This is all going to be WAY too esoteric and technical for most users here.
You might understand it, but you won't have much cause to use it with a stock engine vehicle.
The first thing that crosses my mind,
Why would you use a piston stop on anything that didn't have camshaft in it (Other than to find TDC of the crank when installing a new camshaft or replacing timing set, WHICH should already be located?)
If you are using a spark plug type stop, the heads are on the engine, and you more than likely have a camshaft/timing set in the engine already...
NEVER, EVER, UNDER ANY CIRCUMSTANCES USE A SPARK PLUG TYPE PISTON STOP AND TURN THE ENGINE WITH THE STARTER!
If you do, you stand a VERY good chance of knocking a hole in a piston, ruining the rod bearing/rod, screwing up the head/plug threads, and it's unlikely you will be able to get that bent piston stop out of the head anyway without cutting off the bent part so it will unscrew.
I have a couple of spark plug stops that have 'Whistles' built into them.
When you hit compression stroke, the whistle blows.
They work quite well when turning the engine by hand when you don't have help.
While you are turning the crank, the 'Whistle' alerts you to the start of the compression stroke, so you know you are getting close...
But again, NEVER, EVER TURN AN ENGINE WITH THE STARTER WHEN YOU HAVE A SPARK PLUG HOLE PISTON STOP SCREWED INTO THE HOLE!
Spark plug stops contact the piston at an angle, and no matter how 'Rigid' you *Think* the stop is,
It's going to deflect when the piston contacts it...
And, since there is never a locking bolt on this type of stop, the stop will deflect in the threads, they are NEVER 'Tight', so you will have some variation in your readings from 'Actual' TDC when you use a proper stop over the cylinder with the heads off.
That's why serious racers use a block over the top of the cylinder, with a stop that is in line with the piston travel... No deflection... Better, more accurate readings.
One thing not mentioned here is,
It depends on which way you turn the crank on what reading you will get.
There is slop in the rod bearing, there is slop in the piston wrist pin in all engines, and used engines it's much larger.
When the crank travels across Top Dead Center, every fraction of an inch in 'Slop' translates to degrees on the crank.
Crank shaft direction (Forwards and backwards to normal rotation) will throw in variables in fractions of an inch,
SO you wind up with degrees of inaccuracy at the balancer when doing things like this.
The ONLY COMPLETELY ACCURATE way to do a 'Dead On' crank timing check is to pressurize the block with oil,
Then use an across the bore, inline piston stop.
This takes an ADJUSTABLE timing tab on the front cover, or an adjustable timing mark on the balancer (you have to go with an EXPENSIVE aftermarket balancer for that),
To get a completely accurate TDC setting...
Then you have to consider that most blocks DO NOT have cylinders bored completely perpendicular and completely INLINE with the crank.
Most are angled one way or another to the crank and many are angled off on BOTH axis.
We spend a LOT of time and money correcting just that very problem with high performance engines.
And just because you have ONE cylinder that is accurate, doesn't mean they all are!
Then you have to consider the crankshaft,
Many cranks are ground with the rod journals not in time on the crank.
This presents as cylinders with pistons NOT coming up to TDC at the correct crankshaft degree marks they are supposed to.
So if you get ONE cylinder accurate, it doesn't mean they all are accurate...
Like I said, Swanson has shown you a VERY good way to 'Correct' your balancer.
I highly recommend you use the CORRECTED timing tape on the balancer, it's a VERY good way to see exactly what #1 cylinder is doing in relationship to the timing scale,
But there are so many variables in these older engines it doesn't GUARANTEE you are dead on with the timing scale on any, or any one of, the cylinders.
The GOOD NEWS IS...
Your engines had to be pretty close, or they wouldn't be running this long!
Engines with LOTS of problems, or just one severe problem wouldn't have stayed together this long!
Modern machining and quality testing has eliminated or seriously reduced the flaws you see in older engines,
But if an older engine has stayed together this long, it's got to be VERY close!
And since we aren't running at 9,000 RPM or 28 Lbs of boost, they should continue to stay together with what ever flaws they have...
Another way to locate your 'TDC' with the spark plug stop is to turn the engine one way, make a mark, turn it the other way to the stop, make a mark,
Then back the stop out a little, do it again.
You will see the marks getting closer together as you do this.
And like Shawn said, it will take some of the 'Windward' and 'Leeward' slop out of the wrist pin and rod bearing when you do it this way...
When the marks are VERY close together, it's much easier to figure out where 'Center' is, and that should be where your balancer mark lines up with the 'Zero' on the timing scale.
Once you put a tape on the balancer, you read your timing from the TAPE, not the scale on the front cover.
Once the tape is in place, you simply use the scale on the tape, and the 'Zero' mark on the scale, it's usually more accurate that way...
Most racing engines have a pointer on the cover (TDC/Zero Mark adjusted to the balancer), and a scale on the balancer, and that is normally the best way to do things accurately...
One thing that is good about using the tape, you usually get WAY more degrees showing,
So you can see full advance as a direct read instead of just 12 degrees of advance or so on the typical front cover scale...
A VERY good thing when you are trying to map out your advance curve and make RPM specific or Vacuum specific changes since you can see FULL advance without having to mark up the balancer and try to remember what mark means what...
I'm with Shawn, I'm so used to actual racing dampers with the degree marks on the damper, and just an adjustable pointer on the front of the engine, My brain works that way, not with the scale on the front cover anymore...
Nothing like having 90 degrees of advance as a direct read off a good balancer instead of trying to mark the factory balancer for 'Compensation' and counting marks, then adding in the direct read for your advance total... (Made me tired just typing that!)
Shawn, I'm the all time king of crappy pictures!
I usually take 4 or 5 pictures of EVERYTHING, and often NONE of them are usable, then I have to try again...
And that takes a TON of time out of my projects to take the pictures, run in the house to see if ANY of them turned out fit to publish,
Then go back out and try to pick up where I left off...
Makes for DOUBLING the time of the project when I publish it on the forums,
And a TON of extra work trying to crop, resize, compress, transfer to the web site so I can link to them in the forums,
Then trying to write the project text in a reasonably coherent manner and in terms that a layman understands without his eyes glazing over...
One article on something that took two hours to do can take TWO DAYS to get corrected and published on the forums!
Then some Jack Wagon wants to argue about a .30¢ cost discrepancy in the parts listed on a $200 project, or what lube his uncle once told him to use 20 years ago, ect.!
(But I digress!)
Some people show pictures here I could NEVER do! I don't know how they do it...
Every picture I take is either over-exposed, under-exposed, out of focus, or just plain my Jittery hands screwing things up...
And that's not if I don't have a finger in front of the lens!
I have some EXTREME close ups of my fingers!
Some pretty good shots of my feet, the ground, the ceiling in the office, ect.
Seems the 'Accident' shots ALWAYS turn out better than the ones I'm TRYING to take!
I'm pretty good at getting every thing set up for a picture, then fiddling with the camera, when I FINALLY get ready to take the pic,
Crap has rolled off!
Not in frame anymore, and usually trying to hide in the cracks of the workbench, floor or office chair I use for picture backgrounds!
I'm FINE with gears and wires, but photography is an ART, and I'm no 'Artist'!!!!