Easy Method to find TDC

[Jack B]

This may have already been done, but, here goes, we are building a circuit to log true timing advance, therefore, the need to find a precise TDC. It turned out quite simple, Eastwood's has a device that screws into the spark plug hole and a slide that moves up and down with the pistion. This is fine for adjusting the rockers, but, not accurate enough for timing purposes.

A few thousands on the piston movement means several degrees, therefore, the need to be precise. You only have to take out the valve cover bolt to the rear of cylinder #1, replace the bolt with a 4" 1/4 X 20 bolt. You than mount the dial gauge on a common swivel bracket and bolt it to the 4" bolt.

The dial gauge now travels with the piston. On my car there was no transition, within .001, the engine hit TDC and started to travel back down. Using this method it only takes one revolution to find TDC. That was the easy part, the timing acquisition is next.

[Nadine UK GTS]

Thanks Jack, looks simple but effective, thanks for sharing.

[1TONY1]

True timing advance data with the stock pcm.....that would be nice.

[joe117]

Another way is to use a bumper.

You take an old spark plug and epoxy a steel rod in place of the ceramic insulator.

Make sure the rod reaches far enough to hit the piston.

Then screw it in place, and remove all the other plugs.
Turn the engine, by hand, one way, till it stops against the bumper rod and mark the crank pulley.

Then turn the engine the other way and mark the pulley.

TDC is half way between the marks.

Shade tree, but it works.

[Schulmann]

I already had a similar methode for this but I still don't have a good methode to turn the engin 1/4 turn.

Two days ago I did a leak down test and the engin hit back 2-3 times on my hand (all plugs were out). The engin nearly tore off my wrist.

JackB, do you know a safe way to turn this engin ?

[Joel]

A U tube with fluid in it is a really cheap and accurate way also. I usually find there is a vagueness as the piston transitions from up to down, maybe a degree or so and it is best to measure from a certain distance down the bore on the up stroke to the same distance on the down stroke, then take the middle point.

[Joel]

PS.....but that way is nigh impossible with the U tube

[Johann]

Another way is to use a bumper.

You take an old spark plug and epoxy a steel rod in place of the ceramic insulator.

Make sure the rod reaches far enough to hit the piston.

Then screw it in place, and remove all the other plugs.
Turn the engine, by hand, one way, till it stops against the bumper rod and mark the crank pulley.

Then turn the engine the other way and mark the pulley.

TDC is half way between the marks.

Shade tree, but it works.

This is the only way to find true TDC, the dial indicator reading isn't accurate unless used with a degree wheel.

If close is all you need, the dial indicator is OK.

[Jack B]

Johan:
We did use a degree wheel to mark the the damper, but, why would you say it is needed to mark just TDC. The bearings must be tight, because there was no range at TDC, the piston never dwelled at TDC, it hit the apogee and started back down. We did it several times and it always ended up at the same spot. This saves time over using the locking bolt and splitting the range.

Tony:
We hit a wall with the output of the pcm to coil pack #1, were going to have to filter it, it is too ragged. It won't take long we will be logging true advance soon. I think we will find some interesting facts.

Schulman:
Maybe someone else can chime in, I have heavy valve springs, therefore, it is hard to rotate all the time, but, it doesn't kick back. I don't know what could kick the damper back if you have the plugs removed. I do know one thing you don't want to use a 32mm or 1-1/4" socket for too long. Use a 31 mm socket with a 1/2" drive that has a swivel head and breaker bar handle. I just received the 31 mm socket, I had a hard time finding one that was a shallow depth. The 31mm axle nut sockets are too deep.

Maybe you should take it out of gear - just kidding.

[joe117]

"it hit the apogee and started back down"

If it did that, good for you.

Many engines don't do this. There will be a few degrees where the crank is going over the top where you can't see any change in piston movement.

When you have a short block on a stand with a degree wheel attached and a dial indicator on a piston, you can see what I'm talking about.

In the situation above, the short block, I've always used a bumper of some kind and taken TDC as the halfway point between stops.

[joe117]

Someone with more interest in this question ;-) could do the math on this.
What's the diameter of the circle made by the outside of the rod journal? About 4" or so?
The circumference would be a little over 12.5"

Figure the change for a degree of crank movement on either side of TDC.

What is it? The sine of something or other, right?
I don't remember.

Anyway, this calculation would tell what the dial indicator should see.

[joe117]

The length of the rod will have to be taken into consideration too.

A longer rod will change the movement per degree at the top of the stroke.