 20 years.

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7 minutes ago, Huskydog14 said:

That's it Creeper Jeeper a dial indicator with plug adaptor

And if you don't have the adapter, you can make one out of an old spark plug, just knock the center out and tap for a screw to hold the dial indicator.  You can get a dial indicator at HF for like $15 that works just fine.

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Dial indicator works but you still have to split the difference between two points further down in the stroke.  There is a range of several degrees at the crank where the piston is stationary at TDC.  This is the most important aspect of the job.

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The marks on my bikes are so hard to see I always use a pencil down the spark plug hole.  I pull the crank inspection plug off and put a 17mm socket on the crank nut with a "T" handle.  You can rotate back and forth and feel TDC close enought for any valve/cam work just with your fingers.  I don't need marks or dial indications, since I can tell with way higher precision than a tooth on the timing chain.  Give it a try and see how accurate you can tell TDC.

For TDC on compression stroke, just make sure your cam lobes are facing away from the buckets.

 

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A dial indicator only is accurate through a spark plug hole if the plug hole is square to the piston. It is most of the time but not always. If there is an angle, there will be angular pressure on the tip of the indicator and true TDC will not be found.

Nearly all bikes have marks on the flywheel and a corresponding mark on the cases. The marks on the flywheel (you may find three) typically equate to TDC, idle spark and full advance spark. If I am building an engine with no marks on the flywheel, I make marks on it during the build process (head is off and I can use a dial indicator square to the piston). If needed, I can then use a degree wheel later in the process to make additional marks at key points of rotation.

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How do you find TDC?


A lot of people jumping to conclusions...

Are you talking about for starting purposes? I assume you are based on your other threads, as I tend to research before replying.
Slowly push the starter down until you find the point of maximum resistance. Bring the starter back to the top, careful push it just past the resistance point (TDC on the compression stroke). It's only going to take an inch or 2. Bring the starter back up and give it a full kick. It should fire right up.

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1 hour ago, wielywilly-g said:

The marks on my bikes are so hard to see I always use a pencil down the spark plug hole.  I pull the crank inspection plug off and put a 17mm socket on the crank nut with a "T" handle.  You can rotate back and forth and feel TDC close enought for any valve/cam work just with your fingers.  I don't need marks or dial indications, since I can tell with way higher precision than a tooth on the timing chain.  Give it a try and see how accurate you can tell TDC.

For TDC on compression stroke, just make sure your cam lobes are facing away from the buckets.

 

This.
Simple, easy to do with basic tools and still very accurate.

Synopsis:

1. Pull plug and put pencil down hole. (carefully)
2. Use socket to turn crank (via opening the plug on side of case)
3. Rotate engine till cam lobes are pointing outwards
4. Put finger on end of pencil and rotate engine back and forth slightly till you find "the middle" between where the pencil stops moving up and before it moves down.

Lastly, what is this for?
For the purpose of timing? Or for a simple vale gap check?
If your gapping valves, you don't need to be at exactly dead center, simply "eyeball" the cam lobes and make sure they're sideways. (i just use the back wheel with transmission in gear)
long as they're "out of the way" you can slip a feeler gauge underneath to take measurements.

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8 minutes ago, ohiodrz400sm said:

A lot of people jumping to conclusions...

Ha, this is a very good point.
This poor guy (OP) could have simply been asking how to find TDC for starting the bike :lol:
And here were are giving paragraph long answers!

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Ha, this is a very good point.
This poor guy (OP) could have simply been asking how to find TDC for starting the bike
And here were are giving paragraph long answers!

I saw he had another post about dead engine starts and someone brought up TDC. As soon as I read that I just assumed that's what he meant. I may be the one jumping to conclusions. Hopefully we didn't scare him away.
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I saw he had another post about dead engine starts and someone brought up TDC. As soon as I read that I just assumed that's what he meant. I may be the one jumping to conclusions. Hopefully we didn't scare him away.

Yeah you're right, I was talking about finding TDC on the starting line.
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24 minutes ago, bbix_211 said:


Yeah you're right, I was talking about finding TDC on the starting line.

As you slowly turn the engine over with the kicker, you'll feel it reach a hard spot in the compression as the piston is coming up in the cylinder. Leave pressure on the kicker and as cylinder pressure bleeds off, you feel the kicker drop into a detent with another hard spot. That is TDC for starting purposes. Once there, bring the kicker back to the top and give a good kick.

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