How to get a Top Dead Center mark on a flathead engine

[Bloo]

The lack of a "true" Top Dead Center timing mark is a problem that has come up in several threads recently. This thread aims to show how to get a TDC mark on a completely assembled flathead engine that is still in the car.

 

A true TDC mark is essential when troubleshooting tough driveability problems. Nobody thinks about it until they need it. In fact, I have seen many people building engines without checking it, thinking "I'll just time it with a vacuum gauge" or whatever. That might be fine until the car starts misbehaving, and maybe the cam has jumped time, or the distributor advance has screwed up in some way. There is no excuse for not checking the mark and correcting if necessary while the engine is out and apart.

 

When the engine is not out and apart, and does not at least have the head off, it is a bit more work. The principles here apply to all piston engines, not just flatheads, and there are plenty of reasons you might not have a TDC mark. Maybe you have one of those Buicks that can have the flywheel bolted on one right way or five wrong ways, and the last guy put it on one of the five wrong ways, and you don't want to take the rear axle, torque tube and transmission out of the car to fix it. Maybe you have an engine where some dreaded previous owner mixed parts from different years resulting in no usable marks. Maybe the marks are on the flywheel, inside a hole, and you would like some marks up front you can see with a timing light, preferably without a bunch of parallax error. I fall into that last category.

 

Some common methods of finding TDC on overhead valve engines include attachments with micrometers, dial indicators, etc. The other day someone suggested filling the cylinder with oil, and watching for the oil to rise in a clear tube. All of these things work, but tend to be difficult and slightly ambiguous because the piston is moving so slowly at TDC. Quite a bit of crank movement occurs without much piston movement. For this reason, I prefer the "piston stop" method. If the piston is not at top dead center, it is moving a lot more compared to the crankshaft.

 

What is the piston stop method? A pointer is added to the engine if necessary. Some sort of a stop is put in place and the crank is rotated until the piston hits the stop. Then, a temporary mark is made.

 

Then the crank is rotated the other way until the piston hits the stop again. Another temporary mark is made.

 

A permanent third mark is made exactly halfway between the two marks. This third mark is true TDC.

 

Is this perfect? No. It can be slightly wrong due to offset piston pins or cylinder bores. Long ago when I was in college, and could still remember how, I calculated this error for some common American V8, probably a Chrysler one. The error was less than 1/3 of one degree. I consider it negligible.

 

When engines are apart, usually something like this is used.

 

 

Actually the ones I have seen professional engine builders using were even simpler, usually a piece of steel with a nut welded or brazed on the bottom for a stop. This guy made his fancy and adjustable. Of course you could get really fancy and buy a commercially made anodized aluminum one like this:

But that doesn't help if the head is on. On overhead valve engines you can usually screw something in the spark plug hole. My tool is a spark plug base with short 14mm threads, a piece of steel rod brazed in, and the tip ground approximately hemispherical. It works in more engines than you would expect. There are a million different kinds, and most of the professionally made ones are adjustable length like this:

 

Of course you should be really familiar with the which way the valves move in the engine you are working on. You wouldn't want this thing getting tangled up in the valves. I have seen tools that have a very small diameter protrusion. The downside is the small diameter protrusion would get bent easily.

 

None of this helps us on a Flathead engine though, as hardly any of them have the spark plug hole over the piston. It is almost always somewhere over the valves. But there is a way.

 

More to come....

Edited April 21, 2023 by Bloo (see edit history)

[Bloo]

Enter the giant Zip Tie. I am not the guy who came up with this, and I don't know who did. I have seen it posted various places on the Internet.

 

 

It doesn't necessarily have to be this big. In fact this one is almost too big. I had to file on it a little to get it through a 14mm spark plug hole. The tail of the zip tie needs to be really long though, so you can't possibly lose it in the engine.

 

Here is how it is done:

 

 

The zip tie head follows the piston up at the far side of the cylinder. It gets caught between the piston and the head and stops the crank.

 

I did the same thing once before to the car in the picture above, but it wasn't all disassembled then. Access was terrible. I made a pointer from welding rod to bolt on at the generator slider bolt. It had an offset so it would locate itself against the timing cover in the same place every time, but had to sort of stick up to miss the pulley. Not ideal.

 

 

 

As I recall these marks were made with drops of paint on the end of a kebab skewer, due to the terrible access. 30 and 40 had already washed off when the picture was taken, and are edited back in. That white block over to the left is reflective tape for a Harbor Freight photo-tachometer that never worked.

 

TDC is really the only mark you need if you can use a dialback timing light, but that is a shaky proposition on this car.

 

This time around the pointer will be more solid.

 

 

It is a pointer from a 1949-1954 Pontiac, and after being spaced out 3/8" further than it was on the newer engine, it fits.

Next, the battery gets disconnected if it wasn't already, and all the spark plugs get pulled out. We don't want any uncontrolled movement of the crank.

 

 

The zip tie gets put in, and is arranged so it's head is dragging the cylinder on the far side as previously discussed.

 

 

The engine is turned over gently by hand, with a wrench. The zip tie is guided up the cylinder wall by the piston until it stops. The first temporary mark is made. I'm using pencil here because it is easy to get rid of. These temporary marks will have no meaning once TDC is found. It doesn't matter which direction you go first. Apparently I started with the counterclockwise direction in these pictures.

 

 

Now carefully crank it around the other way, guiding the zip tie up to the same spot as before. It's clockwise this time in these pictures. Make a second temporary mark. Note the first temporary mark.

 

TDC is exactly between these 2 marks. If you are working at the end of a kebab skewer, you might have to get really creative. With things wide open like you see here there are lots of options. One might be a fabric measuring tape. The only thing that makes it the least bit difficult is that the surface is curved.

 

 

I wrapped a piece of paper around the curve and transferred the 2 marks. 62mm. We will be working in millimeters here because the metric side of this ruler has finer graduations.

 

 

Half of that is 31mm in this case, so I make a mark.

 

 

And then took it back to the balancer and transferred the mark.

 

 

Made sure it was square to the pointer....

 

 

And then painted it red. The temporary marks can be erased now.

 

If you can use a dialback timing light on your particular car, TDC is all you need. I'll be making some more useful timing marks anyway. Stay tuned.

 

Edited November 28, 2022 by Bloo (see edit history)

[Hemi Joel]

Great info! I have used piston stops, but never heard of using a zip tie on a flathead. Thanks for the tutorial.

[Bloo]

What else can you do? Well, a lot actually. If there are other marks on your flywheel, like to set valves or something, as long as you know their degree positions, you can duplicate them. The same goes for ignition base timing marks.

 

You could also make TDC marks for all the cylinders. Those, along with a dialback timing light would allow you to see things only guys with distributor machines normally see, like timing errors in the points cam. I have a distributor machine, and already know my points cam is mostly rubbish, but it's easy to do now so why not?

 

On a four stroke engine, as used in almost all cars two rotations are needed to fire all cylinders. On most of them this implies that you only need half as many TDC marks as you have cylinders.

 

For instance a typical inline four cylinder fires every 180 degrees of crank rotation. That means that you only need one more mark, halfway around the balancer or flywheel.

 

The common type of American V8 with the cylinders arranged in a 90 degree vee fires every 90 degrees of crank rotation. In that case you need to make a mark halfway around the balancer or flywheel just like the four cylinder, and then make marks 1/4 of the way around.

 

A typical inline six like our Pontiac guinea pig here fires every 120 degrees of crank rotation. That means we must divide the balancer or flywheel by thirds. This Pontiac balancer is a 6 inch balancer almost exactly. Actually 5.984" if I remember correctly. Let's divide it into thirds.

 

Convert that to metric: 5.98x25.4=151.89mm

Convert diameter to circumference: 151.89x3.14=476.93

Divide by 3 for first mark at 1/3 the circumferencce: 476.93/3=158.98 

Multiply by 2 for the second mark at 2/3 the circumference : 158.98x2=317.95

 

Round those to the nearest whole number and we have marks at 159mm and 318mm from TDC.

 

I made a zero mark and marks at 159mm and 318mm on a piece of paper. Then I wrapped it around the balancer with 0 aligned to TDC.

 

 

Made a mark at 159mm:

 

 

And one at 318mm:

 

 

Then I aligned the 318mm mark to TDC, and wound the paper around the opposite direction to check my work.

 

 

And 159mm aligns to a mark:

 

 

So does 0:

 

 

Looks good. It matches in both directions around the balancer, so there are no errors. I marked them with blue paint to differentiate them from "real" TDC  which is marked in red.

 

What else can we do? How about the factory ignition timing marks? A flathead Pontiac has two of them on the flywheel, 2 degrees and 6 degrees. The circumference is 476.93mm, so:

 

Find how many mm per degree: 476.93/360=1.32mm

Multiply by 2 for 2 degree mark: 1.32x2=2.64mm

Multiply by 6 for 6 degree mark: 1.32x6=7.92mm

 

Round and we have 2.5mm and 8mm. I marked these on a piece of paper and transferred them to the balancer:

 

 

I marked them with white paint. Now what? How about timing tape? With timing tape, you can suss out distributor curves with a plain timing light, one with no dialback. I already had some MSD timing tape here.

 

 

They give you a bunch of different sizes for different size balancers. Fortunately, 6" was one of those sizes. Not for showcars I guess, but pretty handy on anything else.

 

 

I also put the purple dots back in case the tape falls off.

 

Edited November 28, 2022 by Bloo (see edit history)

[Otahuhu]

Would be interested how to check the timing on a Stutz single overhead cam straight eight that the only TDC mark is well hidden under the fire wall There is no exposed pulley on the front of the motor The ignition is dual with two spark plugs per cylinder   Is there a way to check this as mentioned in the other articles ? Len

[john hess]

bloo,  great idea on marking for all cylinders. I'll do that. I was wondering how to check for distributor cam wear without a dwell machine.... thanks... John 

[Bloo]

That depends on what kind of wear. If it is just worn down a lot but evenly, and you set the points with a feeler gauge, and then check with a dwellmeter, the dwell will be off, and low. This is because the tallest spots of the cam have worn down.

 

That isn't what I was referring to in the post above. I meant some cylinders firing ahead of others. I already know there is about 3 degrees variation on my car, because I have checked it on a distributor machine. I don't know how accurate they were when new back in 1936, but I would expect better than that from a postwar car. I'll bet they didn't make it like that and it's uneven cam wear.

 

How to test? Well, with 3 TDC marks, you could put the timing light on #1 and dial the light back to zero. Then you could move the trigger clip to all the other cylinders one by one. They should all be the same. On my car some of them are going to be 3 degrees off. If there was no variation, I could in theory advance the timing 3 degrees. I'm sure that would be noticeable while driving.

 

Edited November 29, 2022 by Bloo (see edit history)

[Bloo]

9 hours ago, Otahuhu said:

Would be interested how to check the timing on a Stutz single overhead cam straight eight that the only TDC mark is well hidden under the fire wall There is no exposed pulley on the front of the motor The ignition is dual with two spark plugs per cylinder   Is there a way to check this as mentioned in the other articles ? Len

I'm not sure how to proceed with that. I'm guessing it's a Vertical Eight? You probably have 2 coils and at least 2 sets of points. I suspect the best way to get all that working in harmony, or to test it, is with a distributor machine. As for the timing mark, is there a toeboard you can take out to see it better? The idea of using a stop to verify that the mark is accurate will work on a Stutz as well as any other car, but I would be very concerned that the stop could get tangled up in the valves somehow. If it were mine, I would not do anything like that without talking to @edinmass first. It's a shame they didn't give you a pulley to put a more visible mark on. I have a Hupp N with no front pulley. They drove everything with silent chains, even the fan.