Identifying TDC

[Josh]

I could figure this out on my own, but i figured I would ask first and potentially save me some effort.

I am currently indexing my 36-1 trigger wheel for MS. I am running it off the crank pulley, so i need to identify where TDC is. It would be easiest to locate it in relation to where the keyway sits. I currently assume the keyway is facing upwards at TDC, as shown i the picture. If someone could confirm or deny this (and tell me where it actually is supposed to be) that would be awesome.

22960

[sdg3205]

I could figure this out on my own, but i figured I would ask first and potentially save me some effort.

I am currently indexing my 36-1 trigger wheel for MS. I am running it off the crank pulley, so i need to identify where TDC is. It would be easiest to locate it in relation to where the keyway sits. I currently assume the keyway is facing upwards at TDC, as shown i the picture. If someone could confirm or deny this (and tell me where it actually is supposed to be) that would be awesome.

22960

Josh,

I found TDC using the method whereby you remove the plug under the water pump and find the rectangular slot. Is your water pump off the car? If so it's super easy and quick.

[Josh]

Josh,

I found TDC using the method whereby you remove the plug under the water pump and find the rectangular slot. Is your water pump off the car? If so it's super easy and quick.

It was a week ago, darn.

Something that came to mind is since the 2.8 is oddfire and the 3.0 is even, the TDC may be different?

[sdg3205]

It was a week ago, darn.

Something that came to mind is since the 2.8 is oddfire and the 3.0 is even, the TDC may be different?

Yeah, i hadn't thought of that. good call.

[David T]

The best way to find TDC is to get #1 cylinder on the compression stroke and, using a dial indicator, rotate the crank until the piston is at TDC.

[Josh]

The best way to find TDC is to get #1 cylinder on the compression stroke and, using a dial indicator, rotate the crank until the piston is at TDC.

yep! That is the plan if I have to figure it out myself.

[WelmoedJ]

yep! That is the plan if I have to figure it out myself.

Use a timing light, or an old spark plug (not the one in there) and plug wire for cylinder 1.
Should be a quicky that way.

[David T]

Use a timing light, or an old spark plug (not the one in there) and plug wire for cylinder 1.
Should be a quicky that way.

And with those items how do you mechanically locate TDC? The OP was not looking to time the ignition system, at least not yet. Before you can do that you need to know where TDC is. TDC (Top Dead Center) is where the piston in cylinder #1 (or whatever cylinder you are trying to time) is at the exact top of it's stroke and is between the compression stroke and the firing stroke. I believe he is trying to find this point so he can set up a crank sensor for an alternate ignition system. Locating TDC (or just verifying it) is a basic technique in preparation to setting ignition timing. It is not good to just assume the timing marks are accurate. They are not always and can be off a little and are not usually adjustable. This is a mechanical procedure and does not involve a running motor or even an ignition system.

[Josh]

Use a timing light, or an old spark plug (not the one in there) and plug wire for cylinder 1.
Should be a quicky that way.

I am also curious about this method, the assumptions David made are correct.

[Spittybug]

Quick and dirty......

Remove plug in #1. Insert a pencil, eraser side down, slowly and carefully until it hits the piston. Put car in 4th gear and gently push/pull car until the pencil sticks out the greatest amount. This means you are either TDC on either the power or the exhaust stroke. To find out which, look to see where your rotor is pointing in the distributor. If pointing to #1, you are on your power stroke.

"Pencil" is a metaphor. Depending on the depth of your plug hole (I don't recall if I've used this trick on my Delorean, but I certainly have on other cars) you may need a long plastic straw or something.... Just make sure it is soft and that you pull it back out again when done!

[David T]

Quick and dirty......

Remove plug in #1. Insert a pencil, eraser side down, slowly and carefully until it hits the piston. Put car in 4th gear and gently push/pull car until the pencil sticks out the greatest amount. This means you are either TDC on either the power or the exhaust stroke. To find out which, look to see where your rotor is pointing in the distributor. If pointing to #1, you are on your power stroke.

"Pencil" is a metaphor. Depending on the depth of your plug hole (I don't recall if I've used this trick on my Delorean, but I certainly have on other cars) you may need a long plastic straw or something.... Just make sure it is soft and that you pull it back out again when done!

This method is good to get "into the ballpark" so to speak. It is not accurate and can be off many degrees in either direction. Because of the dwell (the time in degrees the piston is actually at the top of the stroke) you can be off as much as 5 degrees either way. The more accurate method requires a degree wheel and a dial indicator to find the dwell period and the actual position of the piston. This is part of "dialing in" a motor and accuracy counts. If your basic measurements are not accurate you cannot accurately tune the motor. For someone who is going to install an electronic ignition system, you will never know where you are if the basics are not right. 5 degrees is a LOT and will affect starting, economy, and power. Might as well just assume the existing timing marks are close and leave it at that!

[Ron]

I have an old Proto Tool I use if a dial indicator is too hard/impossible to set up. It is a simple plastic threaded rod, rounded on one end and a hex head on the other. You screw it into #1 spark plug hole and carefully turn the crank until it bumps against the tool, scribe the pulley/balancer, turn the crank the other direction until it bumps again, scribe the pulley again. TDC is centered between the scribe marks (ie halfway).
I've seen a few made out of a gutted spark plug or non-fouler....

[Bitsyncmaster]

I have an old Proto Tool I use if a dial indicator is too hard/impossible to set up. It is a simple plastic threaded rod, rounded on one end and a hex head on the other. You screw it into #1 spark plug hole and carefully turn the crank until it bumps against the tool, scribe the pulley/balancer, turn the crank the other direction until it bumps again, scribe the pulley again. TDC is centered between the scribe marks (ie halfway).
I've seen a few made out of a gutted spark plug or non-fouler....

+1
Those tools can be bought cheaply online. Very accurate way to find TDC but you have to be able to turn the engine over carefully (all plugs removed and breaker bar on the crank). Most are just a metal bolt tapped into a "plug" case.

[mluder]

Pardon my inexperience but couldn't you use a combination procedure?

Insert the pencil in spark 1 and watch for it to top out.
Verify this is the ignition stroke via the distributor rotor location.
Then rotate the engine again to mark the position of the pulley for the start of the top and the position of the pulley when it starts to drop.
Then find the middle and that should be TDC within a degree or two.

Is it safe to "rock" the engine? IN other words, once you think you've found the start of the top of the stroke can you turn the pulley in reverse until it starts to fall again. Going back and forth here would allow you to get pretty close to the mark for the start and stop points of the stroke.

I'm curious as I need to do this too. When I replaced my timing chain cover I had no good way of making sure the timing plate was installed in the correct position.

Cheers
Steve.

[David T]

Pardon my inexperience but couldn't you use a combination procedure?

Insert the pencil in spark 1 and watch for it to top out.
Verify this is the ignition stroke via the distributor rotor location.
Then rotate the engine again to mark the position of the pulley for the start of the top and the position of the pulley when it starts to drop.
Then find the middle and that should be TDC within a degree or two.

Is it safe to "rock" the engine? IN other words, once you think you've found the start of the top of the stroke can you turn the pulley in reverse until it starts to fall again. Going back and forth here would allow you to get pretty close to the mark for the start and stop points of the stroke.

I'm curious as I need to do this too. When I replaced my timing chain cover I had no good way of making sure the timing plate was installed in the correct position.

Cheers
Steve.

You could do it that way. The only problem is the assumption the distributor is set correctly. If the motor was running that is a good assumption. Otherwise you would watch the rockers and see when the valves open and close and you would know you are on compression. Another way is to put your finger over the spark plug hole and when you feel pressure that is the piston coming up on compression. Yes, you can rotate the motor in either direction. This method is close too. OK if you are only verifying marks. If you have to set or make them you should try to do this the most accurate way you can.

[Ron]

...it really wouldn't matter if you were on the compression stroke or not for adjusting the indicator/plate -- TDC is TDC.

[Josh]

Very true! Also something to note is since the premier does not have an advancing distributor the timing marks are non-adjustable.


So thinking about this... If I were to line up the timing mark with zero on the plate and the notch in the crank pulley the engine would be at TDC? I should really just not be lazy and get a dial gauge, lol

[David T]

Very true! Also something to note is since the premier does not have an advancing distributor the timing marks are non-adjustable.


So thinking about this... If I were to line up the timing mark with zero on the plate and the notch in the crank pulley the engine would be at TDC? I should really just not be lazy and get a dial gauge, lol

You're correct, TDC is TDC and it is the same for each stroke. The reason you typically want TDC at the end of the compression stroke (and the beginning if the power stroke) is because you usually need to be there to adjust valves or to install the distributor. If all you are doing is setting the TDC marks (or just verifying them) you do not have to be at the top of any particular stroke. That just puts the cams in a certain position and the valves accordingly. BTW, make double sure you are checking TDC for #1 cylinder!

[Ron]

Very true! Also something to note is since the premier does not have an advancing distributor the timing marks are non-adjustable.


So thinking about this... If I were to line up the timing mark with zero on the plate and the notch in the crank pulley the engine would be at TDC? I should really just not be lazy and get a dial gauge, lol
Assuming the pulley is one solid piece (v/s has a rubber mounted damper that can slip etc), you could then say cylinder #1 ( not "the engine" ) is at TDC, but, it could be at the end of the compression or exhaust stroke...

[Josh]

Yes that does make sense. Sorry about my sloppy terminology. I understand there are two instances of tdc in the 4 stroke cycle and tdc is referring to piston #1. For the purposes of a wasted spark edis6 setup through megasquirt, it does not matter which valve is open, just that piston #1 is at tdc.

The 3.0 crank is one solid piece, just like the original Delorean one.