Top Dead Center

[Member 10617]

Has anyone had any experience with one of those simple Top Dead Center Indicators (a tube with a spark plug type thread and cushioned probe at one end and a scale at the other)?

I know that there are even simpler and cheaper methods (such as a plastic tube with oil in it), but the little TDC indicator looks like it could do a good job, and they are really inexpensive.

 

[Deleted member 8987]

Used similar over the years. However, on something like a Triumph, not much gain in stock form. The "trick" is to insert, roll the engine one way until you have contact with piston. Mark the damper at the pointer. Roll the other way until you have contact with piston, and mark the damper.
Use a tape measure and find the exact center between the two marks, and THAT is TDC.

 

[Member 10617]

TOC,

Not sure I understood what you wrote. The little TDC tool, as I understand it, has a kind of plunger/indicator that moves up as the piston pushes it up and then stops when the piston hits TDC, and then it stays at that position as the piston continues down (losing contact with the plunger). That would appear to be TDC. I'm not sure why I would have to take two measurements and use a tape measure.

Sorry to be in the dark.

 

[Scott_Hower]

TOC,

Not sure I understood what you wrote. The little TDC tool, as I understand it, has a kind of plunger/indicator that moves up as the piston pushes it up and then stops when the piston hits TDC, and then it stays at that position as the piston continues down (losing contact with the plunger). That would appear to be TDC. I'm not sure why I would have to take two measurements and use a tape measure.

Sorry to be in the dark.

Because the "when it stops" method is not a particularly accurate way to measure TRUE TDC. The thingy sticking out of the plug hole may still be moving but ever so slightly at TDC is reached/passed. The procedure mentioned takes a reference going first in one direction (thingy stops moving, make a mark). Continue past TDC a few degrees, then go back the other way (thingy stops moving, make another mark). The distance between the TWO reference points is true TDC. They will invariable be a few degrees apart.

 

[Member 10617]

Scott,

Still in the dark here. If the little rod with a scale on it stops moving at the top of the piston's rise (thingy stops moving, make a mark), I don't see how it will continue to rise (thingy stops moving, make another mark) if the piston is on its way down. Seems to me that the scale sticking out of the rod/tube would just stay where it is and not move up or down.

 

[LBCs_since_1988]

because when the piston stops the con rod and crank are still moving....

check this out

 

[Darrell_Walker]

When the crank journal is near TDC (or BTC), the crank can turn quite a bit for a very small movement in the piston. When the journal is between (3 or 9 o'clock), there is a lot of piston movement relative to the rotation. So measuring below TDC, and splitting the difference, will produce much more accurate results.

 

[Member 10617]

I guess I just don't see it...

The little gauge on the Top Dead Center Indicator that sticks down into the chamber and is pushed up by the piston will be pushed up until the piston reaches its highest point. When the piston starts down (having reached TDC) it leaves the little gauge suspended, marking TDC on its scale.

I wonder if we are talking about the same kind of Top Dead Center Indicator?

 

[TR3driver]

Right. In fact, there is a short zone at the top where the piston does not move at all. Instead the change in sin theta is taking up the clearance in the rod bearing.

A piston stop is also really easy to make. Just take an old spark plug, break the ceramic out, thread it for a bolt, insert the bolt, cut off the head and round the shank.

But honestly, all that precision isn't really necessary in most cases. Ignition timing is always a compromise anyway, the factory timing most likely is not within 4 degrees of the optimum timing. And the cam timing can only be set in increments of about 4 degrees (unless you are using aftermarket equipment like a vernier timing gear).

And just "by eye" will get you within 4 degrees.

 

[Twosheds]

I guess I just don't see it...

The little gauge on the Top Dead Center Indicator that sticks down into the chamber and is pushed up by the piston will be pushed up until the piston reaches its highest point. When the piston starts down (having reached TDC) it leaves the little gauge suspended, marking TDC on its scale.

Due to the clearances between the gudgeon pin, piston, bearings, etc. the piston will stop rising before the crankshaft reaches TDC, and won't fall until the crankshaft is past TDC. So there are a few degrees of possible error with this type of tool.

 

[poolboy]

If the indicator followed the piston back down you'd see that once the piston reached it's highest point, the crank/damper would be able to rotate a few degrees before the piston began to drop.
It's those degrees that have to be split by rotation in the opposite direction that will indicate the true TDC..
Make sense, Ed ?

 

[Member 10617]

.... So.... with the TDC Indicator screwed into the cylinder and the probe extending down into it. the probe will rise with the piston and the scale will rise. Then it will stop as the piston reaches the top, but the crank will continue to move before the piston begins to drop. Then if I turn the engine in the opposite direction, the piston will rise, the TDC Indicator scale will rise until the piston again hits the top. And both of these stopping points will be different? So I have to split the difference for true TDC...?

Sounds to me that you are saying that between "top" (one direction) and "top" (the other direction) there is a "plateau," and that TDC is in the center of this "plateau."

 

[poolboy]

That's a good way to think of it, Ed.

 

[Deleted member 8987]

The play in bearing, piston pin, and piston slop are negated when you come from both directions and measure the centre of the marks you made.
Think about it.
Unless you have one of those very rare 1954 engines that only ever have slop in one direction of rotation.
Just like Randall said, we used to just break out an old plug and spot-weld in a rod (leave it extend out....makes a good handle).

When you see pro racers using that in a shop setting (albeit with a degree wheel on the balancer), you know it works.

 

[Member 10617]

Makes perfect sense to me now! The Top Dead Center Indicator tool looks very simple, and it is cheap, so I think I'll go with that. I've seen a degree wheel used in a video, but didn't really understand what I was seeing.

Many thanks.

Still too cold here in the "sunny south" to go for a ride... so I am tempted to tinker.... Got to give up that tendency.

 

[poolboy]

...fore you break sumtin..

 

[Don Elliott]

Someone once wrote, "If it aint broke - keep at it till you break it!".

 

[Member 10617]

... And: "If it ain't broken... improve it." Back in the TQM days.

 

[Geo Hahn]

And now perhaps someone will explain how a point on the flange of a train wheel actually moves backwards for a moment even as the train itself is moving forward.

One can really over-think these things.

 

[Member 10617]

One note about "tinkering": It gives a certain pleasure and sense of confidence driving down the road to know what is happening or should be happening under the hood. As far as I am concerned, it is part of the experience of these cars and separates the "driver" of one of these cars from someone who is simply steering a machine down the highway.