How to find TDC?

[18Fiatsandcounting]

OK, this comes up time and time again, so I'm interested to hear your tricks: How do you determine TDC on your engine?

I can tell you what I do: Remove the timing belt (or at least loosen it off the camshaft pulleys). Remove the #1 spark plug. Using a screwdriver or the like inserted in the spark plug hole and resting on the piston crown, rotate the crankshaft by hand until the screwdriver reaches its highest point. Now, and this is the critical part and done by feel: Rotate the crankshaft pulley by hand, and you can feel a "sweet spot" where the crankshaft turns fairly easily before getting harder to turn on either side of TDC (i.e., when the piston starts to descend again and the rings are adding friction to the motion). In the middle of that sweet spot is TDC. For my cars, that sweet spot is about a half inch or less of rotation on the crankshaft pulley circumference, or maybe plus minus a few degrees. In other words, if you find the middle of that "sweet spot" in crankshaft rotation, you are probably within a couple degrees of TDC. Close enough in my book.

Quite frankly, given the slop with which my timing belt cover was attached to the engine, you could be off 10 degrees or more depending on how you positioned the shield, or how you looked at the marks which required you to be able to use your x-ray vision to see through the front left fender to view them at the proper angle. I never trusted this method. I'm also not interested in removing my crankshaft pulley to uncover some mythical scribe mark on the crankshaft that may or may not exist.

So how do you determine TDC?

-Bryan

[RRoller123]

Same way, but use a dial indicator with an extension rod.

[18Fiatsandcounting]

Thanks, Pete, and that's an even better solution. Surprisingly, I've never bought myself a dial indicator but maybe now is the time.

-Bryan

[RRoller123]

Really cheap at HF. I think I got my extension rod set there too, but don't quote me. The way things are going lately, I am unsure whether I will continue to buy China. I would like to see extensive general repatriation, and will pay more with no hesitation.

Another really good way is to bolt a small piece of angle iron across the top (if the head is removed) with a bolt centered, head down, into the cylinder maybe a 1/4 inch or so. A "Piston Stop" I believe they call it. Then set a degree wheel, and slowly rotate the crank until the piston just touches the bolt stop. Mark the wheel, rotate around in the other direction, same thing, mark the wheel. Center is halfway between degree readings. (I have not actually done this myself, but it seems a common way with the US hot rodders to find TDC when the head is off.

[dinghyguy]

Dial gauge for me, but i have one circa 1940's that i was given which works fine for this. Next question we should answer for the readers is how to determine if you are on the combustion stroke without looking at the valves......because no one wants to face the dreaded valve cover leaky gasket if they don't have to (although if you are doing a timing belt you probably should check your valve shims too).

Dinghyguy

[SteinOnkel]

Dial gauge for me, but i have one circa 1940's that i was given which works fine for this. Next question we should answer for the readers is how to determine if you are on the combustion stroke without looking at the valves......because no one wants to face the dreaded valve cover leaky gasket if they don't have to (although if you are doing a timing belt you probably should check your valve shims too).

Dinghyguy

That's where this gets interesting. Yes looking at the valves is fine and dandy, but as you pointed out, I do not want to open up the covers unless I absolutely have to.

You could look at the distributor, but chances are if you're determining TDC you're doing it because everything is apart anyways.

Another way to do it is to listen to the engine. I would take all spark plugs out except for cylinder one and then try to determine if that cylinder is breathing in or out. If you've got a manifold off, hold your hand over it and feel for air moving.

[18Fiatsandcounting]

Next question we should answer for the readers is how to determine if you are on the combustion stroke without looking at the valves

I thought the best way was to look at the marks on the cam pulleys? If they are both aligned, either to a bracket pointer or to the nubs on the camshaft housing, that means that cylinder #4 is ready to ignite (or has just ignited since it ignites slightly BTDC) In other words, it's just finished its compression stroke.

Since the firing order is 1-3-4-2, then if you rotate the crankshaft 180 degrees (in the normal direction of course), cylinder #2 has just completed its compression stroke and is ready to fire. Another 180 degrees of crankshaft rotation and now cylinder #1 has finished its compression stroke, and other 180 degrees and it's cylinder #3's turn. And now you've gone full circle and are ready to rinse, wash, repeat.

This assumes of course that the locating pins that align the cam pulleys to the end of the camshafts haven't sheared off, in which case you likely have bigger problems...

-Bryan

[RRoller123]

No, it could also be #1 that is about to fire in that case. You have to look at the valves, to see that they are both closed on Piston #4. Both #1 and #4 pistons are at the same position, or nearly exactly, so close that you can take TDC off either one. As are #2 and #3 pistons, but 180 degrees out.

I suppose you could look at the position of the distributor to see which is about to fire (actually has just fired)? But with the Computronix , I of course couldn't do this, so I pull the valve cover that doesn't have the oil filler, and look through the hole of the one that does have the oil filler (on my 2L). To see that #4 valves are fully closed.

Pete

[SteinOnkel]

It will forever be a mystery to me why "you Fiat guys" have this love affair with timing things on cylinder number 4.

The VWs I'm more familiar with are all inline 4s and have the same firing order - 13's too young, 42's too old. Nowhere have I read anything about cylinder 4 being relevant.

[RRoller123]

Get it wrong and there will be an enormous backfire, from unburnt fuel in the exhaust system, that eventually ignites from the mistimed spark, blowing the air tube off, possibly damaging the AFM and throttle plate, and in my case, it shattered my overflow bottle. Fortunately, no bent or damaged valves, etc. BUT I have a nice shiny new AR overflow bottle now because of it, so there is always something positive that comes out of these things.

[18Fiatsandcounting]

No, it could also be #1 that is about to fire in that case.

Pete, I think you're thinking of the crankshaft marks being at TDC. I'm talking about the marks on the cam pulleys. If those two holes are lined up with the pointer or the nubs, then the crankshaft (well, pistons 1 and 4) should be at TDC, the valves for cylinder #4 are completely closed and it's on the end of the compression stroke, and the distributor rotor should be pointing towards the #4 plug. That's just how Fiat set it up for the DOHC engines.

To answer Steiny's comment, you can use a timing light on either #1 or #4 to set the ignition timing on the engine; it makes no difference. The reason that I like to put my timing gun on the #4 plug wire is that, in addition to setting the ignition timing, you can direct those timing light flashes to the two alignment holes in the cam pulleys and verify that they are equal and just a few degrees advanced. Exactly half of the ignition timing, actually. So, if you set your ignition timing to 10 BTDC, your camshaft timing holes should be 5 degrees before the pointer/nub. So, for a very simple change (timing light on #4 instead of #1), you get twice the bang for the buck.

-Bryan

[RRoller123]

Have to think this through, my engines are both disassembled right now, can't take a pic or video.

I like that timing light idea on the cam wheels! Will definitely try that.

[neoknight88]

I know I'm a newb, but aren't the cams and the crank marked for TDC? If I recall correctly, when the crank is at TDC by the marking on the crank pulley, there are marks on the cam gears that line up with metal fingers that indicate they're lined up correctly.

[SteinOnkel]

There absolutely are.

For some of us who like a little more precision though, it's a good idea to independently verify them.

[18Fiatsandcounting]

I know I'm a newb, but aren't the cams and the crank marked for TDC? If I recall correctly, when the crank is at TDC by the marking on the crank pulley, there are marks on the cam gears that line up with metal fingers that indicate they're lined up correctly.

Yes, as Steiny notes above. But here's the problem. Set your crank to TDC with the cam gear marks also lined up. Now rotate the engine one full revolution on the crankshaft. The crank is again lined up to TDC, but the cam gear marks are 180 degrees from their marks. So which is right? The answer is that when the cam gear marks are lined up and the crank is at TDC, cylinder #4 has closed its valves and is at the end of its compression stroke, and the spark plug is set to fire. When you rotate the crankshaft a full revolution from that point, cylinder #1 is set to fire (and the cam gear marks are 180 degrees off).

-Bryan